Folding nodes in instruction selection - please review

[Demikhovsky, Elena]

Hi,

I analyzed the folding DAG nodes algorithm in LLVM, compared to code generated by Intel compiler and got into conclusion that the LLVM code is not always optimal.
This is an example:

  %b1 = fadd <8 x float> %a1, <float AAA, float AAA, float AAA, float AAA, float AAA, float AAA, float AAA, float AAA >
  %b2 = fadd <8 x float> %a2, <float AAA, float AAA, float AAA, float AAA, float AAA, float AAA, float AAA, float AAA >
  %c = fmul <8 x float> %b1, %b2

The result (1) bellow is not better than (2), because loading constant is not a problem, but spilling %ymm2 that may be required  in (1) is not cheap.

(1)
        vmovaps .LCPI1_0(%rip), %ymm2
        vaddps  %ymm2, %ymm1, %ymm1
        vaddps  %ymm2, %ymm0, %ymm0
        vmulps  %ymm1, %ymm0, %ymm0

(2)
        vaddps  .LCPI1_0(%rip), %ymm1, %ymm1
        vaddps  .LCPI1_0(%rip),  %ymm0, %ymm0
        vmulps  %ymm1, %ymm0, %ymm0

In the attached patch I did a minor thing that does not change the current algorithm, but allows to add some target-specific analysis.

Please review.

Thank you.

-        Elena



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