[CodeGen] Use the zero initializer instead of storing an all zero representation.
Summary:
This change avoids the overhead of storing, and later crawling,
an initializer list of all zeros for arrays. When LLVM
visits this (llvm/IR/Constants.cpp) ConstantArray::getImpl()
it will scan the list looking for an array of all zero.
We can avoid the store, and short-cut the scan, by detecting
all zeros when clang builds-up the initialization representation.
This was brought to my attention when investigating PR36030
Reviewers: majnemer, rjmccall
Reviewed By: rjmccall
Subscribers: cfe-commits
Differential Revision: https://reviews.llvm.org/D42549
llvm-svn: 324776
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index d1b9e13..7b076ea 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -859,9 +859,10 @@
// Copy initializer elements.
SmallVector<llvm::Constant*, 16> Elts;
- Elts.reserve(NumInitableElts + NumElements);
+ Elts.reserve(std::max(NumInitableElts, NumElements));
bool RewriteType = false;
+ bool AllNullValues = true;
for (unsigned i = 0; i < NumInitableElts; ++i) {
Expr *Init = ILE->getInit(i);
llvm::Constant *C = Emitter.tryEmitPrivateForMemory(Init, EltType);
@@ -869,15 +870,22 @@
return nullptr;
RewriteType |= (C->getType() != ElemTy);
Elts.push_back(C);
+ if (AllNullValues && !C->isNullValue())
+ AllNullValues = false;
}
+ // If all initializer elements are "zero," then avoid storing NumElements
+ // instances of the zero representation.
+ if (AllNullValues)
+ return llvm::ConstantAggregateZero::get(AType);
+
RewriteType |= (fillC->getType() != ElemTy);
Elts.resize(NumElements, fillC);
if (RewriteType) {
// FIXME: Try to avoid packing the array
std::vector<llvm::Type*> Types;
- Types.reserve(NumInitableElts + NumElements);
+ Types.reserve(Elts.size());
for (unsigned i = 0, e = Elts.size(); i < e; ++i)
Types.push_back(Elts[i]->getType());
llvm::StructType *SType = llvm::StructType::get(AType->getContext(),