[llvm] [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (PR #148593)
Adar Dagan via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 16 04:12:44 PDT 2025
https://github.com/Adar-Dagan updated https://github.com/llvm/llvm-project/pull/148593
>From 6fa93582e0dd0373eef3ba4717aa02a3641a3860 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 1/2] Expand move trunc through shuffle splat
---
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp | 12 +++++++++---
.../Transforms/InstCombine/trunc-inseltpoison.ll | 4 ++--
llvm/test/Transforms/InstCombine/trunc.ll | 4 ++--
3 files changed, 13 insertions(+), 7 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 033ef8be700eb..16d518dbcec88 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -708,11 +708,17 @@ 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());
- return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
+ auto *const NewTruncTy = VectorType::get(
+ Trunc.getType()->getScalarType(),
+ cast<VectorType>(Shuf->getOperand(0)->getType())->getElementCount());
+ Value *NarrowOp =
+ Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy, Trunc.getName());
+ return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask(),
+ Shuf->getName());
}
return nullptr;
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
>From eb8cac36aa5c870dd19a85e4949e0f700bdc050a Mon Sep 17 00:00:00 2001
From: Adar Dagan <adar.dagan at mobileye.com>
Date: Tue, 15 Jul 2025 15:39:49 +0300
Subject: [PATCH 2/2] Answer comments
---
.../Transforms/InstCombine/InstCombineCasts.cpp | 16 +++++++---------
llvm/test/Transforms/InstCombine/trunc.ll | 13 +++++++++++++
2 files changed, 20 insertions(+), 9 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 16d518dbcec88..f0b2c1048f431 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -708,17 +708,15 @@ 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()->getScalarType() ==
- Shuf->getOperand(0)->getType()->getScalarType()) {
+ 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
- auto *const NewTruncTy = VectorType::get(
- Trunc.getType()->getScalarType(),
- cast<VectorType>(Shuf->getOperand(0)->getType())->getElementCount());
- Value *NarrowOp =
- Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy, Trunc.getName());
- return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask(),
- Shuf->getName());
+ Type *const NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
+ Trunc.getType()->getScalarType());
+ Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy);
+ return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
}
return nullptr;
diff --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index 8f727e365e88e..dfe9d941f840c 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -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>
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