3 ms·

I'm not deep on the weeds of high-performance compilers, but just because ISA evolution hasn't happened doesn't mean compilers can't evolve to use the silicon b

I'm not deep on the weeds of high-performance compilers, but just because ISA evolution hasn't happened doesn't mean compilers can't evolve to use the silicon better. There always was an "Intel advantage" to compilers for decades (admittedly Intel invested in compilers more than AMD, but they also were sneaky about trying to nerf AMD in compilers), but with AMD being such a clear leader for so many years, I would hope at least GCC has started supporting AMD flavors of compilation better. Anyone know if this has happened with GCC and AMD silicon? Or at least is there a better body of knowledge of what GCC flags help AMD more?

scns 5y ago
march=native
aseipp 5y ago
Yes, I consider this to fall under the umbrella of general microarchitectural improvements I mentioned. GCC and LLVM are regularly updated with microarchitectural scheduling models to better emit code that matches the underlying architecture, and have featured these for at least 5-7 years; there can be a big difference between say, Skylake and Zen 2, for instance, so targeting things appropriately is a good idea. You can use the `-march` flag for your compiler to target specific architectures, for instance -march=tigerlake or -march=znver3 But in general I think it's a bit of a red herring for the thrust of my original post; first off you always have to target the benchmark to test a hypothesis, you don't run them in isolation for zero reason. My hypothesis when I ran my own for instance was "General execution of bog standard scalar code is only up by about 50-60%" and using the exact same binary instructions was the baseline criteria for that; it was not "Does targeting a specific microarchitecture scheduling model yield specific gains." If you want to test the second one, you need to run another benchmark. There are too many factors for any particular machine for any such post to be comprehensive, as I'm sure you're aware. I'm just speaking in loose generalities.
- jmgao 5y ago
  > You can use the `-march` flag for your compiler to target specific architectures, for instance -march=tigerlake or -march=znver3 Note that -march will use instructions that might be unavailable on other CPUs of the target. -mtune (which is implied by -march) is the flag that sets the cost tables used by instruction selection, cache line sizes, etc.
  - jiggawatts 5y ago
    I recently watched the CppCon 2019 talk by Matt Godbolt "Compiler Explorer: Behind The Scenes"[1], and a cool feature he presented is the integrated LLVM Machine Code Analyser tool. If you look at the "timeline" and "resource" views of how a Zen 3 executes a typical assembly snippet, it is absolutely mind blowing. That beast of a CPU has so many resources it's just crazy. [1] https://www.youtube.com/watch?v=kIoZDUd5DKw https://www.youtube.com/watch?v=kIoZDUd5DKw

scns 5y ago

march=native

aseipp 5y ago

Yes, I consider this to fall under the umbrella of general microarchitectural improvements I mentioned. GCC and LLVM are regularly updated with microarchitectural scheduling models to better emit code that matches the underlying architecture, and have featured these for at least 5-7 years; there can be a big difference between say, Skylake and Zen 2, for instance, so targeting things appropriately is a good idea. You can use the `-march` flag for your compiler to target specific architectures, for instance -march=tigerlake or -march=znver3 But in general I think it's a bit of a red herring for the thrust of my original post; first off you always have to target the benchmark to test a hypothesis, you don't run them in isolation for zero reason. My hypothesis when I ran my own for instance was "General execution of bog standard scalar code is only up by about 50-60%" and using the exact same binary instructions was the baseline criteria for that; it was not "Does targeting a specific microarchitecture scheduling model yield specific gains." If you want to test the second one, you need to run another benchmark. There are too many factors for any particular machine for any such post to be comprehensive, as I'm sure you're aware. I'm just speaking in loose generalities.

jmgao 5y ago
> You can use the `-march` flag for your compiler to target specific architectures, for instance -march=tigerlake or -march=znver3 Note that -march will use instructions that might be unavailable on other CPUs of the target. -mtune (which is implied by -march) is the flag that sets the cost tables used by instruction selection, cache line sizes, etc.
- jiggawatts 5y ago
  I recently watched the CppCon 2019 talk by Matt Godbolt "Compiler Explorer: Behind The Scenes"[1], and a cool feature he presented is the integrated LLVM Machine Code Analyser tool. If you look at the "timeline" and "resource" views of how a Zen 3 executes a typical assembly snippet, it is absolutely mind blowing. That beast of a CPU has so many resources it's just crazy. [1] https://www.youtube.com/watch?v=kIoZDUd5DKw https://www.youtube.com/watch?v=kIoZDUd5DKw