[PATCH] D55274: [DagCombiner][X86] Simplify a ConcatVectors of a scalar_to_vector with undef.

[Andrea Di Biagio via Phabricator via llvm-commits]

andreadb updated this revision to Diff 176858.
andreadb added a comment.

Patch updated.

Initially, I wanted to move thal combine logic into a separate function.
However, the presence of an early return in the original combine logic ended up causing problems once the code was moved to a separate function.
I was originally under the impression that the change was safe. However - after running more tests - if we don't bail out immediately from the visitCONCAT_VECTOR function, we may end up introducing odd regressions due to the presence of illegal build_vectors dag nodes (when testing for an SSE1 i686 target) (since we potentially trigger other combine rules which may in turn introduce more problematic illegal vector types).

So, I opted for this simpler (and safer) approach.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55274/new/

https://reviews.llvm.org/D55274

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/combine-concatvectors.ll


Index: test/CodeGen/X86/combine-concatvectors.ll
===================================================================
--- test/CodeGen/X86/combine-concatvectors.ll
+++ test/CodeGen/X86/combine-concatvectors.ll
@@ -5,8 +5,6 @@
 ; CHECK-LABEL: PR32957:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    vmovsd {{.*#+}} xmm0 = mem[0],zero
-; CHECK-NEXT:    vxorps %xmm1, %xmm1, %xmm1
-; CHECK-NEXT:    vblendps {{.*#+}} xmm0 = xmm0[0,1],xmm1[2,3]
 ; CHECK-NEXT:    vmovaps %ymm0, (%rsi)
 ; CHECK-NEXT:    vzeroupper
 ; CHECK-NEXT:    retq
Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16502,11 +16502,19 @@
     SDValue In = N->getOperand(0);
     assert(In.getValueType().isVector() && "Must concat vectors");
 
-    // Transform: concat_vectors(scalar, undef) -> scalar_to_vector(sclr).
-    if (In->getOpcode() == ISD::BITCAST &&
-        !In->getOperand(0).getValueType().isVector()) {
-      SDValue Scalar = In->getOperand(0);
+    SDValue Scalar = peekThroughOneUseBitcasts(In);
 
+    // concat_vectors(scalar_to_vector(scalar), undef) ->
+    //     scalar_to_vector(scalar)
+    if (!LegalOperations && Scalar.getOpcode() == ISD::SCALAR_TO_VECTOR &&
+         Scalar.hasOneUse()) {
+      EVT SVT = Scalar.getValueType().getVectorElementType();
+      if (SVT == Scalar.getOperand(0).getValueType())
+        Scalar = Scalar.getOperand(0);
+    }
+
+    // concat_vectors(scalar, undef) -> scalar_to_vector(scalar)
+    if (!Scalar.getValueType().isVector()) {
       // If the bitcast type isn't legal, it might be a trunc of a legal type;
       // look through the trunc so we can still do the transform:
       //   concat_vectors(trunc(scalar), undef) -> scalar_to_vector(scalar)


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