[llvm] 0f26589 - [PatternMatch] match splats from an all-zeroes shufflevector (#228016)

[via llvm-commits]

Author: Graham Hunter
Date: 2026-10-01T13:29:16Z
New Revision: 0f265897950acf12e9924e6e19e8e35ce5807337

URL: https://github.com/llvm/llvm-project/commit/0f265897950acf12e9924e6e19e8e35ce5807337
DIFF: https://github.com/llvm/llvm-project/commit/0f265897950acf12e9924e6e19e8e35ce5807337.diff

LOG: [PatternMatch] match splats from an all-zeroes shufflevector (#228016)

Extends splat pattern matching to recognize
shufflevector(insertelement(elt, 0), any_value, zeroinitializer) in
addition to constant splats.

Added: 
    

Modified: 
    llvm/include/llvm/IR/PatternMatch.h
    llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    llvm/unittests/IR/PatternMatch.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/IR/PatternMatch.h b/llvm/include/llvm/IR/PatternMatch.h
index f2ab36ba7aed2..4b39b940338a9 100644
--- a/llvm/include/llvm/IR/PatternMatch.h
+++ b/llvm/include/llvm/IR/PatternMatch.h
@@ -218,14 +218,28 @@ template <typename SubPattern_t> struct Splat_match {
       auto *Splat = C->getSplatValue();
       return Splat ? SubPattern.match(Splat) : false;
     }
-    // TODO: Extend to other cases (e.g. shufflevectors).
-    return false;
+
+    auto *Shuffle = dyn_cast<ShuffleVectorInst>(V);
+    if (!Shuffle || !Shuffle->isZeroEltSplat())
+      return false;
+
+    // Look for an insertelement.
+    auto *Insert = dyn_cast<InsertElementInst>(Shuffle->getOperand(0));
+    if (!Insert)
+      return false;
+
+    Value *SplatElt = Insert->getOperand(1);
+    ConstantInt *Idx = dyn_cast<ConstantInt>(Insert->getOperand(2));
+    if (!Idx || !Idx->isZero())
+      return false;
+
+    return SubPattern.match(SplatElt);
   }
 };
 
-/// Match a constant splat. TODO: Extend this to non-constant splats.
-template <typename T>
-inline Splat_match<T> m_ConstantSplat(const T &SubPattern) {
+/// Match a vector splat. May be a constant splat or a shufflevector of the
+/// first element.
+template <typename T> inline Splat_match<T> m_Splat(const T &SubPattern) {
   return SubPattern;
 }
 

diff  --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 72adf311850a3..140953ccdcd94 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2470,7 +2470,7 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
           bool SplatLHS) -> Instruction * {
     Value *Idx;
     Constant *Splat, *SubVector, *Dest;
-    if (!match(MaybeSplat, m_ConstantSplat(m_Constant(Splat))) ||
+    if (!match(MaybeSplat, m_Splat(m_Constant(Splat))) ||
         !match(MaybeSubVector,
                m_VectorInsert(m_Constant(Dest), m_Constant(SubVector),
                               m_Value(Idx))))

diff  --git a/llvm/unittests/IR/PatternMatch.cpp b/llvm/unittests/IR/PatternMatch.cpp
index 64ccd1fdc47dd..7bf92f7761a51 100644
--- a/llvm/unittests/IR/PatternMatch.cpp
+++ b/llvm/unittests/IR/PatternMatch.cpp
@@ -1532,6 +1532,17 @@ TEST_F(PatternMatchTest, VectorOps) {
       SP2, m_Shuffle(m_InsertElt(m_Undef(), m_Value(A), m_Zero()),
                      m_Undef(), m_ZeroMask())));
   EXPECT_TRUE(A == Val);
+
+  // Repeat the above with m_Splat.
+  EXPECT_TRUE(match(SI1, m_Splat(m_SpecificInt(1))));
+  EXPECT_FALSE(match(SI2, m_Splat(m_Value())));
+  EXPECT_FALSE(match(SI3, m_Splat(m_Value())));
+  EXPECT_FALSE(match(SI4, m_Splat(m_Value())));
+
+  A = nullptr;
+  EXPECT_TRUE(match(SP1, m_Splat(m_SpecificInt(2))));
+  EXPECT_TRUE(match(SP2, m_Splat(m_Value(A))));
+  EXPECT_TRUE(A == Val);
 }
 
 TEST_F(PatternMatchTest, UndefPoisonMix) {