[IR] move helper function to replace undef constant (elements) with fixed constants

This is the NFC part of D69519.
We had this functionality locally in instcombine, but it can be used
elsewhere, so hoisting it to Constant class.

1 files changed, 25 insertions, 4 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index f792f01..ab4e478 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -31,6 +31,7 @@
 #include <algorithm>
 
 using namespace llvm;
+using namespace PatternMatch;
 
 //===----------------------------------------------------------------------===//
 //                              Constant Class
@@ -259,10 +260,9 @@ bool Constant::isElementWiseEqual(Value *Y) const {
   auto *Cy = dyn_cast<Constant>(Y);
   if (!Cy)
     return false;
-  return PatternMatch::match(ConstantExpr::getICmp(ICmpInst::Predicate::ICMP_EQ,
-                                                   const_cast<Constant *>(this),
-                                                   Cy),
-                             PatternMatch::m_One());
+  return match(ConstantExpr::getICmp(ICmpInst::Predicate::ICMP_EQ,
+                                     const_cast<Constant *>(this), Cy),
+               m_One());
 }
 
 bool Constant::containsUndefElement() const {
@@ -595,6 +595,27 @@ void Constant::removeDeadConstantUsers() const {
   }
 }
 
+Constant *Constant::replaceUndefsWith(Constant *C, Constant *Replacement) {
+  Type *Ty = C->getType();
+  if (C && match(C, m_Undef())) {
+    assert(Ty == Replacement->getType() && "Expected matching types");
+    return Replacement;
+  }
+
+  // Don't know how to deal with this constant.
+  if (!Ty->isVectorTy())
+    return C;
+
+  unsigned NumElts = Ty->getVectorNumElements();
+  SmallVector<Constant *, 32> NewC(NumElts);
+  for (unsigned i = 0; i != NumElts; ++i) {
+    Constant *EltC = C->getAggregateElement(i);
+    assert(EltC->getType() == Replacement->getType() &&
+           "Expected matching types");
+    NewC[i] = EltC && match(EltC, m_Undef()) ? Replacement : EltC;
+  }
+  return ConstantVector::get(NewC);
+}
 
 
 //===----------------------------------------------------------------------===//