[InstSimplify] Constant fold icmp of gep

InstSimplify can fold icmps of gep where the base pointers are the
same and the offsets are constant. It does so by constructing a
constant expression icmp and assumes that it gets folded -- but
this doesn't actually happen, because GEP expressions can usually
only be folded by the target-dependent constant folding layer.
As such, we need to explicitly invoke it here.

Differential Revision: https://reviews.llvm.org/D75407

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index ac79031..45fd623 100644 (file)
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3481,7 +3481,8 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
         SmallVector<Value *, 4> IndicesRHS(GRHS->idx_begin(), GRHS->idx_end());
         Constant *NewRHS = ConstantExpr::getGetElementPtr(
             GLHS->getSourceElementType(), Null, IndicesRHS);
-        return ConstantExpr::getICmp(Pred, NewLHS, NewRHS);
+        Constant *NewICmp = ConstantExpr::getICmp(Pred, NewLHS, NewRHS);
+        return ConstantFoldConstant(NewICmp, Q.DL);
       }
     }
   }

diff --git a/llvm/test/Transforms/InstSimplify/compare.ll b/llvm/test/Transforms/InstSimplify/compare.ll
index c4e791f..3dfdaf2 100644 (file)
--- a/llvm/test/Transforms/InstSimplify/compare.ll
+++ b/llvm/test/Transforms/InstSimplify/compare.ll
@@ -301,7 +301,7 @@ define i1 @gep17() {
 
 define i1 @gep_same_base_constant_indices(i8* %a) {
 ; CHECK-LABEL: @gep_same_base_constant_indices(
-; CHECK-NEXT:    ret i1 icmp slt (i8* getelementptr (i8, i8* null, i64 1), i8* getelementptr (i8, i8* null, i64 10))
+; CHECK-NEXT:    ret i1 true
 ;
   %arrayidx1 = getelementptr inbounds i8, i8* %a, i64 1
   %arrayidx2 = getelementptr inbounds i8, i8* %a, i64 10