[DAGCombiner] Teach DAGCombiner that A-(-B) is A+B.

We already knew A+(-B) is A-B in visitAdd. This does the opposite for visitSub.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337502 91177308-0d34-0410-b5e6-96231b3b80d8

Author: topperc

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -2582,6 +2582,11 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
   if (isAllOnesConstantOrAllOnesSplatConstant(N0))
     return DAG.getNode(ISD::XOR, DL, VT, N1, N0);
 
+  // fold (A - (0-B)) -> A+B
+  if (N1.getOpcode() == ISD::SUB &&
+      isNullConstantOrNullSplatConstant(N1.getOperand(0)))
+    return DAG.getNode(ISD::ADD, DL, VT, N0, N1.getOperand(1));
+
   // fold A-(A-B) -> B
   if (N1.getOpcode() == ISD::SUB && N0 == N1.getOperand(0))
     return N1.getOperand(1);

test/CodeGen/X86/combine-srem.ll

@@ -248,11 +248,9 @@ define <4 x i32> @combine_vec_srem_by_pow2a_neg(<4 x i32> %x) {
 ; SSE-NEXT:    paddd %xmm0, %xmm1
 ; SSE-NEXT:    psrad $2, %xmm1
 ; SSE-NEXT:    pxor %xmm2, %xmm2
-; SSE-NEXT:    pxor %xmm3, %xmm3
-; SSE-NEXT:    psubd %xmm1, %xmm3
-; SSE-NEXT:    pslld $2, %xmm3
-; SSE-NEXT:    psubd %xmm3, %xmm2
-; SSE-NEXT:    psubd %xmm2, %xmm0
+; SSE-NEXT:    psubd %xmm1, %xmm2
+; SSE-NEXT:    pslld $2, %xmm2
+; SSE-NEXT:    paddd %xmm2, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: combine_vec_srem_by_pow2a_neg:
@@ -264,8 +262,7 @@ define <4 x i32> @combine_vec_srem_by_pow2a_neg(<4 x i32> %x) {
 ; AVX-NEXT:    vpxor %xmm2, %xmm2, %xmm2
 ; AVX-NEXT:    vpsubd %xmm1, %xmm2, %xmm1
 ; AVX-NEXT:    vpslld $2, %xmm1, %xmm1
-; AVX-NEXT:    vpsubd %xmm1, %xmm2, %xmm1
-; AVX-NEXT:    vpsubd %xmm1, %xmm0, %xmm0
+; AVX-NEXT:    vpaddd %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %1 = srem <4 x i32> %x, <i32 -4, i32 -4, i32 -4, i32 -4>
   ret <4 x i32> %1