[InstCombine] Use m_Zero instead of isNullValue() when checking if a …

…GEP index is all zeroes to prevent an infinite loop.

The test case here previously infinite looped. Only one element from the GEP is used so SimplifyDemandedVectorElts would replace the other lanes in each index with undef leading to the first index being <0, undef, undef, undef>. But there's a GEP transform that tries to replace an index into a 0 sized type with a zero index. But the zero index check only works on ConstantInt 0 or ConstantAggregateZero so it would turn the index back to zeroinitializer. Resulting in a loop.

The fix is to use m_Zero() to allow a vector of zeroes and undefs.

Differential Revision: https://reviews.llvm.org/D67977

llvm-svn: 373000

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1667,7 +1667,7 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     // to an index of zero, so replace it with zero if it is not zero already.
     Type *EltTy = GTI.getIndexedType();
     if (EltTy->isSized() && DL.getTypeAllocSize(EltTy) == 0)
-      if (!isa<Constant>(*I) || !cast<Constant>(*I)->isNullValue()) {
+      if (!isa<Constant>(*I) || !match(I->get(), m_Zero())) {
         *I = Constant::getNullValue(NewIndexType);
         MadeChange = true;
       }

llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

@@ -651,3 +651,20 @@ define i32* @PR41624(<2 x { i32, i32 }*> %a) {
   %r = extractelement <2 x i32*> %w, i32 0
   ret i32* %r
 }
+
+@global = external global [0 x i32], align 4
+
+; Make sure we don't get stuck in a loop turning the zeroinitializer into
+; <0, undef, undef, undef> and then changing it back.
+define i32* @zero_sized_type_extract(<4 x i64> %arg, i64 %arg1) {
+; CHECK-LABEL: @zero_sized_type_extract(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[TMP:%.*]] = getelementptr inbounds [0 x i32], <4 x [0 x i32]*> <[0 x i32]* @global, [0 x i32]* undef, [0 x i32]* undef, [0 x i32]* undef>, <4 x i64> <i64 0, i64 undef, i64 undef, i64 undef>, <4 x i64> [[ARG:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <4 x i32*> [[TMP]], i64 0
+; CHECK-NEXT:    ret i32* [[TMP2]]
+;
+bb:
+  %tmp = getelementptr inbounds [0 x i32], <4 x [0 x i32]*> <[0 x i32]* @global, [0 x i32]* @global, [0 x i32]* @global, [0 x i32]* @global>, <4 x i64> zeroinitializer, <4 x i64> %arg
+  %tmp2 = extractelement <4 x i32*> %tmp, i64 0
+  ret i32* %tmp2
+}