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

diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index 69493bc2070a833f9fc90d1fce8a83b93a776b5a..89a57b53f3cb4371e0edc4f5ea5593eb051c24c6 100644 (file)
--- 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;
     }

diff --git a/test/CodeGen/X86/jump_sign.ll b/test/CodeGen/X86/jump_sign.ll
index 5fb6ee52548313c5ac7d1bab1884c866e23b3f04..61792d3253ca1c2bc3f6aaf7b235519dd8b2c13e 100644 (file)
--- a/test/CodeGen/X86/jump_sign.ll
+++ b/test/CodeGen/X86/jump_sign.ll
@@ -214,3 +214,19 @@ entry:
   %add. = select i1 %cmp, i32 %add, i32 0
   ret i32 %add.
 }
+; PR13475
+; If we have sub a, b and cmp b, a and the result of cmp is used
+; by sbb, we should not optimize cmp away.
+define i32 @q(i32 %j.4, i32 %w, i32 %el) {
+; CHECK: q:
+; CHECK: sub
+; CHECK: cmp
+; CHECK-NEXT: sbb
+  %tmp532 = add i32 %j.4, %w
+  %tmp533 = icmp ugt i32 %tmp532, %el
+  %tmp534 = icmp ult i32 %w, %el
+  %or.cond = and i1 %tmp533, %tmp534
+  %tmp535 = sub i32 %el, %w
+  %j.5 = select i1 %or.cond, i32 %tmp535, i32 %j.4
+  ret i32 %j.5
+}