[all-commits] [llvm/llvm-project] 459472: [LRE] Adjust order of cases in eliminateDeadDefs (...

[Philip Reames via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 459472eef056ba5119a5da37a5cbb789fac531ae
      https://github.com/llvm/llvm-project/commit/459472eef056ba5119a5da37a5cbb789fac531ae
  Author: Philip Reames <preames at rivosinc.com>
  Date:   2025-10-07 (Tue, 07 Oct 2025)

  Changed paths:
    M llvm/lib/CodeGen/LiveRangeEdit.cpp

  Log Message:
  -----------
  [LRE] Adjust order of cases in eliminateDeadDefs (#162108)

If we have a rematerializable instruction without live virtual register
uses (that we didn't heuristically decide to shrink), but with a physreg
use, we were creating a kill to keep the physreg operand liveness
unchanged. This meant we *weren't* keeping around the remat instruction,
and thus inhibiting future remat within the original live interval. If
we keep the remat instruction, that *also* keeps the physreg use, and
thus we can achieve both objectives at once.

Noticed this via inspection, and we don't currently seem to have any
rematerializable instructions which could observe the difference. This
could in theory happen for both trivial and non-trivial remat, but
requires the rematerialization of an instruction with a physreg use.



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