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

[Adar Dagan via llvm-commits]

https://github.com/Adar-Dagan created https://github.com/llvm/llvm-project/pull/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.

>From e887b3d90d0fd43166e54e095b8f2c5b8c8cdf3f Mon Sep 17 00:00:00 2001
From: Adar Dagan <adar.dagan at mobileye.com>
Date: Mon, 14 Jul 2025 11:00:53 +0300
Subject: [PATCH] Expand move trunc through shuffle splat

---
 llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp   | 8 ++++++--
 llvm/test/Transforms/InstCombine/trunc-inseltpoison.ll | 4 ++--
 llvm/test/Transforms/InstCombine/trunc.ll              | 4 ++--
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 033ef8be700eb..8a98fd3235915 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()) {
+      Shuf->getType()->getScalarType() ==
+          Shuf->getOperand(0)->getType()->getScalarType()) {
     // 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());
+    auto *const NewTruncTy =
+        VectorType::get(Trunc.getType()->getScalarType(),
+                        cast<VectorType>(Shuf->getOperand(0)->getType())->getElementCount());
+    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..8f727e365e88e 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