fix infinite loop in instcombine in the presence of a (malformed) sel…

…f-referencing select inst.

This can happen as long as the instruction is not reachable. Instcombine does generate these unreachable malformed selects when doing RAUW

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160874 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -881,12 +881,16 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
 
   if (SelectInst *TrueSI = dyn_cast<SelectInst>(TrueVal)) {
     if (TrueSI->getCondition() == CondVal) {
+      if (SI.getTrueValue() == TrueSI->getTrueValue())
+        return 0;
       SI.setOperand(1, TrueSI->getTrueValue());
       return &SI;
     }
   }
   if (SelectInst *FalseSI = dyn_cast<SelectInst>(FalseVal)) {
     if (FalseSI->getCondition() == CondVal) {
+      if (SI.getFalseValue() == FalseSI->getFalseValue())
+        return 0;
       SI.setOperand(2, FalseSI->getFalseValue());
       return &SI;
     }

test/Transforms/InstCombine/select-crash.ll

@@ -30,3 +30,20 @@ define <4 x float> @foo(i1 %b, <4 x float> %x, <4 x float> %y, <4 x float> %z) {
   %sel = select i1 %b, <4 x float> %a, <4 x float> %sub 
   ret <4 x float> %sel
 }
+
+; CHECK: @test3
+define i32 @test3(i1 %bool, i32 %a) {
+entry:
+  %cond = or i1 %bool, true
+  br i1 %cond, label %return, label %xpto
+
+; technically reachable, but this malformed IR may appear as a result of constant propagation
+xpto:
+  %select = select i1 %bool, i32 %a, i32 %select
+  %select2 = select i1 %bool, i32 %select2, i32 %a
+  %sum = add i32 %select, %select2
+  ret i32 %sum
+
+return:
+  ret i32 7
+}