[all-commits] [llvm/llvm-project] 82ec2d: [Coalescer] Consider NewMI's subreg index when upd...

[Sander de Smalen via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 82ec2d6aa45fcfbfcf9f12504c10b1e457bda65c
      https://github.com/llvm/llvm-project/commit/82ec2d6aa45fcfbfcf9f12504c10b1e457bda65c
  Author: Sander de Smalen <sander.desmalen at arm.com>
  Date:   2025-01-07 (Tue, 07 Jan 2025)

  Changed paths:
    M llvm/lib/CodeGen/RegisterCoalescer.cpp
    M llvm/test/CodeGen/AArch64/register-coalesce-update-subranges-remat.mir

  Log Message:
  -----------
  [Coalescer] Consider NewMI's subreg index when updating lanemask. (#121780)

The code added in #116191 that updated the lanemasks for rematerialized
values checked if `DefMI`'s destination register had a subreg index.
This seems to have missed the following case:

```
  %0:gpr32 = MOVi32imm 1
  %1:gpr64 = SUBREG_TO_REG 0, %0:gpr32, %subreg.sub_32
```

which during rematerialization would have the following variables set:

```
  DefMI = %0:gpr32 = MOVi32imm 1

  NewMI = %3.sub_32:gpr64 = MOVi32imm 1   (rematerialized value)
```

When checking whether the lanemasks need to be generated, considering
whether DefMI's destination has a subreg index is insufficient, we
should look at DefMI's subreg index instead.

The added tests are a bit more involved, because I was not able to
reconstruct the issue without having some control flow in the test.
These tests come from actual reproducers.



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