X86 Peephole: fix PR13475 in optimizeCompare.

It is possible that an instruction can use and update EFLAGS.
When checking the safety, we should check the usage of EFLAGS first before
declaring it is safe to optimize due to the update.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160912 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 7 insertions, 3 deletions

diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index 69493bc..89a57b5 100644
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -3221,12 +3221,15 @@ optimizeCompareInstr(MachineInstr *CmpInstr, unsigned SrcReg, unsigned SrcReg2,
   MachineBasicBlock::iterator E = CmpInstr->getParent()->end();
   for (++I; I != E; ++I) {
     const MachineInstr &Instr = *I;
-    if (Instr.modifiesRegister(X86::EFLAGS, TRI)) {
+    bool ModifyEFLAGS = Instr.modifiesRegister(X86::EFLAGS, TRI);
+    bool UseEFLAGS = Instr.readsRegister(X86::EFLAGS, TRI);
+    // We should check the usage if this instruction uses and updates EFLAGS.
+    if (!UseEFLAGS && ModifyEFLAGS) {
       // It is safe to remove CmpInstr if EFLAGS is updated again.
       IsSafe = true;
       break;
     }
-    if (!Instr.readsRegister(X86::EFLAGS, TRI))
+    if (!UseEFLAGS && !ModifyEFLAGS)
       continue;
 
     // EFLAGS is used by this instruction.
@@ -3281,7 +3284,8 @@ optimizeCompareInstr(MachineInstr *CmpInstr, unsigned SrcReg, unsigned SrcReg2,
       // instructions will be modified.
       OpsToUpdate.push_back(std::make_pair(&*I, NewOpc));
     }
-    if (Instr.killsRegister(X86::EFLAGS, TRI)) {
+    if (ModifyEFLAGS || Instr.killsRegister(X86::EFLAGS, TRI)) {
+      // It is safe to remove CmpInstr if EFLAGS is updated again or killed.
       IsSafe = true;
       break;
     }