[X86] combineVectorHADDSUB - remove the broken HOP(x,x) merging code (PR51974)

This intention of this code turns out to be superfluous as we can handle this with shuffle combining, and it has a critical flaw in that it doesn't check for dependencies.

Fixes PR51974

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -45014,32 +45014,9 @@ static SDValue combineVectorHADDSUB(SDNode *N, SelectionDAG &DAG,
          "Unexpected horizontal add/sub opcode");
 
   if (!shouldUseHorizontalOp(true, DAG, Subtarget)) {
-    // For slow-hop targets, if we have a hop with a single op, see if we already
-    // have another user that we can reuse and shuffle the result.
     MVT VT = N->getSimpleValueType(0);
     SDValue LHS = N->getOperand(0);
     SDValue RHS = N->getOperand(1);
-    if (VT.is128BitVector() && LHS == RHS) {
-      for (SDNode *User : LHS->uses()) {
-        if (User != N && User->getOpcode() == N->getOpcode()) {
-          MVT ShufVT = VT.isFloatingPoint() ? MVT::v4f32 : MVT::v4i32;
-          if (User->getOperand(0) == LHS && !User->getOperand(1).isUndef()) {
-            return DAG.getBitcast(
-                VT,
-                DAG.getVectorShuffle(ShufVT, SDLoc(N),
-                                     DAG.getBitcast(ShufVT, SDValue(User, 0)),
-                                     DAG.getUNDEF(ShufVT), {0, 1, 0, 1}));
-          }
-          if (User->getOperand(1) == LHS && !User->getOperand(0).isUndef()) {
-            return DAG.getBitcast(
-                VT,
-                DAG.getVectorShuffle(ShufVT, SDLoc(N),
-                                     DAG.getBitcast(ShufVT, SDValue(User, 0)),
-                                     DAG.getUNDEF(ShufVT), {2, 3, 2, 3}));
-          }
-        }
-      }
-    }
 
     // HOP(HOP'(X,X),HOP'(Y,Y)) -> HOP(PERMUTE(HOP'(X,Y)),PERMUTE(HOP'(X,Y)).
     if (LHS != RHS && LHS.getOpcode() == N->getOpcode() &&

llvm/test/CodeGen/X86/horizontal-shuffle-2.ll

@@ -171,6 +171,25 @@ define <4 x float> @test_unpacklo_hadd_v4f32_unary(<4 x float> %0) {
   ret <4 x float> %3
 }
 
+define <8 x i16> @PR51974(<8 x i16> %a0) {
+; SSE-LABEL: PR51974:
+; SSE:       ## %bb.0:
+; SSE-NEXT:    movdqa %xmm0, %xmm1
+; SSE-NEXT:    phaddw %xmm0, %xmm1
+; SSE-NEXT:    phaddw %xmm0, %xmm1
+; SSE-NEXT:    movdqa %xmm1, %xmm0
+; SSE-NEXT:    ret{{[l|q]}}
+;
+; AVX-LABEL: PR51974:
+; AVX:       ## %bb.0:
+; AVX-NEXT:    vphaddw %xmm0, %xmm0, %xmm1
+; AVX-NEXT:    vphaddw %xmm0, %xmm1, %xmm0
+; AVX-NEXT:    ret{{[l|q]}}
+  %r0 = call <8 x i16> @llvm.x86.ssse3.phadd.w.128(<8 x i16> %a0, <8 x i16> %a0)
+  %r1 = call <8 x i16> @llvm.x86.ssse3.phadd.w.128(<8 x i16> %r0, <8 x i16> %a0)
+  ret <8 x i16> %r1
+}
+
 declare <4 x float> @llvm.x86.sse3.hadd.ps(<4 x float>, <4 x float>)
 declare <4 x float> @llvm.x86.sse3.hsub.ps(<4 x float>, <4 x float>)
 declare <2 x double> @llvm.x86.sse3.hadd.pd(<2 x double>, <2 x double>)