[all-commits] [llvm/llvm-project] 9991ea: [CostModel][AArch64] Make extractelement, with fmu...

[Sushant Gokhale via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9991ea28fcd308d5bd357358710e5344e26b46e1
      https://github.com/llvm/llvm-project/commit/9991ea28fcd308d5bd357358710e5344e26b46e1
  Author: Sushant Gokhale <sgokhale at nvidia.com>
  Date:   2024-11-13 (Wed, 13 Nov 2024)

  Changed paths:
    M llvm/include/llvm/Analysis/TargetTransformInfo.h
    M llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
    M llvm/include/llvm/CodeGen/BasicTTIImpl.h
    M llvm/lib/Analysis/TargetTransformInfo.cpp
    M llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
    M llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
    M llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
    M llvm/test/Analysis/CostModel/AArch64/extract_float.ll

  Log Message:
  -----------
  [CostModel][AArch64] Make extractelement, with fmul user, free whenev… (#111479)

…er possible

In case of Neon, if there exists extractelement from lane != 0 such that
  1. extractelement does not necessitate a move from vector_reg -> GPR
  2. extractelement result feeds into fmul
3. Other operand of fmul is a scalar or extractelement from lane 0 or
lane equivalent to 0
then the extractelement can be merged with fmul in the backend and it
incurs no cost.

  e.g. 
  ```
define double @foo(<2 x double> %a) { 
    %1 = extractelement <2 x double> %a, i32 0 
    %2 = extractelement <2 x double> %a, i32 1
    %res = fmul double %1, %2    
    ret double %res
  }
```
  `%2` and `%res` can be merged in the backend to generate:
  `fmul    d0, d0, v0.d[1]`

The change was tested with SPEC FP(C/C++) on Neoverse-v2. 
**Compile time impact**: None
**Performance impact**: Observing 1.3-1.7% uplift on lbm benchmark with -flto depending upon the config.



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