[llvm] 1afb42b - [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (#148593)

[via llvm-commits]

Author: Adar Dagan
Date: 2025-07-28T13:00:37+02:00
New Revision: 1afb42bc10efcf033c1e8b0fda90d1e2956002fa

URL: https://github.com/llvm/llvm-project/commit/1afb42bc10efcf033c1e8b0fda90d1e2956002fa
DIFF: https://github.com/llvm/llvm-project/commit/1afb42bc10efcf033c1e8b0fda90d1e2956002fa.diff

LOG: [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (#148593)

shrinkSplatShuffle in InstCombine would only move truncs up through
shuffles if those shuffles inputs had the exact same type as their
output, this PR weakens this constraint to only requiring that the
scalar type of the input and output match.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
    llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll
    llvm/test/Transforms/InstCombine/trunc.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 033ef8be700eb..a43a6ee1f58b0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -708,10 +708,14 @@ static Instruction *shrinkSplatShuffle(TruncInst &Trunc,
   auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
   if (Shuf && Shuf->hasOneUse() && match(Shuf->getOperand(1), m_Undef()) &&
       all_equal(Shuf->getShuffleMask()) &&
-      Shuf->getType() == Shuf->getOperand(0)->getType()) {
+      ElementCount::isKnownGE(Shuf->getType()->getElementCount(),
+                              cast<VectorType>(Shuf->getOperand(0)->getType())
+                                  ->getElementCount())) {
     // trunc (shuf X, Undef, SplatMask) --> shuf (trunc X), Poison, SplatMask
     // trunc (shuf X, Poison, SplatMask) --> shuf (trunc X), Poison, SplatMask
-    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), Trunc.getType());
+    Type *NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
+        Trunc.getType()->getScalarType());
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy);
     return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
   }
 

diff  --git a/llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll b/llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll
index 33fa2c375f1ec..f83352c94ad89 100644
--- a/llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll
@@ -959,8 +959,8 @@ define <3 x i31> @wide_splat3(<3 x i33> %x) {
 
 define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
 ; CHECK-LABEL: @wide_lengthening_splat(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i16> [[V:%.*]], <4 x i16> poison, <8 x i32> zeroinitializer
-; CHECK-NEXT:    [[TR:%.*]] = trunc <8 x i16> [[SHUF]] to <8 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i16> [[V:%.*]] to <4 x i8>
+; CHECK-NEXT:    [[TR:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> poison, <8 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <8 x i8> [[TR]]
 ;
   %shuf = shufflevector <4 x i16> %v, <4 x i16> %v, <8 x i32> zeroinitializer

diff  --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index a85ce716fbdfa..dfe9d941f840c 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -960,8 +960,8 @@ define <3 x i31> @wide_splat3(<3 x i33> %x) {
 
 define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
 ; CHECK-LABEL: @wide_lengthening_splat(
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <4 x i16> [[V:%.*]], <4 x i16> poison, <8 x i32> zeroinitializer
-; CHECK-NEXT:    [[TR:%.*]] = trunc <8 x i16> [[SHUF]] to <8 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc <4 x i16> [[V:%.*]] to <4 x i8>
+; CHECK-NEXT:    [[TR:%.*]] = shufflevector <4 x i8> [[TMP1]], <4 x i8> poison, <8 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <8 x i8> [[TR]]
 ;
   %shuf = shufflevector <4 x i16> %v, <4 x i16> %v, <8 x i32> zeroinitializer
@@ -969,6 +969,19 @@ define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
   ret <8 x i8> %tr
 }
 
+; This is a negative test, we expect the trunc to remain after the shuffle as it
+; might not be beneficial to preform trunc on a wider type
+define <4 x i8> @wide_shortening_splat(<8 x i16> %v) {
+; CHECK-LABEL: @wide_shortening_splat(
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <8 x i16> [[V:%.*]], <8 x i16> poison, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[TR:%.*]] = trunc <4 x i16> [[SHUF]] to <4 x i8>
+; CHECK-NEXT:    ret <4 x i8> [[TR]]
+;
+  %shuf = shufflevector <8 x i16> %v, <8 x i16> %v, <4 x i32> zeroinitializer
+  %tr = trunc <4 x i16> %shuf to <4 x i8>
+  ret <4 x i8> %tr
+}
+
 define <2 x i8> @narrow_add_vec_constant(<2 x i32> %x) {
 ; CHECK-LABEL: @narrow_add_vec_constant(
 ; CHECK-NEXT:    [[TMP1:%.*]] = trunc <2 x i32> [[X:%.*]] to <2 x i8>