[llvm] 6cff1c5 - [SLP]Look through truncation of shifted extracted sub-fields

[via llvm-commits]

Author: Alexey Bataev
Date: 2026-09-27T12:05:44-04:00
New Revision: 6cff1c56d1678e3d7eb70650d1fe138eac80c05e

URL: https://github.com/llvm/llvm-project/commit/6cff1c56d1678e3d7eb70650d1fe138eac80c05e
DIFF: https://github.com/llvm/llvm-project/commit/6cff1c56d1678e3d7eb70650d1fe138eac80c05e.diff

LOG: [SLP]Look through truncation of shifted extracted sub-fields

The masked sub-field of the shifted truncation of a wider scalar was
matched with the truncation as its source, while the other fields
matched the scalar itself, so the gather of the fields was rejected.
The field is within the truncated bits, look through the truncation.

Fixes #74380

Reviewers: 

Pull Request: https://github.com/llvm/llvm-project/pull/226803

Added: 
    

Modified: 
    llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp
    llvm/test/Transforms/SLPVectorizer/X86/extracted-subfields-shifted-trunc.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp
index a764dec5ad0e7..d639e0d9c7d63 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer/SLPUtils.cpp
@@ -1166,8 +1166,11 @@ matchExtractedField(Value *V) {
     if (match(Val, m_Trunc(m_Shr(m_Value(Src), m_APInt(Amt)))) ||
         match(Val, m_Shr(m_Value(Src), m_APInt(Amt)))) {
       if (std::optional<unsigned> Offset = GetFieldOffset(
-              Amt, Src->getType()->getIntegerBitWidth(), FieldWidth))
+              Amt, Src->getType()->getIntegerBitWidth(), FieldWidth)) {
+        // The truncation of the shifted value keeps the field, look through it.
+        match(Src, m_Trunc(m_Value(Src)));
         return std::make_pair(Src, *Offset);
+      }
       return std::nullopt;
     }
     if (match(Val, m_Trunc(m_Value(Src))) &&

diff  --git a/llvm/test/Transforms/SLPVectorizer/X86/extracted-subfields-shifted-trunc.ll b/llvm/test/Transforms/SLPVectorizer/X86/extracted-subfields-shifted-trunc.ll
index 82e231326d0d1..56bb1404087f1 100644
--- a/llvm/test/Transforms/SLPVectorizer/X86/extracted-subfields-shifted-trunc.ll
+++ b/llvm/test/Transforms/SLPVectorizer/X86/extracted-subfields-shifted-trunc.ll
@@ -8,18 +8,8 @@ define void @store_bytes_i64(i64 %x, ptr %out) {
 ; CHECK-LABEL: define void @store_bytes_i64(
 ; CHECK-SAME: i64 [[X:%.*]], ptr [[OUT:%.*]]) #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
-; CHECK-NEXT:    [[T:%.*]] = trunc i64 [[X]] to i32
-; CHECK-NEXT:    [[TMP0:%.*]] = insertelement <4 x i32> poison, i32 [[T]], i64 0
-; CHECK-NEXT:    [[TMP10:%.*]] = shufflevector <4 x i32> [[TMP0]], <4 x i32> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[TMP2:%.*]] = lshr <4 x i32> [[TMP10]], <i32 0, i32 8, i32 16, i32 24>
-; CHECK-NEXT:    [[TMP3:%.*]] = insertelement <4 x i64> poison, i64 [[X]], i64 0
-; CHECK-NEXT:    [[TMP4:%.*]] = shufflevector <4 x i64> [[TMP3]], <4 x i64> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[TMP5:%.*]] = lshr <4 x i64> [[TMP4]], <i64 32, i64 40, i64 48, i64 56>
-; CHECK-NEXT:    [[TMP6:%.*]] = trunc <4 x i64> [[TMP5]] to <4 x i32>
-; CHECK-NEXT:    [[TMP7:%.*]] = shufflevector <4 x i32> [[TMP2]], <4 x i32> poison, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 poison, i32 poison, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP8:%.*]] = shufflevector <4 x i32> [[TMP6]], <4 x i32> poison, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 poison, i32 poison, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP9:%.*]] = shufflevector <8 x i32> [[TMP7]], <8 x i32> [[TMP8]], <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 8, i32 9, i32 10, i32 11>
-; CHECK-NEXT:    [[TMP1:%.*]] = and <8 x i32> [[TMP9]], <i32 255, i32 255, i32 255, i32 -1, i32 255, i32 255, i32 255, i32 -1>
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i64 [[X]] to <8 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = zext <8 x i8> [[TMP0]] to <8 x i32>
 ; CHECK-NEXT:    store <8 x i32> [[TMP1]], ptr [[OUT]], align 4
 ; CHECK-NEXT:    ret void
 ;