X86 cmp lowering is looking past truncate on the condition node. It should only

do so when the high bits are known zero. This caused a subtle miscompilation.

rdar://12027825 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161451 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 52 insertions, 6 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 8125b4c1ad..d70cdb8a76 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -8741,6 +8741,16 @@ static bool isAllOnes(SDValue V) {
   return C && C->isAllOnesValue();
 }
 
+static bool isTruncWithZeroHighBitsInput(SDValue V, SelectionDAG &DAG) {
+  if (V.getOpcode() != ISD::TRUNCATE)
+    return false;
+
+  SDValue VOp0 = V.getOperand(0);
+  unsigned InBits = VOp0.getValueSizeInBits();
+  unsigned Bits = V.getValueSizeInBits();
+  return DAG.MaskedValueIsZero(VOp0, APInt::getHighBitsSet(InBits,InBits-Bits));
+}
+
 SDValue X86TargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
   bool addTest = true;
   SDValue Cond  = Op.getOperand(0);
@@ -8910,9 +8920,9 @@ SDValue X86TargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
   }
 
   if (addTest) {
-    // Look pass the truncate.
-    if (Cond.getOpcode() == ISD::TRUNCATE)
-      Cond = Cond.getOperand(0);
+    // Look pass the truncate if the high bits are known zero.
+    if (isTruncWithZeroHighBitsInput(Cond, DAG))
+        Cond = Cond.getOperand(0);
 
     // We know the result of AND is compared against zero. Try to match
     // it to BT.
@@ -9219,9 +9229,9 @@ SDValue X86TargetLowering::LowerBRCOND(SDValue Op, SelectionDAG &DAG) const {
   }
 
   if (addTest) {
-    // Look pass the truncate.
-    if (Cond.getOpcode() == ISD::TRUNCATE)
-      Cond = Cond.getOperand(0);
+    // Look pass the truncate if the high bits are known zero.
+    if (isTruncWithZeroHighBitsInput(Cond, DAG))
+        Cond = Cond.getOperand(0);
 
     // We know the result of AND is compared against zero. Try to match
     // it to BT.
diff --git a/test/CodeGen/X86/2012-08-07-CmpISelBug.ll b/test/CodeGen/X86/2012-08-07-CmpISelBug.ll
new file mode 100644
index 0000000000..000b853ab8
--- /dev/null
+++ b/test/CodeGen/X86/2012-08-07-CmpISelBug.ll
@@ -0,0 +1,36 @@
+; RUN: llc < %s -mtriple=x86_64-apple-macosx | FileCheck %s
+
+; Cmp lowering should not look past the truncate unless the high bits are known
+; zero.
+; rdar://12027825
+
+define void @foo(i8 %arg4, i32 %arg5, i32* %arg14) nounwind {
+bb:
+; CHECK: foo:
+; CHECK-NOT: testl
+; CHECK: testb
+  %tmp48 = zext i8 %arg4 to i32
+  %tmp49 = and i32 %tmp48, 32
+  %tmp50 = add i32 %tmp49, 1593371643
+  %tmp55 = sub i32 %tmp50, 0
+  %tmp56 = add i32 %tmp55, 7787538
+  %tmp57 = xor i32 %tmp56, 1601159181
+  %tmp58 = xor i32 %arg5, 1601159181
+  %tmp59 = and i32 %tmp57, %tmp58
+  %tmp60 = add i32 %tmp59, -1263900958
+  %tmp67 = sub i32 %tmp60, 0
+  %tmp103 = xor i32 %tmp56, 13
+  %tmp104 = trunc i32 %tmp103 to i8
+  %tmp105 = sub i8 0, %tmp104
+  %tmp106 = add i8 %tmp105, -103
+  %tmp113 = sub i8 %tmp106, 0
+  %tmp114 = add i8 %tmp113, -72
+  %tmp141 = icmp ne i32 %tmp67, -1263900958
+  %tmp142 = select i1 %tmp141, i8 %tmp114, i8 undef
+  %tmp143 = xor i8 %tmp142, 81
+  %tmp144 = zext i8 %tmp143 to i32
+  %tmp145 = add i32 %tmp144, 2062143348
+  %tmp152 = sub i32 %tmp145, 0
+  store i32 %tmp152, i32* %arg14
+  ret void
+}