The Wiert Corner – irregular stream of stuff

Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘x86’ Category

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The fundamentals of programming, a thread by @isotopp on Twitter

Posted by jpluimers on 2023/03/22

Kristian Kohntöpp publishes great DevOps related threads on Twitter. [Wayback/Archive] Thread by @isotopp “I am Kris, and I am 53 now. I learned programming on a Commodore 64 in 1983. My first real programming language (because C64 isn’t one) was 6502 assembler, forwards and backwards. “ is his response, about a year and a half ago, to a request by Julia Evans (@b0rk) that I also saved: [Wayback/Archive] Thread by @b0rk on Thread Reader App – Thread Reader App.

Her request: [Archive] 🔎Julia Evans🔍 on Twitter: “if you’ve been working in computing for > 15 years — are there fundamentals that you learned “on the job” 15 years ago that you think most people aren’t learning on the job today? (I’m thinking about how for example nobody has ever paid me to write C code)” / Twitter followed by [Archive] 🔎Julia Evans🔍 on Twitter: “I’m especially interested in topics that are still relevant today (like C programming) but are just harder to pick up at work now than they used to be” / Twitter.

The start of his thread is [Archive] Kris on Twitter: “@b0rk I am Kris, and I am 53 now. I learned programming on a Commodore 64 in 1983. My first real programming language (because C64 isn’t one) was 6502 assembler, forwards and backwards.” / Twitter.

Kristian’s story is very similar to mine, though I sooner stepped up the structured programming language ladder as at high school, I had access to an Apple //e with a Z80 card (yes, the SoftCard), so could run CP/M with Turbo Pascal 1.0 (later 2.0 and 3.0) which I partly described in The calculators that got me into programming (via: calculators : Algorithms for the masses – julian m bucknall), followed by early access at the close by university to PC’s running on 8086 and up. The computer science lab, now called Snellius, but back then known as CRI for Centraal RekenInstituut – is now had an educational deal with IBM, which means they switched from the PC/XT to the PC/AT with a 80286 processor as soon as the latter came out).

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Posted in 6502 Assembly, Assembly Language, Development, ESP32, ESP8266, Software Development, x86 | Leave a Comment »

Very useful link: Software optimization resources. C++ and assembly. Windows, Linux, BSD, Mac OS X

Posted by jpluimers on 2023/02/14

If I ever need to go deep into optimisation again, there is lots I can still learn from [Wayback/Archive] Software optimization resources. C++ and assembly. Windows, Linux, BSD, Mac OS X

Thanks [Archive] Kris on Twitter: “@Kharkerlake @unixtippse Agner Fog ist eigentlich ein Anthropologe, aber er reversed interne Strukturen von Intel CPUs, und …, speziell 3. The microarchitecture of Intel, AMD and VIA CPUs: An optimization guide for assembly programmers and compiler makers ist die HPC Bibel.” / Twitter!

Must watch video with Agner about Warlike and Peaceful Societies below the signature.

–jeroen

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Posted in Assembly Language, C++, Development, Software Development, x64, x86 | Leave a Comment »

When floating point code suddenly becomes orders magnitudes slower (via C++ – Why does changing 0.1f to 0 slow down performance by 10x? – Stack Overflow)

Posted by jpluimers on 2022/01/26

When working with converging algorithms, sometimes floating code can become very slow. That is: orders of magnitude slower than you would expect.

A very interesting answer to [Wayback] c++ – Why does changing 0.1f to 0 slow down performance by 10x? – Stack Overflow.

I’ve only quoted a few bits, read the full question and answer for more background information.

Welcome to the world of denormalized floating-point! They can wreak havoc on performance!!!

Denormal (or subnormal) numbers are kind of a hack to get some extra values very close to zero out of the floating point representation. Operations on denormalized floating-point can be tens to hundreds of times slower than on normalized floating-point. This is because many processors can’t handle them directly and must trap and resolve them using microcode.

If you print out the numbers after 10,000 iterations, you will see that they have converged to different values depending on whether 0 or 0.1 is used.

Basically, the convergence uses some values closer to zero than a normal floating point representation dan store, so a trick is used called “denormal numbers or denormalized numbers (now often called subnormal numbers)” as described in Denormal number – Wikipedia:

…

In a normal floating-point value, there are no leading zeros in the significand; rather, leading zeros are removed by adjusting the exponent (for example, the number 0.0123 would be written as 1.23 × 10−2). Denormal numbers are numbers where this representation would result in an exponent that is below the smallest representable exponent (the exponent usually having a limited range). Such numbers are represented using leading zeros in the significand.

…

Since a denormal number is a boundary case, many processors do not optimise for this.

–jeroen

Posted in .NET, Algorithms, ARM, Assembly Language, C, C#, C++, Delphi, Development, Software Development, x64, x86 | Leave a Comment »

Some notes on loosing performance because of using AVX

Posted by jpluimers on 2019/03/20

It looks like AVX can be a curse most of the times. Below are some (many) links that lead me to this conclusion, based on a thread started by Kelly Sommers.

My conclusion

Running AVX instructions will affect the processor frequency, which means that non-AVX code will slow down, so you will only benefit when the gain of using AVX code outweighs the non-AVX loss on anything running on that processor in the same time frame.

In practice, this means you need to long term gain from AVX on many cores. If you don’t, then the performance penalty on all cores, including the initial AVX performance, will degrade, often a lot (dozens of %).

Tweets and pages linked by them

Kelly raised a bunch of interesting questions and remarks because of the above:

I collected the above links because of [WayBack] GitHub – maximmasiutin/FastMM4-AVX: FastMM4 fork with AVX support and multi-threaded enhancements (faster locking), where it is unclear which parts of the gains are because of AVX and which parts are because of other optimizations. It looks like that under heavy loads on data center like conditions, the total gain is about 30%. The loss for traditional processing there has not been measured, but from the above my estimate it is at least 20%.

Full tweets below.

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Posted in Assembly Language, Development, Software Development, x64, x86 | Leave a Comment »

performance – Why is this C++ code faster than my hand-written assembly for testing the Collatz conjecture? – Stack Overflow

Posted by jpluimers on 2019/02/28

Geek pr0n at [WayBack] performance – Why is this C++ code faster than my hand-written assembly for testing the Collatz conjecture? – Stack Overflow

Via: [WayBack] Very nice #Geekpr0n “Why is C++ faster than my hand-written assembly code?” The comments are of high quality i… – Jan Wildeboer – Google+

–jeroen

Posted in Assembly Language, C, C++, Development, Software Development, x64, x86 | Leave a Comment »

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