[lld] r316280 - Assume that mergeable input sections are smaller than 4 GiB.
Rui Ueyama via llvm-commits
llvm-commits at lists.llvm.org
Sun Oct 22 15:52:11 PDT 2017
On Sun, Oct 22, 2017 at 12:25 PM, Sean Silva <chisophugis at gmail.com> wrote:
> 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.
>
This patch didn't make SectionPiece smaller. It is still 16 bytes as before.
But you made a very good point. By removing OffsetMap, we can cut another
566 MiB of memory, and that change actually made lld slightly faster.
That's a surprising result because last time I measured, OffsetMap was
beneficial. I don't know what has changed since the last experiment, but
looks like there's no reason to keep OffsetMap.
-- 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/InputSecti
> on.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/InputSecti
> on.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/SyntheticS
> ections.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->getDa
> ta(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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