[lld] r316280 - Assume that mergeable input sections are smaller than 4 GiB.
Sean Silva via llvm-commits
llvm-commits at lists.llvm.org
Sun Oct 22 12:25:34 PDT 2017
Do you know how much of the savings is from reducing the size of Section
piece vs the OffsetMap? IIRC the offset map was just an optimization, so if
that is turning out to be a major memory hog then maybe we want to
reconsider the approach.
-- Sean Silva
On Oct 21, 2017 4:20 PM, "Rui Ueyama via llvm-commits" <
llvm-commits at lists.llvm.org> wrote:
Author: ruiu
Date: Sat Oct 21 16:20:13 2017
New Revision: 316280
URL: http://llvm.org/viewvc/llvm-project?rev=316280&view=rev
Log:
Assume that mergeable input sections are smaller than 4 GiB.
By assuming that mergeable input sections are smaller than 4 GiB,
lld's memory usage when linking clang with debug info drops from
2.788 GiB to 2.019 GiB (measured by valgrind, and that does not include
memory space for mmap'ed files). I think that's a reasonable assumption
given such a large RAM savings, so this patch.
According to valgrind, gold needs 3.54 GiB of RAM to do the same thing.
NB: This patch does not introduce a limitation on the size of
output sections. You can still create sections larger than 4 GiB.
Modified:
lld/trunk/ELF/InputSection.cpp
lld/trunk/ELF/InputSection.h
lld/trunk/ELF/SyntheticSections.cpp
Modified: lld/trunk/ELF/InputSection.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
InputSection.cpp?rev=316280&r1=316279&r2=316280&view=diff
============================================================
==================
--- lld/trunk/ELF/InputSection.cpp (original)
+++ lld/trunk/ELF/InputSection.cpp Sat Oct 21 16:20:13 2017
@@ -871,13 +871,15 @@ SyntheticSection *MergeInputSection::get
void MergeInputSection::splitStrings(ArrayRef<uint8_t> Data, size_t
EntSize) {
size_t Off = 0;
bool IsAlloc = this->Flags & SHF_ALLOC;
+
while (!Data.empty()) {
size_t End = findNull(Data, EntSize);
if (End == StringRef::npos)
fatal(toString(this) + ": string is not null terminated");
size_t Size = End + EntSize;
- Pieces.emplace_back(Off, !IsAlloc);
- Hashes.push_back(xxHash64(toStringRef(Data.slice(0, Size))));
+
+ Pieces.emplace_back(Off, xxHash64(toStringRef(Data.slice(0, Size))),
+ !IsAlloc);
Data = Data.slice(Size);
Off += Size;
}
@@ -890,17 +892,23 @@ void MergeInputSection::splitNonStrings(
size_t Size = Data.size();
assert((Size % EntSize) == 0);
bool IsAlloc = this->Flags & SHF_ALLOC;
- for (unsigned I = 0, N = Size; I != N; I += EntSize) {
- Hashes.push_back(xxHash64(toStringRef(Data.slice(I, EntSize))));
- Pieces.emplace_back(I, !IsAlloc);
- }
+
+ for (size_t I = 0; I != Size; I += EntSize)
+ Pieces.emplace_back(I, xxHash64(toStringRef(Data.slice(I, EntSize))),
+ !IsAlloc);
}
template <class ELFT>
MergeInputSection::MergeInputSection(ObjFile<ELFT> *F,
const typename ELFT::Shdr *Header,
StringRef Name)
- : InputSectionBase(F, Header, Name, InputSectionBase::Merge) {}
+ : InputSectionBase(F, Header, Name, InputSectionBase::Merge) {
+ // In order to reduce memory allocation, we assume that mergeable
+ // sections are smaller than 4 GiB, which is not an unreasonable
+ // assumption as of 2017.
+ if (Data.size() > UINT32_MAX)
+ error(toString(this) + ": section too large");
+}
// This function is called after we obtain a complete list of input
sections
// that need to be linked. This is responsible to split section contents
@@ -942,8 +950,7 @@ static It fastUpperBound(It First, It La
}
const SectionPiece *MergeInputSection::getSectionPiece(uint64_t Offset)
const {
- uint64_t Size = this->Data.size();
- if (Offset >= Size)
+ if (Data.size() <= Offset)
fatal(toString(this) + ": entry is past the end of the section");
// Find the element this offset points to.
@@ -958,20 +965,20 @@ const SectionPiece *MergeInputSection::g
// Because contents of a mergeable section is not contiguous in output,
// it is not just an addition to a base output offset.
uint64_t MergeInputSection::getOffset(uint64_t Offset) const {
+ if (!this->Live)
+ return 0;
+
// Initialize OffsetMap lazily.
llvm::call_once(InitOffsetMap, [&] {
OffsetMap.reserve(Pieces.size());
- for (const SectionPiece &Piece : Pieces)
- OffsetMap[Piece.InputOff] = Piece.OutputOff;
+ for (size_t I = 0; I < Pieces.size(); ++I)
+ OffsetMap[Pieces[I].InputOff] = I;
});
// Find a string starting at a given offset.
auto It = OffsetMap.find(Offset);
if (It != OffsetMap.end())
- return It->second;
-
- if (!this->Live)
- return 0;
+ return Pieces[It->second].OutputOff;
// If Offset is not at beginning of a section piece, it is not in the
map.
// In that case we need to search from the original section piece vector.
Modified: lld/trunk/ELF/InputSection.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
InputSection.h?rev=316280&r1=316279&r2=316280&view=diff
============================================================
==================
--- lld/trunk/ELF/InputSection.h (original)
+++ lld/trunk/ELF/InputSection.h Sat Oct 21 16:20:13 2017
@@ -214,15 +214,17 @@ private:
// have to be as compact as possible, which is why we don't store the size
(can
// be found by looking at the next one) and put the hash in a side table.
struct SectionPiece {
- SectionPiece(size_t Off, bool Live)
- : InputOff(Off), Live(Live || !Config->GcSections), OutputOff(-1) {}
-
- size_t InputOff : 8 * sizeof(ssize_t) - 1;
- size_t Live : 1;
- ssize_t OutputOff;
+ SectionPiece(size_t Off, uint32_t Hash, bool Live)
+ : InputOff(Off), Hash(Hash), OutputOff(-1),
+ Live(Live || !Config->GcSections) {}
+
+ uint32_t InputOff;
+ uint32_t Hash;
+ uint64_t OutputOff : 63;
+ uint64_t Live : 1;
};
-static_assert(sizeof(SectionPiece) == 2 * sizeof(size_t),
- "SectionPiece is too big");
+
+static_assert(sizeof(SectionPiece) == 16, "SectionPiece is too big");
// This corresponds to a SHF_MERGE section of an input file.
class MergeInputSection : public InputSectionBase {
@@ -252,14 +254,9 @@ public:
LLVM_ATTRIBUTE_ALWAYS_INLINE
llvm::CachedHashStringRef getData(size_t I) const {
size_t Begin = Pieces[I].InputOff;
- size_t End;
- if (Pieces.size() - 1 == I)
- End = this->Data.size();
- else
- End = Pieces[I + 1].InputOff;
-
- StringRef S = {(const char *)(this->Data.data() + Begin), End - Begin};
- return {S, Hashes[I]};
+ size_t End =
+ (Pieces.size() - 1 == I) ? Data.size() : Pieces[I + 1].InputOff;
+ return {toStringRef(Data.slice(Begin, End - Begin)), Pieces[I].Hash};
}
// Returns the SectionPiece at a given input section offset.
@@ -272,9 +269,7 @@ private:
void splitStrings(ArrayRef<uint8_t> A, size_t Size);
void splitNonStrings(ArrayRef<uint8_t> A, size_t Size);
- std::vector<uint32_t> Hashes;
-
- mutable llvm::DenseMap<uint64_t, uint64_t> OffsetMap;
+ mutable llvm::DenseMap<uint32_t, uint32_t> OffsetMap;
mutable llvm::once_flag InitOffsetMap;
llvm::DenseSet<uint64_t> LiveOffsets;
Modified: lld/trunk/ELF/SyntheticSections.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/
SyntheticSections.cpp?rev=316280&r1=316279&r2=316280&view=diff
============================================================
==================
--- lld/trunk/ELF/SyntheticSections.cpp (original)
+++ lld/trunk/ELF/SyntheticSections.cpp Sat Oct 21 16:20:13 2017
@@ -2269,10 +2269,9 @@ void MergeNoTailSection::finalizeContent
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I) {
if (!Sec->Pieces[I].Live)
continue;
- CachedHashStringRef Str = Sec->getData(I);
- size_t ShardId = getShardId(Str.hash());
+ size_t ShardId = getShardId(Sec->Pieces[I].Hash);
if ((ShardId & (Concurrency - 1)) == ThreadId)
- Sec->Pieces[I].OutputOff = Shards[ShardId].add(Str);
+ Sec->Pieces[I].OutputOff = Shards[ShardId].add(Sec->getData(I));
}
}
});
@@ -2294,7 +2293,7 @@ void MergeNoTailSection::finalizeContent
for (size_t I = 0, E = Sec->Pieces.size(); I != E; ++I)
if (Sec->Pieces[I].Live)
Sec->Pieces[I].OutputOff +=
- ShardOffsets[getShardId(Sec->getData(I).hash())];
+ ShardOffsets[getShardId(Sec->Pieces[I].Hash)];
});
}
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