InstCombine: Fix infinite loop when encountering switch on trivial icmp.

The test case feeds the following into InstCombine's visitSelect:
%tobool8 = icmp ne i32 0, 0
%phitmp = select i1 %tobool8, i32 3, i32 0
Then instcombine replaces the right side of the switch with 0, doesn't notice
that nothing changes and tries again indefinitely.

This fixes PR12897.

llvm-svn: 157587

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -498,7 +498,7 @@ Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
 
   // NOTE: if we wanted to, this is where to detect integer MIN/MAX
 
-  if (isa<Constant>(CmpRHS)) {
+  if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS)) {
     if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
       // Transform (X == C) ? X : Y -> (X == C) ? C : Y
       SI.setOperand(1, CmpRHS);

llvm/test/Transforms/InstCombine/2012-05-28-select-hang.ll

@@ -0,0 +1,39 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+@c = common global i8 0, align 1
+@a = common global i8 0, align 1
+@b = common global i8 0, align 1
+
+define void @func() nounwind uwtable ssp {
+entry:
+  %0 = load i8* @c, align 1
+  %conv = zext i8 %0 to i32
+  %or = or i32 %conv, 1
+  %conv1 = trunc i32 %or to i8
+  store i8 %conv1, i8* @a, align 1
+  %conv2 = zext i8 %conv1 to i32
+  %neg = xor i32 %conv2, -1
+  %and = and i32 1, %neg
+  %conv3 = trunc i32 %and to i8
+  store i8 %conv3, i8* @b, align 1
+  %1 = load i8* @a, align 1
+  %conv4 = zext i8 %1 to i32
+  %conv5 = zext i8 %conv3 to i32
+  %tobool = icmp ne i32 %conv4, 0
+  br i1 %tobool, label %land.rhs, label %land.end
+
+land.rhs:                                         ; preds = %entry
+  %tobool8 = icmp ne i32 %conv5, 0
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs, %entry
+  %2 = phi i1 [ false, %entry ], [ %tobool8, %land.rhs ]
+  %land.ext = zext i1 %2 to i32
+  %mul = mul nsw i32 3, %land.ext
+  %conv9 = trunc i32 %mul to i8
+  store i8 %conv9, i8* @a, align 1
+  ret void
+
+; CHECK: @func
+; CHECK-NOT: select
+}