Mark the last use of EFLAGS before the copy's def as a kill if the copy's def operand is itself a kill.

Author: AZero13

llvm/lib/Target/X86/X86FlagsCopyLowering.cpp

@@ -418,6 +418,31 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
  auto Cleanup = make_scope_exit([&] {
  // All uses of the EFLAGS copy are now rewritten, kill the copy into
  // eflags and if dead the copy from.
+
+ // Mark the last use of EFLAGS before the copy's def as a kill if
+ // the copy's def operand is itself a kill.
+ if (CopyDefI.getOperand(1).isKill()) {
+ MachineBasicBlock *DefMBB = CopyDefI.getParent();
+ MachineInstr *LastUse = nullptr;
+
+ // Find the last use of EFLAGS before CopyDefI
+ for (auto MI = std::prev(CopyDefI.getIterator()); MI != DefMBB->begin();
+ --MI) {
+ if (MI->readsRegister(X86::EFLAGS, TRI)) {
+ LastUse = &*MI;
+ break;
+ }
+ }
+
+ // If we found a last use, mark it as kill
+ if (LastUse) {
+ MachineOperand *FlagUse =
+ LastUse->findRegisterUseOperand(X86::EFLAGS, TRI);
+ if (FlagUse)
+ FlagUse->setIsKill(true);
+ }
+ }
+
  CopyI->eraseFromParent();
  if (MRI->use_empty(CopyDefI.getOperand(0).getReg()))
  CopyDefI.eraseFromParent();
@@ -687,9 +712,6 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
 
  rewriteMI(*TestMBB, TestPos, TestLoc, *JmpI, CondRegs);
  }
-
- // FIXME: Mark the last use of EFLAGS before the copy's def as a kill if
- // the copy's def operand is itself a kill.
  }
 
 #ifndef NDEBUG

llvm/test/CodeGen/X86/apx/flags-copy-lowering.mir

@@ -50,7 +50,7 @@ body: |
  ; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64 = COPY $rdi
  ; CHECK-NEXT: [[COPY1:%[0-9]+]]:gr64 = COPY $rsi
  ; CHECK-NEXT: [[SUB64rr_ND:%[0-9]+]]:gr64 = SUB64rr_ND [[COPY]], [[COPY1]], implicit-def $eflags
- ; CHECK-NEXT: [[SETCCr:%[0-9]+]]:gr8 = SETCCr 2, implicit $eflags
+ ; CHECK-NEXT: [[SETCCr:%[0-9]+]]:gr8 = SETCCr 2, implicit killed $eflags
  ; CHECK-NEXT: INLINEASM &nop, 1 /* sideeffect attdialect */, 12 /* clobber */, implicit-def dead $eflags
  ; CHECK-NEXT: dead [[ADD8ri_ND:%[0-9]+]]:gr8 = ADD8ri_ND [[SETCCr]], 255, implicit-def $eflags
  ; CHECK-NEXT: [[SBB64ri32_ND:%[0-9]+]]:gr64 = SBB64ri32_ND [[SUB64rr_ND]], 42, implicit-def $eflags, implicit killed $eflags

llvm/test/CodeGen/X86/flags-copy-lowering.mir

@@ -277,7 +277,7 @@ body: |
  ; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64 = COPY $rdi
  ; CHECK-NEXT: [[COPY1:%[0-9]+]]:gr64 = COPY $rsi
  ; CHECK-NEXT: [[SUB64rr:%[0-9]+]]:gr64 = SUB64rr [[COPY]], [[COPY1]], implicit-def $eflags
- ; CHECK-NEXT: [[SETCCr:%[0-9]+]]:gr8 = SETCCr 2, implicit $eflags
+ ; CHECK-NEXT: [[SETCCr:%[0-9]+]]:gr8 = SETCCr 2, implicit killed $eflags
  ; CHECK-NEXT: INLINEASM &nop, 1 /* sideeffect attdialect */, 12 /* clobber */, implicit-def dead $eflags
  ; CHECK-NEXT: dead [[ADD8ri:%[0-9]+]]:gr8 = ADD8ri [[SETCCr]], 255, implicit-def $eflags
  ; CHECK-NEXT: [[SBB64ri32_:%[0-9]+]]:gr64 = SBB64ri32 [[SUB64rr]], 42, implicit-def $eflags, implicit killed $eflags