4 ms·

> The code uses inline assembly a lot, i.e. optimizer likely failed to achieve much, and for code like this optimizations are very important. Yes, optimization

by ordu 8y ago

> The code uses inline assembly a lot, i.e. optimizer likely failed to achieve much, and for code like this optimizations are very important. Yes, optimizations are important, but not for the question the autor tries to ask. To answer his question he need to control optimizations, to have vectorized code and not vectorized one. The simpliest way to control optimizations is to turn them off and to do them by hand. > Recent CPUs return rdtsc values unrelated to the count of instruction it executed, it’s a counter running at some constant frequency. So this counter is the best fit to measure speed of code. What the point to count number of instructions executed when different instructions take different time to execute? But if rdtsc counts clock ticks, than it fits best. It is OS independant way to measure time. It is compiler dependant way, but it works with gcc and clang, the compilers you can get for free. Alternatively it is not so hard to rewrite two non-empty __asm statements into any asm syntax your compiler understands. There are total of 6 instructions.

justrobert 8y ago
Often you can turn off frequency scaling so you can have rdtsc still be useful.
Const-me 8y ago
> it works with gcc and clang, the compilers you can get for free When I’m building for Windows, I usually use Visual C++ 2017 community edition, the compiler I got for free. > into any asm syntax your compiler understands When building 64 bit binaries, my compiler doesn’t understand any syntax. The portable way to produce that instruction is __rdtsc() intrinsic, it’s supported in all major compilers just in different headers, x86intrin.h for gcc/clang, intrin.h for msvc. > There are total of 6 instructions. Right, but I’m not sure assembly, inline or not, has good compatibility with advanced optimization techniques implemented by all modern compilers. It’s just too low level: harder to inline, impossible to move stuff across registers…
- ordu 8y ago
  > When I’m building for Windows, I usually use Visual C++ 2017 community edition, the compiler I got for free. Is it an obstacle to get one more free compiler? > When building 64 bit binaries, my compiler doesn’t understand any syntax. Is it really? I'm surprised. > The portable way to produce that instruction is __rdtsc() intrinsic, it’s supported in all major compilers just in different headers, x86intrin.h for gcc/clang, intrin.h for msvc. I like to learn it. Thank you. > Right, but I’m not sure assembly, inline or not, has good compatibility with advanced optimization techniques implemented by all modern compilers. It’s just too low level: harder to inline, impossible to move stuff across registers… Its not relevant: this code do not rely on compiler optimizations.
  - Const-me 8y ago
    > Is it an obstacle to get one more free compiler? I have both gcc and clang installed but they’re for Linux. I’m using Windows as my main OS, and prefer running native binaries, e.g. because tools for debugging & profiling. > Its not relevant: this code do not rely on compiler optimizations. Right, and that’s GCC’s problems — MSVC-produced code is optimized, and is free from the described issue. You know how I found that? I’ve made a version portable across compilers, built with VC++ and run on Windows: https://github.com/Const-me/MemoryAccessCosts https://github.com/Const-me/MemoryAccessCosts I don’t have Intel compiler but my expectation is it’ll work even better.
    - ordu 8y ago
      > Right, and that’s GCC’s problems — MSVC-produced code is optimized, and is free from the described issue. > You know how I found that? I’ve made a version portable across compilers, built with VC++ and run on Windows: https://github.com/Const-me/MemoryAccessCosts https://github.com/Const-me/MemoryAccessCosts I don’t have Intel compiler but my expectation is it’ll work even better. Yes! You have found that optimization doesn't allow you to make a comparison. Code is free of the described issue, so you cannot measure effect this issue have on the speed. As I understand from asm listings, you are comparing now not 1 iteration with 16 iteration, but 1 iteration with some vectorized code. In the original article this case is covered, with hand-crafted vectorized code. Do you see? When optimization is turned on you cannot compare 16 iterations with 1 iteration or with vectorized code. It is the basics of experimental method: you need to control independant variable, and to measure dependant one. When using compiler optimizations it is the way harder, you would need to check generated code. Turn off vectorization of code in msvc, and hopefully you'll get the same results.

by ordu 8y ago

justrobert 8y ago

Often you can turn off frequency scaling so you can have rdtsc still be useful.

Const-me 8y ago

> it works with gcc and clang, the compilers you can get for free When I’m building for Windows, I usually use Visual C++ 2017 community edition, the compiler I got for free. > into any asm syntax your compiler understands When building 64 bit binaries, my compiler doesn’t understand any syntax. The portable way to produce that instruction is __rdtsc() intrinsic, it’s supported in all major compilers just in different headers, x86intrin.h for gcc/clang, intrin.h for msvc. > There are total of 6 instructions. Right, but I’m not sure assembly, inline or not, has good compatibility with advanced optimization techniques implemented by all modern compilers. It’s just too low level: harder to inline, impossible to move stuff across registers…

ordu 8y ago
> When I’m building for Windows, I usually use Visual C++ 2017 community edition, the compiler I got for free. Is it an obstacle to get one more free compiler? > When building 64 bit binaries, my compiler doesn’t understand any syntax. Is it really? I'm surprised. > The portable way to produce that instruction is __rdtsc() intrinsic, it’s supported in all major compilers just in different headers, x86intrin.h for gcc/clang, intrin.h for msvc. I like to learn it. Thank you. > Right, but I’m not sure assembly, inline or not, has good compatibility with advanced optimization techniques implemented by all modern compilers. It’s just too low level: harder to inline, impossible to move stuff across registers… Its not relevant: this code do not rely on compiler optimizations.
- Const-me 8y ago
  > Is it an obstacle to get one more free compiler? I have both gcc and clang installed but they’re for Linux. I’m using Windows as my main OS, and prefer running native binaries, e.g. because tools for debugging & profiling. > Its not relevant: this code do not rely on compiler optimizations. Right, and that’s GCC’s problems — MSVC-produced code is optimized, and is free from the described issue. You know how I found that? I’ve made a version portable across compilers, built with VC++ and run on Windows: https://github.com/Const-me/MemoryAccessCosts https://github.com/Const-me/MemoryAccessCosts I don’t have Intel compiler but my expectation is it’ll work even better.
  - ordu 8y ago
    > Right, and that’s GCC’s problems — MSVC-produced code is optimized, and is free from the described issue. > You know how I found that? I’ve made a version portable across compilers, built with VC++ and run on Windows: https://github.com/Const-me/MemoryAccessCosts https://github.com/Const-me/MemoryAccessCosts I don’t have Intel compiler but my expectation is it’ll work even better. Yes! You have found that optimization doesn't allow you to make a comparison. Code is free of the described issue, so you cannot measure effect this issue have on the speed. As I understand from asm listings, you are comparing now not 1 iteration with 16 iteration, but 1 iteration with some vectorized code. In the original article this case is covered, with hand-crafted vectorized code. Do you see? When optimization is turned on you cannot compare 16 iterations with 1 iteration or with vectorized code. It is the basics of experimental method: you need to control independant variable, and to measure dependant one. When using compiler optimizations it is the way harder, you would need to check generated code. Turn off vectorization of code in msvc, and hopefully you'll get the same results.