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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278779 91177308-0d34-0410-b5e6-96231b3b80d8

Author: majnemer

lib/Analysis/InstructionSimplify.cpp

@@ -3597,7 +3597,6 @@ 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()); })) {
@@ -3609,11 +3608,18 @@ 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);
+      }
     }
   }
 

test/Transforms/InstSimplify/compare.ll

@@ -218,19 +218,6 @@ define i1 @gep17() {
 ; CHECK-NEXT: ret i1 true
 }
 
-define i32 @gep18() {
-; CHECK-LABEL: @gep18(
-  %alloca = alloca i32, align 4                                     ; alloca + 0
-  %gep = getelementptr inbounds i32, i32* %alloca, i32 1            ; alloca + 4
-  %bc = bitcast i32* %gep to [4 x i8]*                              ; alloca + 4
-  %pti = ptrtoint i32* %alloca to i32                               ; alloca
-  %sub = sub i32 0, %pti                                            ; -alloca
-  %add = getelementptr [4 x i8], [4 x i8]* %bc, i32 0, i32 %sub     ; alloca + 4 - alloca == 4
-  %add_to_int = ptrtoint i8* %add to i32                            ; 4
-  ret i32 %add_to_int                                               ; 4
-; CHECK-NEXT: ret i32 4
-}
-
 define i1 @zext(i32 %x) {
 ; CHECK-LABEL: @zext(
   %e1 = zext i32 %x to i64