[llvm] [X86] Resolve FIXME: Copy kill flag for eflags (PR #82411)

[via llvm-commits]

https://github.com/AtariDreams updated https://github.com/llvm/llvm-project/pull/82411

>From c17b2ac3cc73cd1b2c2099d042f8d0d0b35cfc54 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Tue, 20 Feb 2024 14:29:42 -0500
Subject: [PATCH] [X86] Resolve FIXME: Copy kill flag for eflags

We now mark the last eflags usage as kill if the def was also kill.
---
 llvm/lib/Target/X86/X86FlagsCopyLowering.cpp | 33 ++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp b/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
index af25b34fbab995..54347e92109c6c 100644
--- a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
+++ b/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
@@ -347,6 +347,31 @@ static X86::CondCode getCondFromFCMOV(unsigned Opcode) {
   }
 }
 
+static bool checkEFLAGSLive(MachineInstr *MI) {
+  if (MI->killsRegister(X86::EFLAGS))
+    return false;
+
+  // The EFLAGS operand of MI might be missing a kill marker.
+  // Figure out whether EFLAGS operand should LIVE after MI instruction.
+  MachineBasicBlock *BB = MI->getParent();
+  MachineBasicBlock::iterator ItrMI = MI;
+
+  // Scan forward through BB for a use/def of EFLAGS.
+  for (auto I = std::next(ItrMI), E = BB->end(); I != E; ++I) {
+    if (I->readsRegister(X86::EFLAGS))
+      return true;
+    if (I->definesRegister(X86::EFLAGS))
+      return false;
+  }
+
+  // We hit the end of the block, check whether EFLAGS is live into a successor.
+  for (MachineBasicBlock *Succ : BB->successors())
+    if (Succ->isLiveIn(X86::EFLAGS))
+      return true;
+
+  return false;
+}
+
 bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** " << getPassName() << " : " << MF.getName()
                     << " **********\n");
@@ -683,6 +708,8 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
     // Now rewrite the jumps that use the flags. These we handle specially
     // because if there are multiple jumps in a single basic block we'll have
     // to do surgery on the CFG.
+    bool CopyDefIsKill = !checkEFLAGSLive(&CopyDefI);
+    MachineOperand *LastEflagsUse = nullptr;
     MachineBasicBlock *LastJmpMBB = nullptr;
     for (MachineInstr *JmpI : JmpIs) {
       // Past the first jump within a basic block we need to split the blocks
@@ -693,10 +720,12 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
         LastJmpMBB = JmpI->getParent();
 
       rewriteCondJmp(*TestMBB, TestPos, TestLoc, *JmpI, CondRegs);
+      if (CopyDefIsKill && JmpI->readsRegister(X86::EFLAGS))
+        LastEflagsUse = JmpI->findRegisterUseOperand(X86::EFLAGS);
     }
 
-    // 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.
+    if (LastEflagsUse && CopyDefIsKill)
+      LastEflagsUse->setIsKill(true);
   }
 
 #ifndef NDEBUG