[LLVMdev] how to disable sse and avx

[zhi chen]

Hello,

I want to disable LLVM to use any SIMD registers like sse and avx. I tried -mno-sse and -mno-avx on my Core i7 machine. It seems that the generated assembly still uses xmm registers. Is there any way for LLVM to only use scalar registers? 

Thanks,

Zhi

[mats petersson]

Sounds like a bug, as far as I can tell, -mno-sse should disable all use of SSE instructions (and thus all uses of xmm registers). Exactly what version of LLVM, and what instructions from what LLVM IR or C source?

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Mats

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[Zhi Chen]

Thanks, Mats. Actually, it is able to generate the assembly now  if I use the follow command: clang++ -O3 -S -mllvm --x86-asm-syntax=intel -mno-sse -o test_nosee.s test.cpp

However, when I use g++ -O3 -o test_nosse test_nosse.s -lm to generate the executable, if gives me the following errors:

 Error: too many memory references for `sub'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: junk `ptr [rsp+112]' after expression

 Error: junk `ptr [rsp+128]' after expression

 Error: junk `(1)' after expression

 Error: operand size mismatch for `fsubrp'

Any idea? Thanks.

--

Zhi

--

PhD Student

Department of Computer Science

University of California, Irvine

[Michael Zolotukhin]

On May 14, 2015, at 11:03 AM, Zhi Chen <zhich...@gmail.com> wrote:

Thanks, Mats. Actually, it is able to generate the assembly now  if I use the follow command: clang++ -O3 -S -mllvm --x86-asm-syntax=intel -mno-sse -o test_nosee.s test.cpp

However, when I use g++ -O3 -o test_nosse test_nosse.s -lm to generate the executable, if gives me the following errors:

 Error: too many memory references for `sub'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: too many memory references for `mov'

 Error: junk `ptr [rsp+112]' after expression

 Error: junk `ptr [rsp+128]' after expression

 Error: junk `(1)' after expression

 Error: operand size mismatch for `fsubrp'

Any idea? Thanks.

I guess the problem is that g++ might expect AT&T syntax in asm files. Could you try changing ‘--x86-asm-syntax=intel’ to ‘--x86-asm-syntax=att’?

Michael

[Eric Christopher]

Probably just issues in our intel syntax generation.

Why are you using that?

-eric

[Reid Kleckner]

Try not using intel assembly syntax, it is the less supported dialect.

[Zhi Chen]

Thanks for all your replies. Yes, it was the because I specified "--x86-asm-syntax=intel". I just wanted to see the performance difference between the code w/ and w/o using SSE/AVX registers.

Best,

Zhi