[PATCH] D30123: [InstCombine] shrink truncated splat shuffle

[Sanjay Patel via Phabricator via llvm-commits]

spatel updated this revision to Diff 89025.
spatel retitled this revision from "[InstCombine] shrink truncated shuffle with constant operand" to "[InstCombine] shrink truncated splat shuffle".
spatel edited the summary of this revision.
spatel added a comment.

Patch upated:

1. Limit the transform to splat shuffles. In this case, we canonicalize the vector to operand 0 (and operand 1 should be undef), so that eliminates the commuted pattern match.
2. Added test for splat with strange types for more coverage of the getSplatValue() implementation.
3. Added test with undef element in mask. Unlike the DAG's ShuffleVectorSDNode::isSplatMask(), Constant::getSplatValue() does not check for undef elements, so we miss this case currently. I'm not sure if this matters in practice currently because we're conservative about changing shuffle masks.


https://reviews.llvm.org/D30123

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/trunc.ll


Index: test/Transforms/InstCombine/trunc.ll
===================================================================
--- test/Transforms/InstCombine/trunc.ll
+++ test/Transforms/InstCombine/trunc.ll
@@ -478,28 +478,26 @@
   ret <4 x i8> %trunc
 }
 
-; FIXME:
 ; trunc (shuffle X, undef, SplatMask) --> shuffle (trunc X), undef, SplatMask
 
 define <4 x i8> @wide_splat1(<4 x i32> %x) {
 ; CHECK-LABEL: @wide_splat1(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> <i32 2, i32 2, i32 2, i32 2>
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc <4 x i32> [[SHUF]] to <4 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i32> %x to <4 x i8>
+; CHECK-NEXT:    [[TRUNC:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> undef, <4 x i32> <i32 2, i32 2, i32 2, i32 2>
 ; CHECK-NEXT:    ret <4 x i8> [[TRUNC]]
 ;
   %shuf = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> <i32 2, i32 2, i32 2, i32 2>
   %trunc = trunc <4 x i32> %shuf to <4 x i8>
   ret <4 x i8> %trunc
 }
 
-; FIXME:
 ; Test weird types.
 ; trunc (shuffle X, undef, SplatMask) --> shuffle (trunc X), undef, SplatMask
 
 define <3 x i31> @wide_splat2(<3 x i33> %x) {
 ; CHECK-LABEL: @wide_splat2(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <3 x i33> %x, <3 x i33> undef, <3 x i32> <i32 1, i32 1, i32 1>
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc <3 x i33> [[SHUF]] to <3 x i31>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <3 x i33> %x to <3 x i31>
+; CHECK-NEXT:    [[TRUNC:%.*]] = shufflevector <3 x i31> [[TMP1]], <3 x i31> undef, <3 x i32> <i32 1, i32 1, i32 1>
 ; CHECK-NEXT:    ret <3 x i31> [[TRUNC]]
 ;
   %shuf = shufflevector <3 x i33> %x, <3 x i33> undef, <3 x i32> <i32 1, i32 1, i32 1>
Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -463,6 +463,23 @@
   return BinaryOperator::Create(LogicOp->getOpcode(), NarrowOp0, NarrowC);
 }
 
+/// Try to narrow the width of a splat shuffle. This could be generalized to any
+/// shuffle with a constant operand, but we limit the transform to avoid
+/// creating a shuffle type that targets may not be able to lower effectively.
+static Instruction *shrinkSplatShuffle(TruncInst &Trunc,
+                                       InstCombiner::BuilderTy &Builder) {
+  auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
+  if (Shuf && Shuf->hasOneUse() && isa<UndefValue>(Shuf->getOperand(1)) &&
+      Shuf->getMask()->getSplatValue()) {
+    // trunc (shuf X, Undef, SplatMask) --> shuf (trunc X), Undef, SplatMask
+    Constant *NarrowUndef = UndefValue::get(Trunc.getType());
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), Trunc.getType());
+    return new ShuffleVectorInst(NarrowOp, NarrowUndef, Shuf->getMask());
+  }
+
+  return nullptr;
+}
+
 Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
@@ -554,6 +571,9 @@
   if (Instruction *I = shrinkBitwiseLogic(CI))
     return I;
 
+  if (Instruction *I = shrinkSplatShuffle(CI, *Builder))
+    return I;
+
   if (Src->hasOneUse() && isa<IntegerType>(SrcTy) &&
       shouldChangeType(SrcTy, DestTy)) {
     // Transform "trunc (shl X, cst)" -> "shl (trunc X), cst" so long as the


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