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Best Tech Deals Reddit

looking for Best prime deals reddit 2020 ?
The best offer of Amazon's Prime Day 2020: the iPhone 11 lowered to 661 $, an all-time low price!
Nokia 3310 Free movies and series Smartwatch Huawei Tesla Model 3 Honor Xiaomi Ambilight Instagram Zoom
WE RECOMMEND YOU Best Amazon Tech Prime Day Deals
The PS5, gutted: the M.2 PCIe 4.0 slot stands out, but watch out for the gigantic 120mm fan and liquid metal for its CPUThe PS5, gutted: the M.2 PCIe 4.0 slot stands out, but watch out for the gigantic 120mm fan and liquid metal for its CPU
Two decades back: I wear a Nokia 3310 when I go out on weekends to avoid distractionsTwo decades back: I wear a Nokia 3310 when I go out on weekends to avoid distractions
'Patria': the first great landing of HBO Spain in fiction occurs with a multifaceted and hard vision of the Basque conflict'Patria': the first great landing of HBO Spain in fiction occurs with a multifaceted and hard vision of the Basque conflict
If you are waiting for Prime Day 2020 to find an iPhone from the still last batch with a good discount , be careful because you can take it with you beforehand: if you choose the 64GB iPhone 11 in red, green, mauve or yellow you will pay 661 $ .
Buy iPhone 11 at the best price reddit
The recommended retail price of the 64GB iPhone 11 is 839 euros and it is difficult to find it in well-known stores below 700 $. In Amazon, the red, green, mauve or yellow models are at different prices and with different discounts , but when you put them in the basket and proceed to payment, you will see that they are at 661 $ .
We detail some screenshots with the model in black, which has now been left out of the promotion. As you can see, the discount is indicated under the price with a message in green letters.
Apple iPhone 11 (64 GB) - (Product) Red
PVP in PcComponentes 739 $
Today on Amazon for $ 761.00Price in El Corte Inglés $ 809
In our analysis of the iPhone 11 we highlight its video stabilization, the game that gives the combination of angular and ultra-angular and the fluidity offered by the A13 Bionic chip, the same one that we find in the iPhone 11 Pro and Pro Max.
Amazon Music Unlimited for 4 months for 0.99 $ . With more than 50 million songs, on demand and without advertising with the possibility of listening to them offline.
Kindle Unlimited free for 3 months, to access more than a million titles to read on Kindle, mobile, tablet or computer.
If you are not an Amazon Prime user , you can try it for free for 30 days, which allows you to take advantage of the Amazon Prime Day offers and have free shipping costs.
Samsung Galaxy S20 + 5G 12 / 128GB for 818 $ in Amazon, Spanish version, it is normal to find it for 900 $ in other stores. High-end this year in 5G version with 6.7 "Dynamic AMOLED panel, Exynos 990 chip, 4500mAh battery. In our analysis we highlight how compact it is despite the size of its screen, the experience of its screen and the sound
It is the best phone of the year in quality price according to the editors of Xataka and in Aliexpress it is available for 333 $ from China. We are talking about the OnePlus Nord , which does not usually lower its official price in Spain. In our analysis we highlight its customization layer, the possibilities of the camera (same as that of the OnePlus 8) and its AMOLED panel at 90Hz
One of the most attractive mid-range on the market and from the Korean firm, cheaper: Samsung Galaxy A71 for 339 $ on Amazon, almost 30 $ less than in stores where it is discounted. It stands out for its large screen, autonomy and balanced design despite its 6.7 "OLED, as we read in the analysis
Redmi Note 9 Pro 6 / 128GB on eBay for 209 $ using PQ42020 , about 30 $ less than on Amazon , where it is also discounted. Ships from Spain in 72 hours, with a two-year warranty. It is the global version. The redmi Xiaomi Note 9 Pro highlighted in our analysis precisely for outstanding autonomy, quality of construction and the speed of fingerprint and facial recognition. And the Redmi Note 9S for 166 $ in its most basic version of 64GB in Aliexpress Plaza, in other stores you can find it for 20 $ more.
Xiaomi Redmi Note 9 Pro 6GB 128GB Smartphone 6.67 & quot; 64MP NFC Global Version
Xiaomi Redmi Note 9 Pro 6GB 128GB Smartphone 6,67 "64MP NFC Global Version
Today on Ebay for $ 233.00
RRP in Media Markt $ 279Today on Amazon for $ 241.99PVP at FNAC $ 253

For more details: https://techbylight.com/best-tech-deals-today/
20 $ discount on the Samsung Galaxy A51 on eBay, where you have it for 253 $ , one of the most attractive phones in quality price like the A71, but in a more compact format. Mid-range with 6.5 "Super AMOLED panel, 48.0 MP + 12.0 MP + 5.0 MP + 5 MP rear camera, 4,000 mAh battery with 15W fast charge
OPPO A72 4 / 128GB for 219 $ at FNAC, in other stores where it is discounted it costs 10 $ more. A mid-range with 6.5 "FHD + panel, Snapdragon 665 chip, 48MP + 8MP + 2MP + 2MP rear camera, 5,000 mAh battery, NFC
10 $ discount on one of the most sought after phones, the 64GB POCO X3 for 220 $ on eBay, it is the global version, it is sent from Spain by a seller with 99.7% positive votes in more than 8,000 transactions and offers a two-year warranty. The POCO X3 stands out for offering a 6.67 "FullHD + IPS panel with a 120 Hz refresh rate, the premiere of Qualcomm's new Snapdragon 732G chip and a large 5,160 mAh battery with 33W mAh fast charge. .
Save more than 10 $ on the Motorola Moto G8 Power , at 169 $ on Amazon and PcComponentes , an entry-level smartphone - medium with a brutal autonomy of 3 days
Redmi 9 4 / 64GB for 137 $ , in stores like Media Markt or FNAC is around 150 $. A simple terminal with outstanding autonomy, good appearance and extras to be appreciated in this price range such as NFC, here you can read our review
Hon
Mark the discount coupon to benefit from a discount of 10 $ on the new Honor Watch ES , a watch reminiscent in form factor of the Huawei Watch but cheaper: it stays at 89 $ . With 1.64-inch AMOLED panel, 5ATM water resistance, oxygen saturation and heart rate monitoring, 95 training modes and up to 10 days of battery according to the manufacturer
HONOR Watch ES Smartwatch Women Men Smart Watches, SpO2 and Heart Rate Monitoring, 5ATM Call and SMS Notification, 95 Training Mode 1.64''AMOLED Sports Watch, Black
HONOR Watch ES Smartwatch Women Men Smart Watches, SpO2 and Heart Rate Monitoring, 5ATM Call and SMS Notification, 95 Training Mode 1.64''AMOLED Sports Watch, Black
RRP on Amazon $ 99.00
HiHonor RRP $ 99
Put the Garmin Fenix ​​6S in the basket in Media Markt and pay 429 $ , in Amazon it costs 466 $. Downgrade for this high-end multisport GPS watch
Small discount on Apple's best value for money smartwatch: Apple Watch SE 40mm for 285 $ and the 44mm for 309 $ . Form factor and processor of the Apple Watch Series 5 and an approximate autonomy of 18 hours
Attractive launch pack: Huawei Watch T2 Pro and FreeBuds 3i for 299 $ at Amazon and Huawei . For what the latest and most ambitious smartwatch of the firm costs, you get the headphones as a gift. With more than 100 training modes, 1.39 "screen with AMOLED technology and sapphire crystal
Samsung Galaxy Watch Active 2 Special Edition Under Armor in AliExpress Plaza for the 44 mm version is 209 $, but applying the discount code "OCT25" it stays at 184 $ . With minimalist design and bold colors in this special edition, with a 1.4 Super AMOLED circular screen, with sensors to measure heart rate, accelerometer, gyroscope, barometer and electrocardiogram (ECG)
Ticwatch S Sport for 120 $ on Amazon, down 80 $ at once. A GPS and Android Wear 2.0 smartwatch with OLED touch screen that measures heart rate and integrated fitness assistant
The Huawei Band 4 drops 5 $ , now for 19.90 $ . Quantifying bracelet with 0.96 "TFT color screen, with 9 sport modes, waterproof, SpO2 estimation
Xiaomi Mi Band 4C at a historical price: 15.90 $ . A basic bracelet with a 1.08-inch color TFT screen with a resolution of 128 * 220 pixels, a flat pad and a silicone strap. Autonomy of two weeks and measurement of activity and five sports: running, walking, cycling, treadmill and free training.
Mini
Use the coupon PQ42020 and pay 332 $ for the 64GB iPad Mini 5 (2019) , a very interesting tablet for general use with a 7.9-inch screen with 2,048 x 1,536 Retina resolution, A12 chip and compatible with Apple Pencil
New Apple iPad Mini 5 Wifi 64GB Space Gray
New Apple iPad Mini 5 Wifi 64GB Space Gray
Today on Ebay for $ 369.00
Today on Amazon for $ 429.00Price in Amazon$ 449
The ambitious Huawei Matepad Pro for 449 $ on the official website and with a 50-$ discount if you buy a case and keyboard, the same price (but without that promo) as in Amazon and PcComponentes , 150 $ less than in El Corte Inglés. The most premium tablet of the brand, with a high quality screen, the most powerful hardware and outstanding audio. No Google services
If you are looking for a simple and cheap tablet, perhaps for the little ones in the house or for someone not very technological, the veteran Lenovo TAB4 2/16 drops 30 $ in Amazon: 79 $ costs this 8 "tablet with HD resolution, Snapdragon 425 chip , with Android 7.1.1 and Wifi + Bluetooth 4.0 connectivity
Airpods Pro 02
The AirPods Pro for 199 $ on eBay, 20 $ different from Amazon where they are discounted. New design and active noise cancellation technology for Apple's most ambitious wireless earphones.
Apple AirPods Pro
Apple AirPods Pro
Today on Ebay for $ 199.99
Today on Amazon for $ 219.90Price in Amazon$ 239
Details here https://techbylight.com/best-tech-deals-today/
On the official website, the HUAWEI FreeBuds 3 cost 99 $ . Active noise canceling, low latency wireless headphones, 14mm drivers, wireless charging. Very complete at this price, offering a good experience both to play and to listen to music from your devices
Last day to use the coupon and get the TWS Aukey headphones with active noise cancellation with a discount of 20%: from 59.99 $ to 48 $ . With 4 microphones to neutralize external noise, autonomy of up to 35 hours with the case, with fast charging via USB-C and IPX5 certification
From 35 $ to 23.14 $ the Samsung EVO Select 128 GB memory card , optimized for use in smartphones, tablets, 360 ° cameras, Full HD action cameras and drones. Class UHS-I U3
Are you a fan of the most famous plumber of all time? For your Switch but also for other devices such as cameras, phones and tablets, the SanDisk microSDXC UHS-I card for Nintendo Switch 128GB with Nintendo License, drops from 33 to 21 $
Computers and accessories
Lg
200 $ of automatic discount when processing the order in the ultrabook LG gram 17Z90N-V-AA78B , which stays at 1,199 $ on Amazon and Media Markt . A laptop for intensive use in mobility due to its lightness and autonomy, in addition to hardware, it is well served to work hours on demanding tasks (not editing): 17 "WQXGA IPS screen, Intel Core i7-1065G7 chip, 16GB of RAM, 512GB SSD and Windows 10 Home +
LG gram 17Z90N-V-AA78B - 17 "WQXGA IPS Ultralight Laptop (Intel Core i7-1065G7, 16GB RAM, 512GB SSD, Windows 10 Home +) Silver - Spanish QWERTY Keyboard
LG gram 17Z90N-V-AA78B - 17 "WQXGA IPS Ultralight Laptop (Intel Core i7-1065G7, 16GB RAM, 512GB SSD, Windows 10 Home +) Silver - Spanish QWERTY Keyboard
RRP in Media Markt $ 1199
Today on Amazon for $ 1,399.00PVP in PcComponentes 1599 $PVP at FNAC $ 1,487
Put this Asus Tuf Gaming A15 FA506IU-HN278 in the basket in Media Markt and pay 999 $ , brutal at that price for its processor, an AMD Ryzen 7 4800H, its 16GB of RAM, 1TB SSD and its graphics, a GTX1660Ti, which It does not have ray tracing but it is solvent to move games and offer a good experience. In other stores such as PcComponentes it costs 200 $ more
The HP Pavilion Gaming 16-a0025ns in Amazoncontinues for less than a thousand $ . For 999 $ you get a solvent configuration for gaming, with a Core i7 10750H processor, 16GB RAM, 512GB SSD, a GeForce RTX 2060 Max-Q 6GB compatible with ray tracing and a 16.1 "FHD panel that comes out of the common formats 15.6 "and 17.3". Without Operating System
At PcComponentes, if you buy a ConceptD 5, ConceptD 7, ConceptD 7 Ezel or ConceptD 500 professional laptop , you get a 500 $ gift card during this month of October
In the HONOR store there is the Honor MagicBook Pro reduced by more than 100 $, now for 749 $ . Careful design with 16.1 "panel with AMD Ryzen 5 4600H processor, 8 GB of RAM, 256 GB SSD, approximate 11 hours of autonomy, fingerprint reader and pop-up webcam, interesting for general use
ASUS ZenBook 14 UM431DA-AM022 for 699 $ at Amazon, several hundred $ less than in other stores. A lightweight laptop, with good autonomy, design and hardware for intensive use, thanks to details such as its ergolift hinge and its resistant chassis. Inside it comes with a veteran but powerful AMD Ryzen 7 3700U, 16GB of RAM, 512GB SSD and AMD Radeon RX Vega 10 graphics.
If you need a laptop for multitasking with dedicated graphics, in PcComponentes the HP Pavilion Gaming 16-A0023NS is reduced from 749 $ to 669 $ . With the modern Intel Core i5-10300H, 16GB of RAM, 512GB SSD, a veteran GTX 1050 and 16.1 "panel, a combination suitable for playing a game every now and then.
A neat laptop for general and office use: Asus P1510CDA-BR772R for 499 $ on eBay with Windows 10 Pro installed. Comes with an AMD Ryzen 3 3250U processor, 8GB of RAM, and 256GB SSD
Ody More Details https://techbylight.com/best-tech-deals-today/
Over 100 $ down on the impressive Samsung C49HG90DMU , now for 699 $ on Amazon. 49-inch curved gaming monitor of QLED type with 32: 9 format, resolution 3840 x 1080 pixels, refresh rate of 144 Hz, response time of 1 ms, with HDR and FreeSync, a monitor to enjoy at a high level
Samsung C49HG90DMU - 49 ”Curved Gaming Monitor (QLED, 2 x Full HD, 32: 9, 144 Hz, 1 ms, HDR, 3000: 1, 1800R, FreeSync)
Samsung C49HG90DMU - 49 ”Curved Gaming Monitor (QLED, 2 x Full HD, 32: 9, 144 Hz, 1 ms, HDR, 3000: 1, 1800R, FreeSync)
PVP in PcComponentes 799 $
Today on Amazon for $ 806.48
More than 100 $ discount on the BenQ EW3270UE in PcComponentes, where it costs 329 $ . A 31.5-inch general purpose monitor, 4K UHD resolution with FreeSync, four levels of HDR, 16: 9 aspect ratio, 4 ms response time
Discount of 60 $ on the HP 25x gaming monitor , 189 $ on Amazon and Amazon(in this case with an additional voucher of 15% of its price), in Media Markt the 23.8 " monitor costs 209 $. With panel 24.5 "TN with Full HD resolution, 1ms response time, 144Hz refresh rate, AMD FreeSync, Low Blue Light and height adjustable
HP 27fw for 169 $ in El Corte Inglés, 20 $ less than in Amazon, and also with a voucher with 15% of its price. A 27-inch FHD general-purpose monitor with thin borders, integrated audio and gaming details like AMD FreeSync
Razer
From 100 to 69 $ the Razer Nari Essential gaming headsets, comfortable wireless headphones, with RGB chroma lighting, THX Spatial Audio technology and autonomy of approximately 16 hours. They work for PC, Xbox One, PS4 and Switch.
Razer Nari Essential Wireless Gaming Headset, Uncompromising Comfort, 16 Hours of Battery Life, THX Spatial Audio and RGB Chroma Lighting for PC, Xbox One, PS4 and Switch
Razer Nari Essential Wireless Gaming Headset, Uncompromising Comfort, 16 Hours of Battery Life, THX Spatial Audio and RGB Chroma Lighting for PC, Xbox One, PS4 and Switch
RRP on Amazon $ 69.99
PVP in PcComponentes $ 69.99PVP at FNAC $ 113
The HP Laser MFP 135w drops from 150 to 118 $ at Amazon, a very interesting single-color multifunction laser printer for those who frequently print documents. Up to 20 ppm, with Wi-Fi connectivity
Apply the discount coupon of 20 $ and take the Honor Router 3 for 58 $ . Compact and minimalist design, with support for Wi-Fi 6+, four antennas, dual band, four gigabit ports, LAN / WAN Ethernet port
In a few days the Tenda Nova Mw3 Mesh has dropped by 20 $ on Amazon, now for 50 $ . An AC1200 mesh router Wi-Fi system. It is a pack of two units with dual band, easy to configure, compact and minimalist design, very minimal technology
Reduction of 25 $ in the AMD Ryzen 7 2700X processor : 179 $ costs this veteran chip with 8-core Wraith Prism heatsink, speed of 4.35 GhZ and consumption of 105 W
Image and sound
Olee
One of the most attractive OLED televisions in quality price for gaming and multimedia consumption, cheaper: LG OLED55CX6LA for 1,481 $ in Amazon, in Amazonit costs 1,499 $ but you get a voucher with 15% of its value (224 $ !). With 55 "10-bit 4K UHD OLED panel, Dolby Vision, HDR10 and HLG modes, Alpha 9 3rd generation with AI, with very interesting features to play like the previous version (which in our analysis we consider as a candidate for last year's TV for its high quality-price ratio) such as the HDMI 2.1 standard or compatibility with Nvidia G-SYNC and the new webOS 5.0 as OS. And if you prefer it larger, the 65 " LG OLED65CX for 2249 $ in El Corte Inglés, in other stores it is for more than 2,500 $
LG TV OLED 55CX6 4K UHD
RRP on Amazon $ 1,481.00
Retail price in Amazon$ 1499PVP in PcComponentes $ 1529RRP in Mielectro $ 1499
Almost 800 $ discount on the Philips 55OLED934 / 12 in El Corte Inglés, where it costs 1,499 $ and an additional voucher worth 15% of its price. From the best of the Dutch firm last year, with 55 "OLED panel, 10-bit panel, compatible with HDR10, HDR10 +, HLG and Dolby Vision, Android TV as OS, Dolby Atmos sound tuned by Bowers & Wilkins
One of the cheapest OLEDs you will find: the Philips 55OLED754 for 1098 $ in PcComponentes, 200 $ less than in other stores. Simplest model from last year with 55 "panel with Ambilight, wide mode compatibility: HDR10, HLG, Dolby Vision, HDR10 + and Dolby Atmos for immersive sound, with built-in Alexa and Saphi as the operating system
Samsung QE65Q75T for 1199 $ in Amazonand an additional voucher worth 15% of its price, in stores where it is discounted costs 100 $ more. This year's upper mid-range with a 65 "QLED panel and interesting features for both content viewing and gaming. With HDR10, HDR10 +, HLG and Quantum HDR modes and the intuitiveness of Tizen 5.5 on the one hand and with features like HDMI 2.1, low latency auto mode and AMD Freesync for gaming.
From a range immediately lower than the previous one and at 55 ", the Samsung QE55Q70T for 820 $ on eBay, in other stores it reaches 900 $. With a 55" VA panel with Edge LED backlighting, HDR10, HDR10 +, HLG and Quantum modes HDR, Tizen 5.5 as the operating system. As in the previous case, it has HDMI 2.1 and low latency automatic mode.
Brutal in value for money the TCL 55C715 at 448 $ when you put it in the basket in Media Markt. QLED VA panel with 55 "Direct LED backlighting, Dolby Vision, HDR10 +, HDR10, HLG modes and Dolby Atmos sound. With Android TV as OS
Sam details https://techbylight.com/best-tech-deals-today/
Super Bargain: Samsung QE43Q60T for 474 $ on eBay, a smart TV 4K QLED of this year with 43 "VA panel with Edge LED backlight, HDR, HDR10, HDR10 +, Quantum HDR, Tizen 5.5 as OS. With low latency automatic mode for gaming, built-in Alexa and Tap View to cast content
Samsung QE43Q60T 43 "QLED UltraHD 4K TV new model 2020 WITH integrated ALEXA
Today on Ebay for $ 474.00
Today on Amazon for $ 583.00Price in PcComponentes 583 $Price at Amazon$ 719
Savings of 200 $ compared to other stores in the Samsung Crystal UHD 75TU7125 , at 999 $ in Media Markt when putting it in the basket. This year's mid-range model with a 75 "VA panel with Edge LED backlighting, 4K resolution and Crystal UHD nanocrystals to improve color reproduction, with HDR10 + and HLG modes, One Remote control, is compatible with Google Assistant and Alexa. This TV from Samsung's simple range for 2020 integrates the new Tizen 5.5 operating system as an operating system and functions such as Tap View , which allows you to cast content directly from your mobile
LG 65NANO866NA for 999 $ in Amazonand an additional voucher worth 15% of its price, in other stores it costs 100 - 300 $ plus this mid-range 4K smart TV - high of 2020 with 65 "IPS LED panel with Edge LED and NanoCell 4K backlighting with Technicolor, HDR10, HLG Dolby Vision and Dolby Atmos sound. And if you want it for gaming, it includes AMD Free Sync and HDMI 2.1 8K input. With WebOS 5.0 as OS, a complete model for content consumption and gaming
More than 300 $ discount on the Panasonic TX-58GX810E in El Corte Inglés: 599 $ and an additional voucher worth 15% of its price. A smart TV with a 58 "VA panel with Edge LED backlighting, HDR10, HDR10 +, HLG, HDR Photo and Dolby Vision with Dolvy Atmos sound and Cinema Sorround Pro, a very attractive set for watching movies and series with the plus of been calibrated in Hollywood. With My Home Screen 4.0 as OS
Big diagonal at a reasonable price: SHARP 65BL2EA for 579 $ on Worten's eBay account. An entry-level smart TV with a 65 "panel VA type with Direct LED backlighting, HDR10 and Android TV as OS
LG 65UM7050PLA for 479 $ in Media Markt when putting it in the basket, in other stores it is around 600 $. An entry-level smart TV that has the strengths of being this year, the size of its 65 "panel, HDR10 + and Web OS 4.5. With Direct led backlighting
Philips 58PUS7555 for 449 $ , this year's model that in other stores exceeds 500 $. 58-inch 4K Smart TV with Dolby Atmos sound and wide support for high dynamic range (HDR 10+ and Dolby Vision), with Saphi as the operating system
Super attractive for its diagonal and price: last year's HISENSE 55B7300 for 293 $ on Amazon. A simple smart TV, but with advantages such as a 55 "panel of VA type with Direct LED backlighting, compatible with HDR10, HDR10 + and HLG and Dolby Audio sound. It carries the operating system of the brand, VIDAA U.3.0. In other stores it costs 100 $ more.
From the official 449 $ of the 55 "Xiaomi Mi TV 4S it drops to 384 $ on eBay. It is shipped from Spain, two years warranty and invoice. In the Xiaomi Mi TV 4S analysis we highlight its attractive design, the fluidity of its operating system (Android) and its good performance with cinema and gaming.
Screenshot 2020 10 08 At 11 59 44
Samsung Sound Bar | Harman Kardon HW-Q60R for 396 $ in Amazonand an additional voucher worth 15% of its price, in other stores this model with 5.1Ch configuration and 360W power, wireless subwoofer and Acoustic Beam technology exceeds 500 $
Samsung Sound Bar | Harman Kardon HW-Q60R 5.1Ch 360W with Wireless Subwoofer and Acoustic Beam Technology
Price at Amazon$ 396
Today on Amazon for $ 447.22
In Amazonthe Samsung 2020 HW-T550 soundbar costs 228 $ (and an additional voucher worth 15% of its price) and in Mielectro , 40 $ less than in stores like FNAC. With Dolby Digital 2.1 sound system for its two channels, multi-connection via Bluetooth 4.2, three-dimensional sound with DTS Virtual: X and Game Mode for gamers
Xiaomi Mi TV Box for 44.99 $ on eBay using PQ42020 , a set top box with Android TV upgradeable to Android TV 9, Chromecast integrated to send content from your smartphone, tablet or computer. Allows content playback in high quality 4K HDR, and is compatible with Google Assistant
Consoles and video games
Series
If you pre-buy the Xbox Series S at Amazonat its RRP 299 $ (its official price), you get 15% of its value back in a voucher. Interesting not only because they are more than 44 $, but also because in other stores it is no longer available
Xbox Series S
Price in Amazon$ 299.90
Nintendo's most versatile console, priced and discounted: Nintendo Switch 2019 with the Super Kirby Clash game and 90 days of Nintendo Switch online for 324 $ on eBay, for what it costs in other stores without games, here you take it with entertainment hours and hours. And if you are a Fortnite fan, the Nintendo Switch Limited Edition of Fortnite with the game installed + downloadable content for 319 $ on eBay
Save a few bucks on the official Nintendo Joy-Con for Switch in neon green / pink colors, on Amazon they cost 67 $ , elsewhere they exceed 70 $
It goes on sale next week but you can buy it cheaper: Mario Kart Live: Home Circuit (Luigi) for 89 $ , 10 $ less
Lower the FIFA 21 Legacy Edition for Nintendo Switch slightly , for 44.90 $ on Amazon, the latest delivery in physical format
Connected home
Slim
It is not a robot, but at this price it is brutal for the quality of its filter and materials: Dyson V6 Slim Origin for 149 $ on eBay. A lightweight cordless vacuum cleaner with 20 minutes of autonomy and 350W of power, interesting for cleaning small spaces. It is sold by the official Dyson store and you have a two-year warranty
DYSON V6 Slim Origin Aspirateur balai - 21,6V - 400 ml - Bleu
Today on Ebay for $ 149.00
Today on Amazon for $ 285.00
In Amazonyou have one of the robot vacuum cleaners with the best value for money on the market and from iRobot itself, very well priced: the Roomba e5 for 319 $ and an additional voucher worth 15% of its price, in other stores it is round the 400 $. A mid-range with random navigation (but with the efficient iRobot algorithm), high-end rubber brushes and an app to manage it from the mobile
Almost 100 $ less in Aliexpress Plaza the Xiaomi Mi Robot 1 Original robot vacuum cleaner , where it costs 195 $ compared to 280 on Amazon. Navigation by laser, app to control it remotely and 1800 Pa, difficult to see a robot vacuum cleaner with intelligent navigation in this price range
Echo Show 8 at an all-time low price: from 129.99 $ to 69.99 $ , almost half the price. The Show 8 is a "smart" speaker with an 8 "screen with a resolution of 1,280 x 800 pixels, a size and quality that is very functional for viewing content or making video calls. You can listen to Alexa responses, music or have conversations thanks to its acoustic configuration, composed of two 2 "loudspeakers with 10W per channel. For more information, see our analysis . The Echo Show 5 , same form factor but with a 5.5 "panel, also drops almost half of its RRP , now for 49.99 $
Introducing the Echo Show 8 - Stay in Touch with Help from Alexa, Anthracite Fabric
RRP on Amazon $ 69.99
The new 4th generation Echo is cheaper and comes with a gift : from 119 $ to 99 $ and with a Philips Hue smart bulb. The fourth generation arrives with obvious aesthetic changes, but also with improved acoustic architecture and Zigbee connectivity
Google Nest Hub drops from 89.99 $ to 59 $ in El Corte Inglés, with a 7 "touch screen with a resolution of 1,024 x 600 pixels, Chromecast integrated and Google Assistant
The simple and compact 3rd gen Echo Dot is lowered from 59.99 $ to 39.99 $ , all the muscle of Alexa in pill format
In PcComponentes, the Lenovo Smart Clock is less than half its RRP: 39.95 $ , a smart device with a screen designed to be used as an alarm clock. With 4 "touch panel and Google assistant
The Philips Hue E14 drops more than 10 $ , now for 29.98 $ . A 6.5W LED connected bulb that delivers white and colored light, compatible with Alexa, Siri and Google Assistant. It requires an intermediary hub between the router and your mobile
More offers?
Quality
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If you join Amazon Prime , you have a 30-day free trial (after 36 $ per year) to enjoy free fast shipping, priority accessory to offers, services such as Prime Video , Prime Music , Prime Reading and unlimited photo storage. Also, if you are a student, becoming a Prime Student costs half and you have a 90-day trial.
Amazon Music Unlimited for 4 months for 0.99 $ . With more than 50 million songs, on demand and without advertising with the possibility of listening to them offline.
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https://techbylight.com/best-tech-deals-today/
submitted by nigosid10 to BestAmazonPrimeDeals

AJ ALMENDINGER

glimpse into the future of Roblox

Our vision to bring the world together through play has never been more relevant than it is now. As our founder and CEO, David Baszucki (a.k.a. Builderman), mentioned in his keynote, more and more people are using Roblox to stay connected with their friends and loved ones. He hinted at a future where, with our automatic machine translation technology, Roblox will one day act as a universal translator, enabling people from different cultures and backgrounds to connect and learn from each other.
During his keynote, Builderman also elaborated upon our vision to build the Metaverse; the future of avatar creation on the platform (infinitely customizable avatars that allow any body, any clothing, and any animation to come together seamlessly); more personalized game discovery; and simulating large social gatherings (like concerts, graduations, conferences, etc.) with tens of thousands of participants all in one server. We’re still very early on in this journey, but if these past five months have shown us anything, it’s clear that there is a growing need for human co-experience platforms like Roblox that allow people to play, create, learn, work, and share experiences together in a safe, civil 3D immersive space.
Up next, our VP of Developer Relations, Matt Curtis (a.k.a. m4rrh3w), shared an update on all the things we’re doing to continue empowering developers to create innovative and exciting content through collaboration, support, and expertise. He also highlighted some of the impressive milestones our creator community has achieved since last year’s RDC. Here are a few key takeaways:
  • Adopt Me! now has over 10 billion plays and surpassed 1.6 million concurrent users.
  • Piggy, launched in January 2020, has close to 5 billion visits in just over six months.
  • Developers are on track to earn over $250 million in 2020.
  • In June 2020, developers earned nearly $2 million from Premium Payouts, which rewards them based on the amount of engagement time Premium subscribers spend in their games.
  • There are now 345,000 developers on the platform who are monetizing their games.
And lastly, our VP of Engineering, Technology, Adam Miller (a.k.a. rbadam), unveiled a myriad of cool and upcoming features developers will someday be able to sink their teeth into. We saw a glimpse of procedural skies, skinned meshes, more high-quality materials, new terrain types, more fonts in Studio, a new asset type for in-game videos, haptic feedback on mobile, real-time CSG operations, and many more awesome tools that will unlock the potential for even bigger, more immersive experiences on Roblox.

Vibin’

Despite the virtual setting, RDC just wouldn’t have been the same without any fun party activities and networking opportunities. So, we invited special guests DJ Hyper Potions and cyber mentalist Colin Cloud for some truly awesome, truly mind-bending entertainment. Yoga instructor Erin Gilmore also swung by to inspire attendees to get out of their chair and get their body moving. And of course, we even had virtual rooms dedicated to karaoke and head-to-head social games, like trivia and Pictionary.
Over on the networking side, Team Adopt Me, Red Manta, StyLiS Studios, and Summit Studios hosted a virtual booth for attendees to ask questions, submit resumes, and more. We also had a networking session where three participants would be randomly grouped together to get to know each other.

What does Roblox mean to you?

We all know how talented the Roblox community is from your creations. We’ve heard plenty of stories over the years about how Roblox has touched your lives, how you’ve made friendships, learned new skills, or simply found a place where you can be yourself. We wanted to hear more. So, we asked attendees: What does Roblox mean to you? How has Roblox connected you? How has Roblox changed your life? Then, over the course of RDC, we incorporated your responses into this awesome mural.
📷
Created by Alece Birnbach at Graphic Recording Studio

Knowledge is power

This year’s breakout sessions included presentations from Roblox developers and staff members on the latest game development strategies, a deep dive into the Roblox engine, learning how to animate with Blender, tools for working together in teams, building performant game worlds, and the new Creator Dashboard. Dr. Michael Rich, Associate Professor at Harvard Medical School and Physician at Boston Children’s Hospital, also led attendees through a discussion on mental health and how to best take care of you and your friends’ emotional well-being, especially now during these challenging times.
📷
Making the Dream Work with Teamwork (presented by Roblox developer Myzta)
In addition to our traditional Q&A panel with top product and engineering leaders at Roblox, we also held a special session with Builderman himself to answer the community’s biggest questions.
📷
Roblox Product and Engineering Q&A Panel

2020 Game Jam

The Game Jam is always one of our favorite events of RDC. It’s a chance for folks to come together, flex their development skills, and come up with wildly inventive game ideas that really push the boundaries of what’s possible on Roblox. We had over 60 submissions this year—a new RDC record.
Once again, teams of up to six people from around the world had less than 24 hours to conceptualize, design, and publish a game based on the theme “2020 Vision,” all while working remotely no less! To achieve such a feat is nothing short of awe-inspiring, but as always, our dev community was more than up for the challenge. I’ve got to say, these were some of the finest creations we’ve seen.
WINNERS
Best in Show: Shapescape Created By: GhettoMilkMan, dayzeedog, maplestick, theloudscream, Brick_man, ilyannna You awaken in a strange laboratory, seemingly with no way out. Using a pair of special glasses, players must solve a series of anamorphic puzzles and optical illusions to make their escape.
Excellence in Visual Art: agn●sia Created By: boatbomber, thisfall, Elttob An obby experience unlike any other, this game is all about seeing the world through a different lens. Reveal platforms by switching between different colored lenses and make your way to the end.
Most Creative Gameplay: Visions of a perspective reality Created By: Noble_Draconian and Spathi Sometimes all it takes is a change in perspective to solve challenges. By switching between 2D and 3D perspectives, players can maneuver around obstacles or find new ways to reach the end of each level.
Outstanding Use of Tech: The Eyes of Providence Created By: Quenty, Arch_Mage, AlgyLacey, xJennyBeanx, Zomebody, Crykee This action/strategy game comes with a unique VR twist. While teams fight to construct the superior monument, two VR players can support their minions by collecting resources and manipulating the map.
Best Use of Theme: Sticker Situation Created By: dragonfrosting and Yozoh Set in a mysterious art gallery, players must solve puzzles by manipulating the environment using a magic camera and stickers. Snap a photograph, place down a sticker, and see how it changes the world.
OTHER TOP PICKS
HONORABLE MENTIONS
For the rest of the 2020 Game Jam submissions, check out the list below:
20-20 Vision | 20/20 Vision | 2020 Vision, A Crazy Perspective | 2020 Vision: Nyon | A Wild Trip! | Acuity | Best Year Ever | Better Half | Bloxlabs | Climb Stairs to 2021 | Double Vision (Team hey apple) | Eyebrawl | Eyeworm Exam | FIRE 2020 | HACKED | Hyperspective | Lucid Scream | Mystery Mansion | New Years at the Museum | New Year’s Bash | Poor Vision | Predict 2020 | RBC News | Retrovertigo | Second Wave | see no evil | Sight Fight | Sight Stealers | Spectacles Struggle | Specter Spectrum | Survive 2020 | The Lost Chicken Leg | The Outbreak | The Spyglass | Time Heist | Tunnel Vision | Virtual RDC – The Story | Vision (Team Freepunk) | Vision (Team VIP People ####) | Vision Developers Conference 2020 | Vision Is Key | Vision Perspective | Vision Racer | Visions | Zepto
And last but not least, we wanted to give a special shout out to Starboard Studios. Though they didn’t quite make it on time for our judges, we just had to include Dave’s Vision for good measure. 📷
Thanks to everyone who participated in the Game Jam, and congrats to all those who took home the dub in each of our categories this year. As the winners of Best in Show, the developers of Shapescape will have their names forever engraved on the RDC Game Jam trophy back at Roblox HQ. Great work!

‘Til next year

And that about wraps up our coverage of the first-ever digital RDC. Thanks to all who attended! Before we go, we wanted to share a special “behind the scenes” video from the 2020 RDC photoshoot.
Check it out:
It was absolutely bonkers. Getting 350 of us all in one server was so much fun and really brought back the feeling of being together with everyone again. That being said, we can’t wait to see you all—for real this time—at RDC next year. It’s going to be well worth the wait. ‘Til we meet again, my friends.
© 2020 Roblox Corporation. All Rights Reserved.

Improving Simulation and Performance with an Advanced Physics Solver

August

05, 2020

by chefdeletat
PRODUCT & TECH
📷In mid-2015, Roblox unveiled a major upgrade to its physics engine: the Projected Gauss-Seidel (PGS) physics solver. For the first year, the new solver was optional and provided improved fidelity and greater performance compared to the previously used spring solver.
In 2016, we added support for a diverse set of new physics constraints, incentivizing developers to migrate to the new solver and extending the creative capabilities of the physics engine. Any new places used the PGS solver by default, with the option of reverting back to the classic solver.
We ironed out some stability issues associated with high mass differences and complex mechanisms by the introduction of the hybrid LDL-PGS solver in mid-2018. This made the old solver obsolete, and it was completely disabled in 2019, automatically migrating all places to the PGS.
In 2019, the performance was further improved using multi-threading that splits the simulation into jobs consisting of connected islands of simulating parts. We still had performance issues related to the LDL that we finally resolved in early 2020.
The physics engine is still being improved and optimized for performance, and we plan on adding new features for the foreseeable future.

Implementing the Laws of Physics

📷
The main objective of a physics engine is to simulate the motion of bodies in a virtual environment. In our physics engine, we care about bodies that are rigid, that collide and have constraints with each other.
A physics engine is organized into two phases: collision detection and solving. Collision detection finds intersections between geometries associated with the rigid bodies, generating appropriate collision information such as collision points, normals and penetration depths. Then a solver updates the motion of rigid bodies under the influence of the collisions that were detected and constraints that were provided by the user.
📷
The motion is the result of the solver interpreting the laws of physics, such as conservation of energy and momentum. But doing this 100% accurately is prohibitively expensive, and the trick to simulating it in real-time is to approximate to increase performance, as long as the result is physically realistic. As long as the basic laws of motion are maintained within a reasonable tolerance, this tradeoff is completely acceptable for a computer game simulation.

Taking Small Steps

The main idea of the physics engine is to discretize the motion using time-stepping. The equations of motion of constrained and unconstrained rigid bodies are very difficult to integrate directly and accurately. The discretization subdivides the motion into small time increments, where the equations are simplified and linearized making it possible to solve them approximately. This means that during each time step the motion of the relevant parts of rigid bodies that are involved in a constraint is linearly approximated.
📷📷
Although a linearized problem is easier to solve, it produces drift in a simulation containing non-linear behaviors, like rotational motion. Later we’ll see mitigation methods that help reduce the drift and make the simulation more plausible.

Solving

📷
Having linearized the equations of motion for a time step, we end up needing to solve a linear system or linear complementarity problem (LCP). These systems can be arbitrarily large and can still be quite expensive to solve exactly. Again the trick is to find an approximate solution using a faster method. A modern method to approximately solve an LCP with good convergence properties is the Projected Gauss-Seidel (PGS). It is an iterative method, meaning that with each iteration the approximate solution is brought closer to the true solution, and its final accuracy depends on the number of iterations.
📷
This animation shows how a PGS solver changes the positions of the bodies at each step of the iteration process, the objective being to find the positions that respect the ball and socket constraints while preserving the center of mass at each step (this is a type of positional solver used by the IK dragger). Although this example has a simple analytical solution, it’s a good demonstration of the idea behind the PGS. At each step, the solver fixes one of the constraints and lets the other be violated. After a few iterations, the bodies are very close to their correct positions. A characteristic of this method is how some rigid bodies seem to vibrate around their final position, especially when coupling interactions with heavier bodies. If we don’t do enough iterations, the yellow part might be left in a visibly invalid state where one of its two constraints is dramatically violated. This is called the high mass ratio problem, and it has been the bane of physics engines as it causes instabilities and explosions. If we do too many iterations, the solver becomes too slow, if we don’t it becomes unstable. Balancing the two sides has been a painful and long process.

Mitigation Strategies

📷A solver has two major sources of inaccuracies: time-stepping and iterative solving (there is also floating point drift but it’s minor compared to the first two). These inaccuracies introduce errors in the simulation causing it to drift from the correct path. Some of this drift is tolerable like slightly different velocities or energy loss, but some are not like instabilities, large energy gains or dislocated constraints.
Therefore a lot of the complexity in the solver comes from the implementation of methods to minimize the impact of computational inaccuracies. Our final implementation uses some traditional and some novel mitigation strategies:
  1. Warm starting: starting with the solution from a previous time-step to increase the convergence rate of the iterative solver
  2. Post-stabilization: reprojecting the system back to the constraint manifold to prevent constraint drift
  3. Regularization: adding compliance to the constraints ensuring a solution exists and is unique
  4. Pre-conditioning: using an exact solution to a linear subsystem, improving the stability of complex mechanisms
Strategies 1, 2 and 3 are pretty traditional, but 3 has been improved and perfected by us. Also, although 4 is not unheard of, we haven’t seen any practical implementation of it. We use an original factorization method for large sparse constraint matrices and a new efficient way of combining it with the PGS. The resulting implementation is only slightly slower compared to pure PGS but ensures that the linear system coming from equality constraints is solved exactly. Consequently, the equality constraints suffer only from drift coming from the time discretization. Details on our methods are contained in my GDC 2020 presentation. Currently, we are investigating direct methods applied to inequality constraints and collisions.

Getting More Details

Traditionally there are two mathematical models for articulated mechanisms: there are reduced coordinate methods spearheaded by Featherstone, that parametrize the degrees of freedom at each joint, and there are full coordinate methods that use a Lagrangian formulation.
We use the second formulation as it is less restrictive and requires much simpler mathematics and implementation.
The Roblox engine uses analytical methods to compute the dynamic response of constraints, as opposed to penalty methods that were used before. Analytics methods were initially introduced in Baraff 1989, where they are used to treat both equality and non-equality constraints in a consistent manner. Baraff observed that the contact model can be formulated using quadratic programming, and he provided a heuristic solution method (which is not the method we use in our solver).
Instead of using force-based formulation, we use an impulse-based formulation in velocity space, originally introduced by Mirtich-Canny 1995 and further improved by Stewart-Trinkle 1996, which unifies the treatment of different contact types and guarantees the existence of a solution for contacts with friction. At each timestep, the constraints and collisions are maintained by applying instantaneous changes in velocities due to constraint impulses. An excellent explanation of why impulse-based simulation is superior is contained in the GDC presentation of Catto 2014.
The frictionless contacts are modeled using a linear complementarity problem (LCP) as described in Baraff 1994. Friction is added as a non-linear projection onto the friction cone, interleaved with the iterations of the Projected Gauss-Seidel.
The numerical drift that introduces positional errors in the constraints is resolved using a post-stabilization technique using pseudo-velocities introduced by Cline-Pai 2003. It involves solving a second LCP in the position space, which projects the system back to the constraint manifold.
The LCPs are solved using a PGS / Impulse Solver popularized by Catto 2005 (also see Catto 2009). This method is iterative and considers each individual constraints in sequence and resolves it independently. Over many iterations, and in ideal conditions, the system converges to a global solution.
Additionally, high mass ratio issues in equality constraints are ironed out by preconditioning the PGS using the sparse LDL decomposition of the constraint matrix of equality constraints. Dense submatrices of the constraint matrix are sparsified using a method we call Body Splitting. This is similar to the LDL decomposition used in Baraff 1996, but allows more general mechanical systems, and solves the system in constraint space. For more information, you can see my GDC 2020 presentation.
The architecture of our solver follows the idea of Guendelman-Bridson-Fedkiw, where the velocity and position stepping are separated by the constraint resolution. Our time sequencing is:
  1. Advance velocities
  2. Constraint resolution in velocity space and position space
  3. Advance positions
This scheme has the advantage of integrating only valid velocities, and limiting latency in external force application but allowing a small amount of perceived constraint violation due to numerical drift.
An excellent reference for rigid body simulation is the book Erleben 2005 that was recently made freely available. You can find online lectures about physics-based animation, a blog by Nilson Souto on building a physics engine, a very good GDC presentation by Erin Catto on modern solver methods, and forums like the Bullet Physics Forum and GameDev which are excellent places to ask questions.

In Conclusion

The field of game physics simulation presents many interesting problems that are both exciting and challenging. There are opportunities to learn a substantial amount of cool mathematics and physics and to use modern optimizations techniques. It’s an area of game development that tightly marries mathematics, physics and software engineering.
Even if Roblox has a good rigid body physics engine, there are areas where it can be improved and optimized. Also, we are working on exciting new projects like fracturing, deformation, softbody, cloth, aerodynamics and water simulation.
Neither Roblox Corporation nor this blog endorses or supports any company or service. Also, no guarantees or promises are made regarding the accuracy, reliability or completeness of the information contained in this blog.
This blog post was originally published on the Roblox Tech Blog.
© 2020 Roblox Corporation. All Rights Reserved.

Using Clang to Minimize Global Variable Use

July

23, 2020

by RandomTruffle
PRODUCT & TECH
Every non-trivial program has at least some amount of global state, but too much can be a bad thing. In C++ (which constitutes close to 100% of Roblox’s engine code) this global state is initialized before main() and destroyed after returning from main(), and this happens in a mostly non-deterministic order. In addition to leading to confusing startup and shutdown semantics that are difficult to reason about (or change), it can also lead to severe instability.
Roblox code also creates a lot of long-running detached threads (threads which are never joined and just run until they decide to stop, which might be never). These two things together have a very serious negative interaction on shutdown, because long-running threads continue accessing the global state that is being destroyed. This can lead to elevated crash rates, test suite flakiness, and just general instability.
The first step to digging yourself out of a mess like this is to understand the extent of the problem, so in this post I’m going to talk about one technique you can use to gain visibility into your global startup flow. I’m also going to discuss how we are using this to improve stability across the entire Roblox game engine platform by decreasing our use of global variables.

Introducing -finstrument-functions

Nothing excites me more than learning about a new obscure compiler option that I’ve never had a use for before, so I was pretty happy when a colleague pointed me to this option in the Clang Command Line Reference. I’d never used it before, but it sounded very cool. The idea being that if we could get the compiler to tell us every time it entered and exited a function, we could filter this information through a symbolizer of some kind and generate a report of functions that a) occur before main(), and b) are the very first function in the call-stack (indicating it’s a global).
Unfortunately, the documentation basically just tells you that the option exists with no mention of how to use it or if it even actually does what it sounds like it does. There’s also two different options that sound similar to each other (-finstrument-functions and -finstrument-functions-after-inlining), and I still wasn’t entirely sure what the difference was. So I decided to throw up a quick sample on godbolt to see what happened, which you can see here. Note there are two assembly outputs for the same source listing. One uses the first option and the other uses the second option, and we can compare the assembly output to understand the differences. We can gather a few takeaways from this sample:
  1. The compiler is injecting calls to __cyg_profile_func_enter and __cyg_profile_func_exit inside of every function, inline or not.
  2. The only difference between the two options occurs at the call-site of an inline function.
  3. With -finstrument-functions, the instrumentation for the inlined function is inserted at the call-site, whereas with -finstrument-functions-after-inlining we only have instrumentation for the outer function. This means that when using-finstrument-functions-after-inlining you won’t be able to determine which functions are inlined and where.
Of course, this sounds exactly like what the documentation said it did, but sometimes you just need to look under the hood to convince yourself.
To put all of this another way, if we want to know about calls to inline functions in this trace we need to use -finstrument-functions because otherwise their instrumentation is silently removed by the compiler. Sadly, I was never able to get -finstrument-functions to work on a real example. I would always end up with linker errors deep in the Standard C++ Library which I was unable to figure out. My best guess is that inlining is often a heuristic, and this can somehow lead to subtle ODR (one-definition rule) violations when the optimizer makes different inlining decisions from different translation units. Luckily global constructors (which is what we care about) cannot possibly be inlined anyway, so this wasn’t a problem.
I suppose I should also mention that I still got tons of linker errors with -finstrument-functions-after-inlining as well, but I did figure those out. As best as I can tell, this option seems to imply –whole-archive linker semantics. Discussion of –whole-archive is outside the scope of this blog post, but suffice it to say that I fixed it by using linker groups (e.g. -Wl,–start-group and -Wl,–end-group) on the compiler command line. I was a bit surprised that we didn’t get these same linker errors without this option and still don’t totally understand why. If you happen to know why this option would change linker semantics, please let me know in the comments!

Implementing the Callback Hooks

If you’re astute, you may be wondering what in the world __cyg_profile_func_enter and __cyg_profile_func_exit are and why the program is even successfully linking in the first without giving undefined symbol reference errors, since the compiler is apparently trying to call some function we’ve never defined. Luckily, there are some options that allow us to see inside the linker’s algorithm so we can find out where it’s getting this symbol from to begin with. Specifically, -y should tell us how the linker is resolving . We’ll try it with a dummy program first and a symbol that we’ve defined ourselves, then we’ll try it with __cyg_profile_func_enter .
[email protected]:~/src/sandbox$ cat instr.cpp int main() {} [email protected]:~/src/sandbox$ clang++-9 -fuse-ld=lld -Wl,-y -Wl,main instr.cpp /usbin/../lib/gcc/x86_64-linux-gnu/crt1.o: reference to main /tmp/instr-5b6c60.o: definition of main
No surprises here. The C Runtime Library references main(), and our object file defines it. Now let’s see what happens with __cyg_profile_func_enter and -finstrument-functions-after-inlining.
[email protected]:~/src/sandbox$ clang++-9 -fuse-ld=lld -finstrument-functions-after-inlining -Wl,-y -Wl,__cyg_profile_func_enter instr.cpp /tmp/instr-8157b3.o: reference to __cyg_profile_func_enter /lib/x86_64-linux-gnu/libc.so.6: shared definition of __cyg_profile_func_enter
Now, we see that libc provides the definition, and our object file references it. Linking works a bit differently on Unix-y platforms than it does on Windows, but basically this means that if we define this function ourselves in our cpp file, the linker will just automatically prefer it over the shared library version. Working godbolt link without runtime output is here. So now you can kind of see where this is going, however there are still a couple of problems left to solve.
  1. We don’t want to do this for a full run of the program. We want to stop as soon as we reach main.
  2. We need a way to symbolize this trace.
The first problem is easy to solve. All we need to do is compare the address of the function being called to the address of main, and set a flag indicating we should stop tracing henceforth. (Note that taking the address of main is undefined behavior[1], but for our purposes it gets the job done, and we aren’t shipping this code, so ¯\_(ツ)_/¯). The second problem probably deserves a little more discussion though.

Symbolizing the Traces

In order to symbolize these traces, we need two things. First, we need to store the trace somewhere on persistent storage. We can’t expect to symbolize in real time with any kind of reasonable performance. You can write some C code to save the trace to some magic filename, or you can do what I did and just write it to stderr (this way you can pipe stderr to some file when you run it).
Second, and perhaps more importantly, for every address we need to write out the full path to the module the address belongs to. Your program loads many shared libraries, and in order to translate an address into a symbol, we have to know which shared library or executable the address actually belongs to. In addition, we have to be careful to write out the address of the symbol in the file on disk. When your program is running, the operating system could have loaded it anywhere in memory. And if we’re going to symbolize it after the fact we need to make sure we can still reference it after the information about where it was loaded in memory is lost. The linux function dladdr() gives us both pieces of information we need. A working godbolt sample with the exact implementation of our instrumentation hooks as they appear in our codebase can be found here.

Putting it All Together

Now that we have a file in this format saved on disk, all we need to do is symbolize the addresses. addr2line is one option, but I went with llvm-symbolizer as I find it more robust. I wrote a Python script to parse the file and symbolize each address, then print it in the same “visual” hierarchical format that the original output file is in. There are various options for filtering the resulting symbol list so that you can clean up the output to include only things that are interesting for your case. For example, I filtered out any globals that have boost:: in their name, because I can’t exactly go rewrite boost to not use global variables.
The script isn’t as simple as you would think, because simply crawling each line and symbolizing it would be unacceptably slow (when I tried this, it took over 2 hours before I finally killed the process). This is because the same address might appear thousands of times, and there’s no reason to run llvm-symbolizer against the same address multiple times. So there’s a lot of smarts in there to pre-process the address list and eliminate duplicates. I won’t discuss the implementation in more detail because it isn’t super interesting. But I’ll do even better and provide the source!
So after all of this, we can run any one of our internal targets to get the call tree, run it through the script, and then get output like this (actual output from a Roblox process, source file information removed):
excluded_symbols = [‘.\boost.*’]* excluded_modules = [‘/usr.\’]* /uslib/x86_64-linux-gnu/libLLVM-9.so.1: 140 unique addresses InterestingRobloxProcess: 38928 unique addresses /uslib/x86_64-linux-gnu/libstdc++.so.6: 1 unique addresses /uslib/x86_64-linux-gnu/libc++.so.1: 3 unique addresses Printing call tree with depth 2 for 29276 global variables. __cxx_global_var_init.5 (InterestingFile1.cpp:418:22) RBX::InterestingRobloxClass2::InterestingRobloxClass2() (InterestingFile2.cpp.:415:0) __cxx_global_var_init.19 (InterestingFile2.cpp:183:34) (anonymous namespace)::InterestingRobloxClass2::InterestingRobloxClass2() (InterestingFile2.cpp:171:0) __cxx_global_var_init.274 (InterestingFile3.cpp:2364:33) RBX::InterestingRobloxClass3::InterestingRobloxClass3()
So there you have it: the first half of the battle is over. I can run this script on every platform, compare results to understand what order our globals are actually initialized in in practice, then slowly migrate this code out of global initializers and into main where it can be deterministic and explicit.

Future Work

It occurred to me sometime after implementing this that we could make a general purpose profiling hook that exposed some public symbols (dllexport’ed if you speak Windows), and allowed a plugin module to hook into this dynamically. This plugin module could filter addresses using whatever arbitrary logic that it was interested in. One interesting use case I came up for this is that it could look up the debug information, check if the current address maps to the constructor of a function local static, and write out the address if so. This effectively allows us to gain a deeper understanding of the order in which our lazy statics are initialized. The possibilities are endless here.

Further Reading

If you’re interested in this kind of thing, I’ve collected a couple of my favorite references for this kind of topic.
  1. Various: The C++ Language Standard
  2. Matt Godbolt: The Bits Between the Bits: How We Get to main()
  3. Ryan O’Neill: Learning Linux Binary Analysis
  4. Linkers and Loaders: John R. Levine
  5. https://eel.is/c++draft/basic.exec#basic.start.main-3
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