[LLVMdev] use AVX automatically if present

[Henning Thielemann]

I wonder why AVX is not used automatically if available at the host 
machine. In contrast to that, SSE41 instructions (like pmulld) are 
automatically used if the host machine supports SSE41.

E.g.

$ cat avx.ll

define void @_fun1(<8 x float>*, <8 x float>*) {
_L1:
   %x = load <8 x float>* %0
   %y = load <8 x float>* %1
   %z = fadd <8 x float> %x, %y
   store <8 x float> %z, <8 x float>* %0
   ret void
}


$ llc -o - avx.ll
         .file   "avx.ll"
         .text
         .globl  _fun1
         .align  16, 0x90
         .type   _fun1, at function
_fun1:                                  # @_fun1
         .cfi_startproc
# BB#0:                                 # %_L1
         movaps  (%rdi), %xmm0
         movaps  16(%rdi), %xmm1
         addps   (%rsi), %xmm0
         addps   16(%rsi), %xmm1
         movaps  %xmm1, 16(%rdi)
         movaps  %xmm0, (%rdi)
         ret
.Ltmp0:
         .size   _fun1, .Ltmp0-_fun1
         .cfi_endproc


         .section        ".note.GNU-stack","", at progbits



$ llc -o - -mattr avx avx.ll
         .file   "avx.ll"
         .text
         .globl  _fun1
         .align  16, 0x90
         .type   _fun1, at function
_fun1:                                  # @_fun1
         .cfi_startproc
# BB#0:                                 # %_L1
         pushq   %rbp
.Ltmp2:
         .cfi_def_cfa_offset 16
.Ltmp3:
         .cfi_offset %rbp, -16
         movq    %rsp, %rbp
.Ltmp4:
         .cfi_def_cfa_register %rbp
         vmovaps (%rdi), %ymm0
         vaddps  (%rsi), %ymm0, %ymm0
         vmovaps %ymm0, (%rdi)
         popq    %rbp
         vzeroupper
         ret
.Ltmp5:
         .size   _fun1, .Ltmp5-_fun1
         .cfi_endproc


         .section        ".note.GNU-stack","", at progbits




I guess your answer is that I did not specify a target triple. However why 
is SSE41 automatically detected and AVX is not?