[DAG][X86] Fold mgather/mscatter/etc with splat index (llvm#65980)

A splat index means the operation is reading from (writing to) the same
memory location. Generally, zero is the cheapest value to splat. As
such, we'd prefer to add the splatted value to the base, and use a
constant zero as the index operand.

Author: preames

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11637,8 +11637,6 @@ static SDValue ConvertSelectToConcatVector(SDNode *N, SelectionDAG &DAG) {
 
 bool refineUniformBase(SDValue &BasePtr, SDValue &Index, bool IndexIsScaled,
                        SelectionDAG &DAG, const SDLoc &DL) {
-  if (Index.getOpcode() != ISD::ADD)
-    return false;
 
   // Only perform the transformation when existing operands can be reused.
   if (IndexIsScaled)
@@ -11648,6 +11646,21 @@ bool refineUniformBase(SDValue &BasePtr, SDValue &Index, bool IndexIsScaled,
     return false;
 
   EVT VT = BasePtr.getValueType();
+
+  if (SDValue SplatVal = DAG.getSplatValue(Index);
+      SplatVal && !isNullConstant(SplatVal) &&
+      SplatVal.getValueType() == VT) {
+    if (isNullConstant(BasePtr))
+      BasePtr = SplatVal;
+    else
+      BasePtr = DAG.getNode(ISD::ADD, DL, VT, BasePtr, SplatVal);
+    Index = DAG.getSplat(Index.getValueType(), DL, DAG.getConstant(0, DL, VT));
+    return true;
+  }
+
+  if (Index.getOpcode() != ISD::ADD)
+    return false;
+
   if (SDValue SplatVal = DAG.getSplatValue(Index.getOperand(0));
       SplatVal && SplatVal.getValueType() == VT) {
     if (isNullConstant(BasePtr))

llvm/test/CodeGen/X86/masked_gather.ll

@@ -1747,29 +1747,27 @@ define <8 x i32> @gather_v8i32_v8i32(<8 x i32> %trigger) {
 ; AVX512F-NEXT:    vptestnmd %zmm0, %zmm0, %k0
 ; AVX512F-NEXT:    kshiftlw $8, %k0, %k0
 ; AVX512F-NEXT:    kshiftrw $8, %k0, %k1
-; AVX512F-NEXT:    vpbroadcastd {{.*#+}} zmm0 = [12,12,12,12,12,12,12,12,12,12,12,12,12,12,12,12]
+; AVX512F-NEXT:    vpxor %xmm0, %xmm0, %xmm0
 ; AVX512F-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX512F-NEXT:    vpxor %xmm2, %xmm2, %xmm2
 ; AVX512F-NEXT:    kmovw %k1, %k2
-; AVX512F-NEXT:    vpgatherdd c(,%zmm0), %zmm2 {%k2}
-; AVX512F-NEXT:    vpbroadcastd {{.*#+}} zmm0 = [28,28,28,28,28,28,28,28,28,28,28,28,28,28,28,28]
-; AVX512F-NEXT:    vpgatherdd c(,%zmm0), %zmm1 {%k1}
-; AVX512F-NEXT:    vpaddd %ymm1, %ymm2, %ymm0
-; AVX512F-NEXT:    vpaddd %ymm1, %ymm0, %ymm0
+; AVX512F-NEXT:    vpgatherdd c+12(,%zmm0), %zmm1 {%k2}
+; AVX512F-NEXT:    vpxor %xmm2, %xmm2, %xmm2
+; AVX512F-NEXT:    vpgatherdd c+28(,%zmm0), %zmm2 {%k1}
+; AVX512F-NEXT:    vpaddd %ymm2, %ymm1, %ymm0
+; AVX512F-NEXT:    vpaddd %ymm2, %ymm0, %ymm0
 ; AVX512F-NEXT:    retq
 ;
 ; AVX512VL-LABEL: gather_v8i32_v8i32:
 ; AVX512VL:       # %bb.0:
 ; AVX512VL-NEXT:    vptestnmd %ymm0, %ymm0, %k1
 ; AVX512VL-NEXT:    vpxor %xmm0, %xmm0, %xmm0
-; AVX512VL-NEXT:    vpbroadcastd {{.*#+}} ymm1 = [12,12,12,12,12,12,12,12]
 ; AVX512VL-NEXT:    kmovw %k1, %k2
+; AVX512VL-NEXT:    vpxor %xmm1, %xmm1, %xmm1
+; AVX512VL-NEXT:    vpgatherdd c+12(,%ymm0), %ymm1 {%k2}
 ; AVX512VL-NEXT:    vpxor %xmm2, %xmm2, %xmm2
-; AVX512VL-NEXT:    vpgatherdd c(,%ymm1), %ymm2 {%k2}
-; AVX512VL-NEXT:    vpbroadcastd {{.*#+}} ymm1 = [28,28,28,28,28,28,28,28]
-; AVX512VL-NEXT:    vpgatherdd c(,%ymm1), %ymm0 {%k1}
-; AVX512VL-NEXT:    vpaddd %ymm0, %ymm2, %ymm1
-; AVX512VL-NEXT:    vpaddd %ymm0, %ymm1, %ymm0
+; AVX512VL-NEXT:    vpgatherdd c+28(,%ymm0), %ymm2 {%k1}
+; AVX512VL-NEXT:    vpaddd %ymm2, %ymm1, %ymm0
+; AVX512VL-NEXT:    vpaddd %ymm2, %ymm0, %ymm0
 ; AVX512VL-NEXT:    retq
   %1 = icmp eq <8 x i32> %trigger, zeroinitializer
   %2 = call <8 x i32> @llvm.masked.gather.v8i32.v8p0(<8 x ptr> getelementptr (%struct.a, <8 x ptr> <ptr @c, ptr @c, ptr @c, ptr @c, ptr @c, ptr @c, ptr @c, ptr @c>, <8 x i64> zeroinitializer, i32 0, <8 x i64> <i64 3, i64 3, i64 3, i64 3, i64 3, i64 3, i64 3, i64 3>), i32 4, <8 x i1> %1, <8 x i32> undef)