Modified to avoid creating an instruction that causes an infinite loop.

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

InstCombinerImpl::foldCmpLoadFromIndexedGlobal(GetElementPtrInst *GEP,
   if (!GEP->isInBounds()) {
     // If the index is larger than the pointer size of the target, truncate the
     // index down like the GEP would do implicitly.  We don't have to do this
     // for an inbounds GEP because the index can't be out of range.
     Type *IntPtrTy = DL.getIntPtrType(GEP->getType());
     unsigned PtrSize = IntPtrTy->getIntegerBitWidth();
     if (Idx->getType()->getPrimitiveSizeInBits().getFixedSize() > PtrSize)
       Idx = Builder.CreateTrunc(Idx, IntPtrTy);
-
-    unsigned ElementSize =
-        DL.getTypeAllocSize(Init->getType()->getArrayElementType());
+  }
 
-    // If inbounds keyword is not present, Idx * ElementSize can overflow.
-    // Let's assume that ElementSize is 2 and the wanted value is at offset 0.
-    // Then, there are two possible values for Idx to match offset 0:
-    // 0x00..00, 0x80..00.
-    // Emitting 'icmp eq Idx, 0' isn't correct in this case because the
-    // comparison is false if Idx was 0x80..00.
-    // We need to erase the highest countTrailingZeros(ElementSize) bits of Idx.
-    if (countTrailingZeros(ElementSize) != 0) {
-      Value *Mask = ConstantInt::getSigned(Idx->getType(), -1);
+  unsigned ElementSize =
+      DL.getTypeAllocSize(Init->getType()->getArrayElementType());
+  auto MaskIdx = [&](Value* Idx){
+    if (!GEP->isInBounds() && countTrailingZeros(ElementSize) != 0) {
+      Value *Mask = ConstantInt::get(Idx->getType(), -1);
       Mask = Builder.CreateLShr(Mask, countTrailingZeros(ElementSize));
       Idx = Builder.CreateAnd(Idx, Mask);
     }
-  }
+    return Idx;
+  };
 
   // If the comparison is only true for one or two elements, emit direct
   // comparisons.
   if (SecondTrueElement != Overdefined) {
+    Idx = MaskIdx(Idx);
     // None true -> false.
     if (FirstTrueElement == Undefined)
       return replaceInstUsesWith(ICI, Builder.getFalse());
 
     Value *FirstTrueIdx = ConstantInt::get(Idx->getType(), FirstTrueElement);
 
     // True for one element -> 'i == 47'.
     if (SecondTrueElement == Undefined)
       return new ICmpInst(ICmpInst::ICMP_EQ, Idx, FirstTrueIdx);
 
     // True for two elements -> 'i == 47 | i == 72'.
     Value *C1 = Builder.CreateICmpEQ(Idx, FirstTrueIdx);
     Value *SecondTrueIdx = ConstantInt::get(Idx->getType(), SecondTrueElement);
     Value *C2 = Builder.CreateICmpEQ(Idx, SecondTrueIdx);
     return BinaryOperator::CreateOr(C1, C2);
   }
 
   // If the comparison is only false for one or two elements, emit direct
   // comparisons.
   if (SecondFalseElement != Overdefined) {
+    Idx = MaskIdx(Idx);
     // None false -> true.
     if (FirstFalseElement == Undefined)
       return replaceInstUsesWith(ICI, Builder.getTrue());
 
     Value *FirstFalseIdx = ConstantInt::get(Idx->getType(), FirstFalseElement);
 
     // False for one element -> 'i != 47'.
     if (SecondFalseElement == Undefined)
       return new ICmpInst(ICmpInst::ICMP_NE, Idx, FirstFalseIdx);
 
     // False for two elements -> 'i != 47 & i != 72'.
     Value *C1 = Builder.CreateICmpNE(Idx, FirstFalseIdx);
     Value *SecondFalseIdx = ConstantInt::get(Idx->getType(),SecondFalseElement);
     Value *C2 = Builder.CreateICmpNE(Idx, SecondFalseIdx);
     return BinaryOperator::CreateAnd(C1, C2);
   }
 
   // If the comparison can be replaced with a range comparison for the elements
   // where it is true, emit the range check.
   if (TrueRangeEnd != Overdefined) {
     assert(TrueRangeEnd != FirstTrueElement && "Should emit single compare");
+    Idx = MaskIdx(Idx);
 
     // Generate (i-FirstTrue) <u (TrueRangeEnd-FirstTrue+1).
     if (FirstTrueElement) {
       Value *Offs = ConstantInt::get(Idx->getType(), -FirstTrueElement);
       Idx = Builder.CreateAdd(Idx, Offs);
     }
 
     Value *End = ConstantInt::get(Idx->getType(),
                                   TrueRangeEnd-FirstTrueElement+1);
     return new ICmpInst(ICmpInst::ICMP_ULT, Idx, End);
   }
 
   // False range check.
   if (FalseRangeEnd != Overdefined) {
     assert(FalseRangeEnd != FirstFalseElement && "Should emit single compare");
+    Idx = MaskIdx(Idx);
     // Generate (i-FirstFalse) >u (FalseRangeEnd-FirstFalse).
     if (FirstFalseElement) {
       Value *Offs = ConstantInt::get(Idx->getType(), -FirstFalseElement);
       Idx = Builder.CreateAdd(Idx, Offs);
     }
 
     Value *End = ConstantInt::get(Idx->getType(),
                                   FalseRangeEnd-FirstFalseElement);

llvm/test/Transforms/InstCombine/load-cmp.ll

 ;
   %p = getelementptr inbounds [4 x %Foo], [4 x %Foo]* @GStructArr, i64 0, i64 %x, i32 2
   %q = load i32, i32* %p
   %r = icmp eq i32 %q, 9
   ret i1 %r
 }
 
 define i1 @test10_struct_arr_noinbounds_i16(i16 %x) {
-; CHECK-LABEL: @test10_struct_arr_noinbounds_i16(
 ; CHECK-NEXT:    [[TMP1:%.*]] = sext i16 [[X:%.*]] to i32
 ; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP1]], 268435455
 ; CHECK-NEXT:    [[R:%.*]] = icmp ne i32 [[TMP2]], 1
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %p = getelementptr [4 x %Foo], [4 x %Foo]* @GStructArr, i32 0, i16 %x, i32 2
   %q = load i32, i32* %p
   %r = icmp eq i32 %q, 9