[all-commits] [llvm/llvm-project] 02187e: [GlobalISel] Combine G_SELECTs of the form (cond ?...

[Jessica Paquette via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 02187ed45a4302db5f6721c3233988fef7819701
      https://github.com/llvm/llvm-project/commit/02187ed45a4302db5f6721c3233988fef7819701
  Author: Jessica Paquette <jpaquette at apple.com>
  Date:   2020-03-23 (Mon, 23 Mar 2020)

  Changed paths:
    M llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
    M llvm/include/llvm/Target/GlobalISel/Combine.td
    M llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
    A llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-select.mir

  Log Message:
  -----------
  [GlobalISel] Combine G_SELECTs of the form (cond ? x : x) into x

When we find something like this:

```
%a:_(s32) = G_SOMETHING ...
...
%select:_(s32) = G_SELECT %cond(s1), %a, %a
```

We can remove the select and just replace it entirely with `%a` because it's
always going to result in `%a`.

Same if we have

```
%select:_(s32) = G_SELECT %cond(s1), %a, %b
```

where we can deduce that `%a == %b`.

This implements the following cases:

- `%select:_(s32) = G_SELECT %cond(s1), %a, %a` -> `%a`

- `%select:_(s32) = G_SELECT %cond(s1), %a, %some_copy_from_a` -> `%a`

- `%select:_(s32) = G_SELECT %cond(s1), %a, %b` -> `%a` when `%a` and `%b`
   are defined by identical instructions

This gives a few minor code size improvements on CTMark at -O3 for AArch64.

Differential Revision: https://reviews.llvm.org/D76523