[all-commits] [llvm/llvm-project] 30f6f8: RegisterPressure: Remove dead defs correctly

[Matt Arsenault via All-commits]

  Branch: refs/heads/users/arsenm/codegen/registerpressure-remove-dead-defs
  Home:   https://github.com/llvm/llvm-project
  Commit: 30f6f8641c48d53abb78bcd5f03bdb74d5b6ff47
      https://github.com/llvm/llvm-project/commit/30f6f8641c48d53abb78bcd5f03bdb74d5b6ff47
  Author: Matt Arsenault <Matthew.Arsenault at amd.com>
  Date:   2026-09-10 (Thu, 10 Sep 2026)

  Changed paths:
    M llvm/lib/CodeGen/RegisterPressure.cpp
    A llvm/test/CodeGen/X86/dead-register-pr76416.mir
    A llvm/test/CodeGen/X86/inline-asm-pr149144.ll
    A llvm/test/CodeGen/X86/inline-asm-pr155807.ll
    A llvm/test/CodeGen/X86/inline-asm-pr76416.ll
    M llvm/test/CodeGen/X86/masked-udiv.ll
    M llvm/test/CodeGen/X86/min-legal-vector-width.ll
    A llvm/test/CodeGen/X86/pr205272.ll
    M llvm/test/CodeGen/X86/ssub_sat_plus.ll
    M llvm/test/CodeGen/X86/udiv_fix.ll
    M llvm/test/CodeGen/X86/udiv_fix_sat.ll
    M llvm/test/CodeGen/X86/usub_sat_plus.ll
    M llvm/test/CodeGen/X86/vector-idiv-strictfp.ll
    M llvm/test/CodeGen/X86/vector-idiv-udiv-512.ll
    M llvm/test/CodeGen/X86/xmulo.ll

  Log Message:
  -----------
  RegisterPressure: Remove dead defs correctly

When an instruction has overlapping register defs where only some carry the
dead flag (for example a dead super-register def alongside a live sub-register
def), the collector left the shared register units in both the live and dead def
sets. That produced a PressureDiff decrement with no matching increment and tripped
the "PSet overflow/underflow" assertion in getUpwardPressureDelta.

A register unit is dead if any def covering it is dead, regardless of operand
order. Reconcile by subtracting the dead defs from the live defs instead of the
reverse: removeRegLanes only clears the overlapping units, so a def keeps any
units a dead def does not cover.

Fixes #155807.
Fixes #149144.
Fixes #76416.
Fixes #205272.

Co-authored-by: XChy <xxs_chy at outlook.com>
Co-authored-by: Claude claude-opus-4.8 <noreply at anthropic.com>



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