[all-commits] [llvm/llvm-project] 50efbf: [GlobalISel] Narrow binops feeding into G_AND with...

[Jessica Paquette via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 50efbf9cbeed9d202e9f66a0d154489811918944
      https://github.com/llvm/llvm-project/commit/50efbf9cbeed9d202e9f66a0d154489811918944
  Author: Jessica Paquette <jpaquette at apple.com>
  Date:   2021-08-13 (Fri, 13 Aug 2021)

  Changed paths:
    M llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
    M llvm/include/llvm/CodeGen/TargetLowering.h
    M llvm/include/llvm/Target/GlobalISel/Combine.td
    M llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
    A llvm/test/CodeGen/AArch64/GlobalISel/prelegalizer-combiner-narrow-binop-feeding-add.mir

  Log Message:
  -----------
  [GlobalISel] Narrow binops feeding into G_AND with a mask

This is a fairly common pattern:

```
%mask = G_CONSTANT iN <mask val>
%add = G_ADD %lhs, %rhs
%and = G_AND %add, %mask
```

We have combines to eliminate G_AND with a mask that does nothing.

If we combined the above to this:

```
%mask = G_CONSTANT iN <mask val>
%narrow_lhs = G_TRUNC %lhs
%narrow_rhs = G_TRUNC %rhs
%narrow_add = G_ADD %narrow_lhs, %narrow_rhs
%ext = G_ZEXT %narrow_add
%and = G_AND %ext, %mask
```

We'd be able to take advantage of those combines using the trunc + zext.

For this to work (or be beneficial in the best case)

- The operation we want to narrow then widen must only be used by the G_AND
- The G_TRUNC + G_ZEXT must be free
- Performing the operation at a narrower width must not produce a different
  value than performing it at the original width *after masking.*

Example comparison between SDAG + GISel: https://godbolt.org/z/63jzb1Yvj

At -Os for AArch64, this is a 0.2% code size improvement on CTMark/pairlocalign.

Differential Revision: https://reviews.llvm.org/D107929