Any way to prevent gc emitting AVX256 and AVX512 instructions for amd64?

[Amnon Baron Cohen]

I am trying to avoid running any AVX instructions in order to prevent Intel's dynamic frequency scaling 
reducing my CPU base frequency.

Thanks,
   Amnon

[Amnon Baron Cohen]

apart from GOARCH=386 ?

[Ian Lance Taylor]

On Tue, May 14, 2019 at 2:18 AM Amnon Baron Cohen <amn...@gmail.com> wrote:
>
> I am trying to avoid running any AVX instructions in order to prevent Intel's dynamic frequency scaling
> reducing my CPU base frequency.

You should be able to avoid AVX instructions at runtime by setting the
environment variable GODEBUG=cpu.avx=off. For the complete list of
available GODEBUG=cpu options available on amd64 see the options
variable in internal/cpu/cpu_x86.go.

Ian

[Amnon Baron Cohen]

Thanks!

> 

> go version

go version go1.12.5 linux/amd64

> GODEBUG=cpu.avx=off go build hello.go 

> objdump -d hello | grep '%ymm'

  4021bd: c5 fe 6f 16          vmovdqu (%rsi),%ymm2

  4021c1: c5 fe 6f 1f          vmovdqu (%rdi),%ymm3

  4021c5: c5 fe 6f 66 20        vmovdqu 0x20(%rsi),%ymm4

> GODEBUG=cpu.avx=off,cpu.avx2=off go build hello.go 

> objdump -d hello | grep '%ymm' |head

  4021bd: c5 fe 6f 16          vmovdqu (%rsi),%ymm2

  4021c1: c5 fe 6f 1f          vmovdqu (%rdi),%ymm3

  4021c5: c5 fe 6f 66 20        vmovdqu 0x20(%rsi),%ymm4

  4021ca: c5 fe 6f 6f 20        vmovdqu 0x20(%rdi),%ymm5

but the sledgehammer GOARCH=386 does work

> GOARCH=386 go build hello.go 

> objdump -d hello | grep '%ymm' 

> 

I'll keep digging.

- Amnon

[Ian Lance Taylor]

On Tue, May 14, 2019 at 7:03 AM Amnon Baron Cohen <amn...@gmail.com> wrote:

> > go version
> go version go1.12.5 linux/amd64
> > GODEBUG=cpu.avx=off go build hello.go
> > objdump -d hello | grep '%ymm'
> 4021bd: c5 fe 6f 16 vmovdqu (%rsi),%ymm2
> 4021c1: c5 fe 6f 1f vmovdqu (%rdi),%ymm3
> 4021c5: c5 fe 6f 66 20 vmovdqu 0x20(%rsi),%ymm4
>
> > GODEBUG=cpu.avx=off,cpu.avx2=off go build hello.go
> > objdump -d hello | grep '%ymm' |head
> 4021bd: c5 fe 6f 16 vmovdqu (%rsi),%ymm2
> 4021c1: c5 fe 6f 1f vmovdqu (%rdi),%ymm3
> 4021c5: c5 fe 6f 66 20 vmovdqu 0x20(%rsi),%ymm4
> 4021ca: c5 fe 6f 6f 20 vmovdqu 0x20(%rdi),%ymm5

Yes: GODEBUG=cpu.avx=off affects runtime execution. It does not
affect the compiler. You should set it when running the program. It
will override the CPU detection to say that AVX instructions are not
available on this processor.

There is no way to change what the compiler generates.

Ian

[Amnon Baron Cohen]

OK.
Thanks for the explanation and pointers.

[keith....@gmail.com]

By the way, these instructions are not generated by the compiler. They are part of the assembly in the stdlib (runtime or internal/bytealg, probably).