[PATCH] D99481: [InstCombine] Fix miscompile on GEP+load to icmp fold (PR45210)

[Hyeongyu Kim via Phabricator via llvm-commits]

hyeongyukim updated this revision to Diff 352330.
hyeongyukim added a comment.

Rebase


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D99481/new/

https://reviews.llvm.org/D99481

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/load-cmp.ll


Index: llvm/test/Transforms/InstCombine/load-cmp.ll
===================================================================
--- llvm/test/Transforms/InstCombine/load-cmp.ll
+++ llvm/test/Transforms/InstCombine/load-cmp.ll
@@ -293,8 +293,9 @@
 }
 
 define i1 @test10_struct_arr_noinbounds_i16(i16 %x) {
-; CHECK-LABEL: @test10_struct_arr_noinbounds_i16(
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i16 [[X:%.*]], 1
+; 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
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -279,9 +279,28 @@
       Idx = Builder.CreateTrunc(Idx, IntPtrTy);
   }
 
+  // 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.
+  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());
@@ -302,6 +321,7 @@
   // 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());
@@ -323,6 +343,7 @@
   // 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) {
@@ -338,6 +359,7 @@
   // 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);
@@ -364,6 +386,7 @@
       Ty = DL.getSmallestLegalIntType(Init->getContext(), ArrayElementCount);
 
     if (Ty) {
+      Idx = MaskIdx(Idx);
       Value *V = Builder.CreateIntCast(Idx, Ty, false);
       V = Builder.CreateLShr(ConstantInt::get(Ty, MagicBitvector), V);
       V = Builder.CreateAnd(ConstantInt::get(Ty, 1), V);


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