[all-commits] [llvm/llvm-project] ee0bf6: [InstCombine] try to fold mul by neg-power-of-2 to...

[Sanjay Patel via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ee0bf6472291bd40687046ab2990f987a022a940
      https://github.com/llvm/llvm-project/commit/ee0bf6472291bd40687046ab2990f987a022a940
  Author: Sanjay Patel <spatel at rotateright.com>
  Date:   2022-09-21 (Wed, 21 Sep 2022)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    M llvm/test/Transforms/InstCombine/add.ll
    M llvm/test/Transforms/InstCombine/neg-alloca.ll
    M llvm/test/Transforms/Reassociate/basictest.ll

  Log Message:
  -----------
  [InstCombine] try to fold mul by neg-power-of-2 to shl

`(A * -2**C) + B --> B - (A << C)`

https://alive2.llvm.org/ce/z/A6BWkf

This inverts what Negator was doing before:
D134310 / 0f32a5dea0e9

Analysis and codegen are generally better without multiply,
so we should favor this form even if we trade add for sub
(because those are generally equivalent cost operations).