[lld] r314094 - Speeds up CU vector creation.

[Rafael Avila de Espindola via llvm-commits]

I just benchmarked this series of patches on the clang gdb index
case. It is a 1.159X speedup!

Thanks,
Rafael

Rui Ueyama via llvm-commits <llvm-commits at lists.llvm.org> writes:

> Author: ruiu
> Date: Sun Sep 24 21:55:27 2017
> New Revision: 314094
>
> URL: http://llvm.org/viewvc/llvm-project?rev=314094&view=rev
> Log:
> Speeds up CU vector creation.
>
> We used to use std::set to uniquify CU vector elements, but as we know,
> std::set is pretty slow. Fortunately we didn't actually have to use a
> std::set here. This patch replaces it with std::vector.
>
> With this patch, lld's -gdb-index overhead when linking a clang debug
> build is now about 1 second (8.65 seconds without -gdb-index vs 9.60
> seconds with -gdb-index). Since gold takes more than 6 seconds to create
> a .gdb_index for the same output, our number isn't that bad.
>
> Modified:
>     lld/trunk/ELF/SyntheticSections.cpp
>     lld/trunk/ELF/SyntheticSections.h
>
> Modified: lld/trunk/ELF/SyntheticSections.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/SyntheticSections.cpp?rev=314094&r1=314093&r2=314094&view=diff
> ==============================================================================
> --- lld/trunk/ELF/SyntheticSections.cpp (original)
> +++ lld/trunk/ELF/SyntheticSections.cpp Sun Sep 24 21:55:27 2017
> @@ -1784,8 +1784,8 @@ void GdbIndexSection::fixCuIndex() {
>    }
>  }
>  
> -std::vector<std::set<uint32_t>> GdbIndexSection::createCuVectors() {
> -  std::vector<std::set<uint32_t>> Ret;
> +std::vector<std::vector<uint32_t>> GdbIndexSection::createCuVectors() {
> +  std::vector<std::vector<uint32_t>> Ret;
>    uint32_t Idx = 0;
>    uint32_t Off = 0;
>  
> @@ -1795,13 +1795,18 @@ std::vector<std::set<uint32_t>> GdbIndex
>        if (!Sym) {
>          Sym = make<GdbSymbol>(GdbSymbol{Ent.Name.hash(), Off, Ret.size()});
>          Off += Ent.Name.size() + 1;
> -        Ret.resize(Ret.size() + 1);
> +        Ret.push_back({});
>        }
> -      Ret[Sym->CuVectorIndex].insert((Ent.Type << 24) | Idx);
> +      Ret[Sym->CuVectorIndex].push_back((Ent.Type << 24) | Idx);
>      }
>      Idx += Chunk.CompilationUnits.size();
>    }
>  
> +  for (std::vector<uint32_t> &V : Ret) {
> +    std::sort(V.begin(), V.end());
> +    V.erase(std::unique(V.begin(), V.end()), V.end());
> +  }
> +
>    StringPoolSize = Off;
>    return Ret;
>  }
> @@ -1870,9 +1875,9 @@ GdbIndexSection::GdbIndexSection(std::ve
>    ConstantPoolOffset = SymtabOffset + GdbSymtab.size() * 8;
>  
>    size_t Off = 0;
> -  for (std::set<uint32_t> &CuVec : CuVectors) {
> +  for (ArrayRef<uint32_t> Vec : CuVectors) {
>      CuVectorOffsets.push_back(Off);
> -    Off += (CuVec.size() + 1) * 4;
> +    Off += (Vec.size() + 1) * 4;
>    }
>    StringPoolOffset = ConstantPoolOffset + Off;
>  }
> @@ -1922,10 +1927,10 @@ void GdbIndexSection::writeTo(uint8_t *B
>    }
>  
>    // Write the CU vectors.
> -  for (std::set<uint32_t> &CuVec : CuVectors) {
> -    write32le(Buf, CuVec.size());
> +  for (ArrayRef<uint32_t> Vec : CuVectors) {
> +    write32le(Buf, Vec.size());
>      Buf += 4;
> -    for (uint32_t Val : CuVec) {
> +    for (uint32_t Val : Vec) {
>        write32le(Buf, Val);
>        Buf += 4;
>      }
>
> Modified: lld/trunk/ELF/SyntheticSections.h
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/SyntheticSections.h?rev=314094&r1=314093&r2=314094&view=diff
> ==============================================================================
> --- lld/trunk/ELF/SyntheticSections.h (original)
> +++ lld/trunk/ELF/SyntheticSections.h Sun Sep 24 21:55:27 2017
> @@ -27,8 +27,6 @@
>  #include "llvm/ADT/MapVector.h"
>  #include "llvm/MC/StringTableBuilder.h"
>  
> -#include <set>
> -
>  namespace lld {
>  namespace elf {
>  
> @@ -536,14 +534,14 @@ public:
>  
>  private:
>    void fixCuIndex();
> -  std::vector<std::set<uint32_t>> createCuVectors();
> +  std::vector<std::vector<uint32_t>> createCuVectors();
>    std::vector<GdbSymbol *> createGdbSymtab();
>  
>    // A symbol table for this .gdb_index section.
>    std::vector<GdbSymbol *> GdbSymtab;
>  
>    // CU vector is a part of constant pool area of section.
> -  std::vector<std::set<uint32_t>> CuVectors;
> +  std::vector<std::vector<uint32_t>> CuVectors;
>  
>    // Symbol table contents.
>    llvm::DenseMap<llvm::CachedHashStringRef, GdbSymbol *> Symbols;
>
>
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