[llvm] [CodeGen] Correctly handle non-standard cases in RemoveLoadsIntoFakeUses (PR #111551)
Stephen Tozer via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 8 14:44:36 PDT 2024
================
@@ -109,31 +112,38 @@ bool RemoveLoadsIntoFakeUses::runOnMachineFunction(MachineFunction &MF) {
// reload of a spilled register.
if (MI.getRestoreSize(TII)) {
Register Reg = MI.getOperand(0).getReg();
- assert(Reg.isPhysical() && "VReg seen in function with NoVRegs set?");
// Don't delete live physreg defs, or any reserved register defs.
if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
continue;
- // There should be an exact match between the loaded register and the
- // FAKE_USE use. If not, this is a load that is unused by anything? It
- // should probably be deleted, but that's outside of this pass' scope.
- if (RegFakeUses.contains(Reg)) {
+ // There should typically be an exact match between the loaded register
+ // and the FAKE_USE, but sometimes regalloc will choose to load a larger
+ // value than is needed. Therefore, as long as the load isn't used by
+ // anything except at least one FAKE_USE, we will delete it. If it isn't
+ // used by any fake uses, it should still be safe to delete but we
+ // choose to ignore it so that this pass has no side effects unrelated
+ // to fake uses.
+ SmallDenseSet<MachineInstr *> FakeUsesToDelete;
+ for (MCRegUnit Unit : TRI->regunits(Reg)) {
+ if (!RegFakeUses.contains(Unit))
+ continue;
+ for (MachineInstr *FakeUse : RegFakeUses[Unit])
+ FakeUsesToDelete.insert(FakeUse);
+ RegFakeUses.erase(Unit);
----------------
SLTozer wrote:
We don't want to erase any FAKE_USE here unless we know we're deleting the load which feeds into it; we only delete the FAKE_USEs here since leaving it around after the load is deleted would make it a fake use of the wrong value.
https://github.com/llvm/llvm-project/pull/111551
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