[InstSimplify] Fold gep (gep V, C), (xor V, -1) to C-1

llvm-svn: 278779

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 95273a78..3a1b272 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3645,7 +3645,6 @@ static Value *SimplifyGEPInst(Type *SrcTy, ArrayRef<Value *> Ops,
     }
   }
 
-  // gep (gep V, C), (sub 0, V) -> C
   if (Q.DL.getTypeAllocSize(LastType) == 1 &&
       all_of(Ops.slice(1).drop_back(1),
              [](Value *Idx) { return match(Idx, m_Zero()); })) {
@@ -3657,11 +3656,18 @@ static Value *SimplifyGEPInst(Type *SrcTy, ArrayRef<Value *> Ops,
           Ops[0]->stripAndAccumulateInBoundsConstantOffsets(Q.DL,
                                                             BasePtrOffset);
 
+      // gep (gep V, C), (sub 0, V) -> C
       if (match(Ops.back(),
                 m_Sub(m_Zero(), m_PtrToInt(m_Specific(StrippedBasePtr))))) {
         auto *CI = ConstantInt::get(GEPTy->getContext(), BasePtrOffset);
         return ConstantExpr::getIntToPtr(CI, GEPTy);
       }
+      // gep (gep V, C), (xor V, -1) -> C-1
+      if (match(Ops.back(),
+                m_Xor(m_PtrToInt(m_Specific(StrippedBasePtr)), m_AllOnes()))) {
+        auto *CI = ConstantInt::get(GEPTy->getContext(), BasePtrOffset - 1);
+        return ConstantExpr::getIntToPtr(CI, GEPTy);
+      }
     }
   }