[llvm] Mark the last use of EFLAGS before the copy's def as a kill if the copy's def operand is itself a kill. (PR #135726)

[via llvm-commits]

https://github.com/AZero13 created https://github.com/llvm/llvm-project/pull/135726

None

>From bf4b910d43851ce3e7785561c4d721115e54377f Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Sun, 13 Apr 2025 23:49:53 -0400
Subject: [PATCH] Mark the last use of EFLAGS before the copy's def as a kill
 if the copy's def operand is itself a kill.

---
 llvm/lib/Target/X86/X86FlagsCopyLowering.cpp  | 28 +++++++++++++++++--
 .../CodeGen/X86/apx/flags-copy-lowering.mir   |  2 +-
 llvm/test/CodeGen/X86/flags-copy-lowering.mir |  2 +-
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp b/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
index ca953d6008b27..4e1b81324afc7 100644
--- a/llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
+++ b/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
diff --git a/llvm/test/CodeGen/X86/apx/flags-copy-lowering.mir b/llvm/test/CodeGen/X86/apx/flags-copy-lowering.mir
index 4002906795dc8..ab48243b0e4e1 100644
--- a/llvm/test/CodeGen/X86/apx/flags-copy-lowering.mir
+++ b/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
diff --git a/llvm/test/CodeGen/X86/flags-copy-lowering.mir b/llvm/test/CodeGen/X86/flags-copy-lowering.mir
index a08c63c285e15..69256cbd43337 100644
--- a/llvm/test/CodeGen/X86/flags-copy-lowering.mir
+++ b/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