Reduce x - y to -y when we know the 'x' part will get masked off anyways.

llvm-svn: 53271

llvm/lib/Transforms/Scalar/InstructionCombining.cpp

@@ -3461,6 +3461,17 @@ Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
         // ((A ^ N) - B) & AndRHS -> (A - B) & AndRHS iff N&AndRHS == 0
         if (Value *V = FoldLogicalPlusAnd(Op0LHS, Op0RHS, AndRHS, true, I))
           return BinaryOperator::CreateAnd(V, AndRHS);
+
+        // (A - N) & AndRHS -> -N & AndRHS where A & AndRHS == 0
+        if (Op0I->hasOneUse() && MaskedValueIsZero(Op0LHS, AndRHSMask)) {
+          ConstantInt *A = dyn_cast<ConstantInt>(Op0LHS);
+          if (!A || !A->isZero()) {
+            Instruction *NewNeg = BinaryOperator::CreateNeg(Op0RHS);
+            InsertNewInstBefore(NewNeg, I);
+            return BinaryOperator::CreateAnd(NewNeg, AndRHS);
+          }
+        }
+
         break;
       }
 
@@ -3780,7 +3791,7 @@ Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
           }
     }
   }
-      
+
   return Changed ? &I : 0;
 }
 

llvm/test/Transforms/InstCombine/2008-07-08-SubAnd.ll

@@ -0,0 +1,9 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep -v {i32 8}
+; PR2330
+
+define i32 @a(i32 %a) nounwind  {
+entry:
+	%tmp2 = sub i32 8, %a		; <i32> [#uses=1]
+	%tmp3 = and i32 %tmp2, 7		; <i32> [#uses=1]
+	ret i32 %tmp3
+}