[InstCombine] Don't miscompile select to poison

If we have (select a, b, c), it is sometimes valid to simplify this to a
single select operand.  However, doing so is only valid if the
computation doesn't inject poison into the computation.

It might be helpful to consider the following example:
  (select (icmp ne %i, INT_MAX), (add nsw %i, 1), INT_MIN)

The select is equivalent to (add %i, 1) but not (add nsw %i, 1).

Self hosting on x86_64 revealed that this occurs very, very rarely so
bailing out is hopefully pretty reasonable.

llvm-svn: 239215

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -292,6 +292,19 @@ static Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
 
   // If this is a binary operator, try to simplify it with the replaced op.
   if (BinaryOperator *B = dyn_cast<BinaryOperator>(I)) {
+    // Consider:
+    //   %cmp = icmp eq i32 %x, 2147483647
+    //   %add = add nsw i32 %x, 1
+    //   %sel = select i1 %cmp, i32 -2147483648, i32 %add
+    //
+    // We can't replace %sel with %add unless we strip away the flags.
+    if (isa<OverflowingBinaryOperator>(B))
+      if (B->hasNoSignedWrap() || B->hasNoUnsignedWrap())
+        return nullptr;
+    if (isa<PossiblyExactOperator>(B))
+      if (B->isExact())
+        return nullptr;
+
     if (B->getOperand(0) == Op)
       return SimplifyBinOp(B->getOpcode(), RepOp, B->getOperand(1), DL, TLI);
     if (B->getOperand(1) == Op)

llvm/test/Transforms/InstCombine/select.ll

@@ -1532,3 +1532,16 @@ define i32 @test_max_of_min(i32 %a) {
   %s1 = select i1 %c1, i32 %s0, i32 -1
   ret i32 %s1
 }
+
+
+define i32 @PR23757(i32 %x) {
+; CHECK-LABEL: @PR23757
+; CHECK:      %[[cmp:.*]] = icmp eq i32 %x, 2147483647
+; CHECK-NEXT: %[[add:.*]] = add nsw i32 %x, 1
+; CHECK-NEXT: %[[sel:.*]] = select i1 %[[cmp]], i32 -2147483648, i32 %[[add]]
+; CHECK-NEXT: ret i32 %[[sel]]
+  %cmp = icmp eq i32 %x, 2147483647
+  %add = add nsw i32 %x, 1
+  %sel = select i1 %cmp, i32 -2147483648, i32 %add
+  ret i32 %sel
+}