[DAGCombiner] Require same type of splat & element for build_vector #88284

fzou1 · 2024-04-10T15:36:47Z

Only allow to change build_vector to concat_vector when the splat type and vector element type is same. It's to fix assertion of failing to bitcast types of different sizes.

…ector Only allow to change build_vector to concat_vector when the splat type and vector element type is same. It's to fix assertion of failing to bitcast types of different sizes.

github-actions · 2024-04-10T15:37:04Z

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llvmbot · 2024-04-10T15:37:32Z

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-llvm-selectiondag

Author: Feng Zou (fzou1)

Changes

Only allow to change build_vector to concat_vector when the splat type and vector element type is same. It's to fix assertion of failing to bitcast types of different sizes.

Full diff: https://github.com/llvm/llvm-project/pull/88284.diff

2 Files Affected:

(modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+16-11)
(added) llvm/test/CodeGen/X86/buildvec-bitcast.ll (+24)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 8fe074666a3dc9..ee90ca8eaa7d7f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -23429,17 +23429,22 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
   // TODO: Maybe this is useful for non-splat too?
   if (!LegalOperations) {
     if (SDValue Splat = cast<BuildVectorSDNode>(N)->getSplatValue()) {
-      Splat = peekThroughBitcasts(Splat);
-      EVT SrcVT = Splat.getValueType();
-      if (SrcVT.isVector()) {
-        unsigned NumElts = N->getNumOperands() * SrcVT.getVectorNumElements();
-        EVT NewVT = EVT::getVectorVT(*DAG.getContext(),
-                                     SrcVT.getVectorElementType(), NumElts);
-        if (!LegalTypes || TLI.isTypeLegal(NewVT)) {
-          SmallVector<SDValue, 8> Ops(N->getNumOperands(), Splat);
-          SDValue Concat = DAG.getNode(ISD::CONCAT_VECTORS, SDLoc(N),
-                                       NewVT, Ops);
-          return DAG.getBitcast(VT, Concat);
+      EVT SplatVT = Splat.getValueType();
+      // Only change build_vector to a concat_vector if the splat value type is
+      // same as the vector element type.
+      if (SplatVT == VT.getVectorElementType()) {
+        Splat = peekThroughBitcasts(Splat);
+        EVT SrcVT = Splat.getValueType();
+        if (SrcVT.isVector()) {
+          unsigned NumElts = N->getNumOperands() * SrcVT.getVectorNumElements();
+          EVT NewVT = EVT::getVectorVT(*DAG.getContext(),
+                                       SrcVT.getVectorElementType(), NumElts);
+          if (!LegalTypes || TLI.isTypeLegal(NewVT)) {
+            SmallVector<SDValue, 8> Ops(N->getNumOperands(), Splat);
+            SDValue Concat =
+                DAG.getNode(ISD::CONCAT_VECTORS, SDLoc(N), NewVT, Ops);
+            return DAG.getBitcast(VT, Concat);
+          }
         }
       }
     }
diff --git a/llvm/test/CodeGen/X86/buildvec-bitcast.ll b/llvm/test/CodeGen/X86/buildvec-bitcast.ll
new file mode 100644
index 00000000000000..9f7167f2946bf9
--- /dev/null
+++ b/llvm/test/CodeGen/X86/buildvec-bitcast.ll
@@ -0,0 +1,24 @@
+; RUN: llc < %s -mtriple=x86_64 -mattr=avx512bw | FileCheck %s
+
+; Verify that the DAGCombiner doesn't change build_vector to concat_vectors if
+; the vector element type is different than splat type. The example here:
+;   v8i1 = build_vector (i8 (bitcast (v8i1 X))), ..., (i8 (bitcast (v8i1 X))))
+
+; CHECK:      foo:
+; CHECK:      # %bb.0: # %entry
+; CHECK-NEXT: retq
+
+define void @foo(<8 x i1> %mask.i1) {
+entry:
+  %0 = and <8 x i1> %mask.i1, <i1 true, i1 false, i1 false, i1 false, i1 false, i1 false, i1 false, i1 false>
+  %1 = bitcast <8 x i1> %0 to i8
+  %2 = icmp ne i8 %1, 0
+  %insert54 = insertelement <8 x i1> zeroinitializer, i1 %2, i64 0
+  %splat55 = shufflevector <8 x i1> %insert54, <8 x i1> zeroinitializer, <8 x i32> zeroinitializer
+  %3 = and <8 x i1> %0, %splat55
+  br label %end
+
+end:                           ; preds = %entry
+  %4 = select <8 x i1> %3, <8 x i1> zeroinitializer, <8 x i1> zeroinitializer
+  ret void
+}

RKSimon · 2024-04-10T15:52:23Z

llvm/test/CodeGen/X86/buildvec-bitcast.ll

@@ -0,0 +1,24 @@
+; RUN: llc < %s -mtriple=x86_64 -mattr=avx512bw | FileCheck %s


Regenerate with utils/update_llc_test_checks.py

Done. Thanks.

RKSimon · 2024-04-10T17:25:14Z

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

+      EVT SplatVT = Splat.getValueType();
+      // Only change build_vector to a concat_vector if the splat value type is
+      // same as the vector element type.
+      if (SplatVT == VT.getVectorElementType()) {


SplatVT is only used once - move into if (Splat.getValueType() == VT.getVectorElementType())

KanRobert · 2024-04-11T02:47:57Z

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

-          SDValue Concat = DAG.getNode(ISD::CONCAT_VECTORS, SDLoc(N),
-                                       NewVT, Ops);
-          return DAG.getBitcast(VT, Concat);
+      // Only change build_vector to a concat_vector if the splat value type is


Minor suggestion.

SDValue Splat = cast<BuildVectorSDNode>(N)->getSplatValue(); if (Splat && Splat.getValueType() == VT.getVectorElementType())

to eliminate nested braces.

Right. Updated.

KanRobert · 2024-04-11T02:52:43Z

llvm/test/CodeGen/X86/buildvec-bitcast.ll

+; the vector element type is different than splat type. The example here:
+;   v8i1 = build_vector (i8 (bitcast (v8i1 X))), ..., (i8 (bitcast (v8i1 X))))
+
+define void @foo(<8 x i1> %mask.i1) {


Should we change the function prototype to <8 x i1> @foo (<8 x i1> %mask.i1) and return %4 at line 23?

In the current variant, the function has no return value and has no side effects. I doubt it may be optimized to

define void @foo(<8 x i1> %mask.i1) { entry: return void }

before ISEL in the future, then it checks nothing for DAG.

phoebewang · 2024-04-11T03:28:06Z

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -23429,17 +23429,21 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
  // TODO: Maybe this is useful for non-splat too?
  if (!LegalOperations) {
    if (SDValue Splat = cast<BuildVectorSDNode>(N)->getSplatValue()) {


The old code seems problematic, if we use cast, we don't need to use if.

It's correct, I think. cast is used for casting N to BuildeVectorSDNode type and if is to check the return value of getSplatValue().

Sorry, my mistake. The check is for getSplatValue() not cast. We can't change it to dyn_cast.

KanRobert

LGTM

RKSimon

LGTM

fzou1 · 2024-04-12T01:33:26Z

Thanks. Is it okay to merge? If so, can someone help merge this?

KanRobert · 2024-04-12T02:10:02Z

Thanks. Is it okay to merge? If so, can someone help merge this?

Waiting for result of pre-commit tests.

github-actions · 2024-04-12T02:14:14Z

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[DAGCombiner] Require same type of splat & vector element for build_v…

fde2538

…ector Only allow to change build_vector to concat_vector when the splat type and vector element type is same. It's to fix assertion of failing to bitcast types of different sizes.

llvmbot added backend:X86 llvm:SelectionDAG SelectionDAGISel as well labels Apr 10, 2024

RKSimon reviewed Apr 10, 2024

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Address comments

e6a4c76

KanRobert self-requested a review April 11, 2024 02:44

KanRobert reviewed Apr 11, 2024

View reviewed changes

phoebewang reviewed Apr 11, 2024

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Update test and code per comments.

e939d87

KanRobert approved these changes Apr 11, 2024

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RKSimon approved these changes Apr 11, 2024

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KanRobert merged commit 6b6f272 into llvm:main Apr 12, 2024
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fzou1 deleted the fzou1-dagcombiner branch April 12, 2024 02:23

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[DAGCombiner] Require same type of splat & element for build_vector #88284

[DAGCombiner] Require same type of splat & element for build_vector #88284

fzou1 commented Apr 10, 2024

github-actions bot commented Apr 10, 2024

llvmbot commented Apr 10, 2024 •

edited

RKSimon Apr 10, 2024

fzou1 Apr 10, 2024

RKSimon Apr 10, 2024

fzou1 Apr 10, 2024

KanRobert Apr 11, 2024

fzou1 Apr 11, 2024

KanRobert Apr 11, 2024

fzou1 Apr 11, 2024

phoebewang Apr 11, 2024

fzou1 Apr 11, 2024 •

edited

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edited

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fzou1 commented Apr 12, 2024

KanRobert commented Apr 12, 2024

github-actions bot commented Apr 12, 2024

		@@ -0,0 +1,24 @@
		; RUN: llc < %s -mtriple=x86_64 -mattr=avx512bw \| FileCheck %s

[DAGCombiner] Require same type of splat & element for build_vector #88284

[DAGCombiner] Require same type of splat & element for build_vector #88284

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