Index: llvm/include/llvm/IR/PatternMatch.h
===================================================================
--- llvm/include/llvm/IR/PatternMatch.h
+++ llvm/include/llvm/IR/PatternMatch.h
@@ -152,8 +152,10 @@
 
 struct apint_match {
   const APInt *&Res;
+  bool AllowUndef;
 
-  apint_match(const APInt *&R) : Res(R) {}
+  apint_match(const APInt *&Res, bool AllowUndef)
+    : Res(Res), AllowUndef(AllowUndef) {}
 
   template <typename ITy> bool match(ITy *V) {
     if (auto *CI = dyn_cast<ConstantInt>(V)) {
@@ -162,7 +164,8 @@
     }
     if (V->getType()->isVectorTy())
       if (const auto *C = dyn_cast<Constant>(V))
-        if (auto *CI = dyn_cast_or_null<ConstantInt>(C->getSplatValue())) {
+        if (auto *CI = dyn_cast_or_null<ConstantInt>(
+                C->getSplatValue(AllowUndef))) {
           Res = &CI->getValue();
           return true;
         }
@@ -174,7 +177,11 @@
 // function for both apint/apfloat.
 struct apfloat_match {
   const APFloat *&Res;
-  apfloat_match(const APFloat *&R) : Res(R) {}
+  bool AllowUndef;
+
+  apfloat_match(const APFloat *&Res, bool AllowUndef)
+      : Res(Res), AllowUndef(AllowUndef) {}
+
   template <typename ITy> bool match(ITy *V) {
     if (auto *CI = dyn_cast<ConstantFP>(V)) {
       Res = &CI->getValueAPF();
@@ -182,7 +189,8 @@
     }
     if (V->getType()->isVectorTy())
       if (const auto *C = dyn_cast<Constant>(V))
-        if (auto *CI = dyn_cast_or_null<ConstantFP>(C->getSplatValue())) {
+        if (auto *CI = dyn_cast_or_null<ConstantFP>(
+                C->getSplatValue(AllowUndef))) {
           Res = &CI->getValueAPF();
           return true;
         }
@@ -192,11 +200,37 @@
 
 /// Match a ConstantInt or splatted ConstantVector, binding the
 /// specified pointer to the contained APInt.
-inline apint_match m_APInt(const APInt *&Res) { return Res; }
+inline apint_match m_APInt(const APInt *&Res) {
+  // Forbid undefs by default to maintain previous behavior.
+  return apint_match(Res, /* AllowUndef */ false);
+}
+
+/// Match APInt while allowing undefs in splat vector constants.
+inline apint_match m_APIntAllowUndef(const APInt *&Res) {
+  return apint_match(Res, /* AllowUndef */ true);
+}
+
+/// Match APInt while forbidding undefs in splat vector constants.
+inline apint_match m_APIntForbidUndef(const APInt *&Res) {
+  return apint_match(Res, /* AllowUndef */ false);
+}
 
 /// Match a ConstantFP or splatted ConstantVector, binding the
 /// specified pointer to the contained APFloat.
-inline apfloat_match m_APFloat(const APFloat *&Res) { return Res; }
+inline apfloat_match m_APFloat(const APFloat *&Res) {
+  // Forbid undefs by default to maintain previous behavior.
+  return apfloat_match(Res, /* AllowUndef */ false);
+}
+
+/// Match APFloat while allowing undefs in splat vector constants.
+inline apfloat_match m_APFloatAllowUndef(const APFloat *&Res) {
+  return apfloat_match(Res, /* AllowUndef */ true);
+}
+
+/// Match APFloat while forbidding undefs in splat vector constants.
+inline apfloat_match m_APFloatForbidUndef(const APFloat *&Res) {
+  return apfloat_match(Res, /* AllowUndef */ false);
+}
 
 template <int64_t Val> struct constantint_match {
   template <typename ITy> bool match(ITy *V) {
Index: llvm/unittests/IR/PatternMatch.cpp
===================================================================
--- llvm/unittests/IR/PatternMatch.cpp
+++ llvm/unittests/IR/PatternMatch.cpp
@@ -1045,6 +1045,43 @@
   EXPECT_TRUE(match(ScalarZero, m_Zero()));
   EXPECT_TRUE(match(VectorZero, m_Zero()));
   EXPECT_TRUE(match(VectorZeroUndef, m_Zero()));
+
+  const APInt *C;
+  // Regardless of whether undefs are allowed,
+  // a fully undef constant does not match.
+  EXPECT_FALSE(match(ScalarUndef, m_APInt(C)));
+  EXPECT_FALSE(match(ScalarUndef, m_APIntForbidUndef(C)));
+  EXPECT_FALSE(match(ScalarUndef, m_APIntAllowUndef(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APInt(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APIntForbidUndef(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APIntAllowUndef(C)));
+
+  // We can always match simple constants and simple splats.
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APInt(C)));
+  EXPECT_TRUE(C->isNullValue());
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APIntForbidUndef(C)));
+  EXPECT_TRUE(C->isNullValue());
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APIntAllowUndef(C)));
+  EXPECT_TRUE(C->isNullValue());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APInt(C)));
+  EXPECT_TRUE(C->isNullValue());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APIntForbidUndef(C)));
+  EXPECT_TRUE(C->isNullValue());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APIntAllowUndef(C)));
+  EXPECT_TRUE(C->isNullValue());
+
+  // Whether splats with undef can be matched depends on the matcher.
+  EXPECT_FALSE(match(VectorZeroUndef, m_APInt(C)));
+  EXPECT_FALSE(match(VectorZeroUndef, m_APIntForbidUndef(C)));
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZeroUndef, m_APIntAllowUndef(C)));
+  EXPECT_TRUE(C->isNullValue());
 }
 
 TEST_F(PatternMatchTest, VectorUndefFloat) {
@@ -1073,6 +1110,43 @@
   EXPECT_TRUE(match(ScalarZero, m_AnyZeroFP()));
   EXPECT_TRUE(match(VectorZero, m_AnyZeroFP()));
   EXPECT_TRUE(match(VectorZeroUndef, m_AnyZeroFP()));
+
+  const APFloat *C;
+  // Regardless of whether undefs are allowed,
+  // a fully undef constant does not match.
+  EXPECT_FALSE(match(ScalarUndef, m_APFloat(C)));
+  EXPECT_FALSE(match(ScalarUndef, m_APFloatForbidUndef(C)));
+  EXPECT_FALSE(match(ScalarUndef, m_APFloatAllowUndef(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APFloat(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APFloatForbidUndef(C)));
+  EXPECT_FALSE(match(VectorUndef, m_APFloatAllowUndef(C)));
+
+  // We can always match simple constants and simple splats.
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APFloat(C)));
+  EXPECT_TRUE(C->isZero());
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APFloatForbidUndef(C)));
+  EXPECT_TRUE(C->isZero());
+  C = nullptr;
+  EXPECT_TRUE(match(ScalarZero, m_APFloatAllowUndef(C)));
+  EXPECT_TRUE(C->isZero());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APFloat(C)));
+  EXPECT_TRUE(C->isZero());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APFloatForbidUndef(C)));
+  EXPECT_TRUE(C->isZero());
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZero, m_APFloatAllowUndef(C)));
+  EXPECT_TRUE(C->isZero());
+
+  // Whether splats with undef can be matched depends on the matcher.
+  EXPECT_FALSE(match(VectorZeroUndef, m_APFloat(C)));
+  EXPECT_FALSE(match(VectorZeroUndef, m_APFloatForbidUndef(C)));
+  C = nullptr;
+  EXPECT_TRUE(match(VectorZeroUndef, m_APFloatAllowUndef(C)));
+  EXPECT_TRUE(C->isZero());
 }
 
 TEST_F(PatternMatchTest, FloatingPointFNeg) {