[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 350101.
hyeongyukim added a comment.

Avoid creating an meaningless instruction that causes an infinte loop.


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
@@ -300,9 +300,7 @@
 
 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:    [[R:%.*]] = icmp ne i16 [[X:%.*]], 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
@@ -288,8 +288,20 @@
     // 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);
+    // Unnecessary 'lshr' and 'and' can cause infinite loops, it is added only 
+    // when overflow can occur.
+    Value *IdxBeforeSext;
+    bool IsOverflows = false;
+    if (match(Idx, m_SExt(m_Value(IdxBeforeSext))))
+      IsOverflows =
+          IdxBeforeSext->getType()->getPrimitiveSizeInBits().getFixedSize() >=
+          PtrSize;
+    else
+      IsOverflows =
+          Idx->getType()->getPrimitiveSizeInBits().getFixedSize() >= PtrSize;
+
+    if (IsOverflows && countTrailingZeros(ElementSize) != 0) {
+      Value *Mask = ConstantInt::get(Idx->getType(), -1);
       Mask = Builder.CreateLShr(Mask, countTrailingZeros(ElementSize));
       Idx = Builder.CreateAnd(Idx, Mask);
     }


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