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avoid allocations in SymbolTable::create

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speeds up parsing by ~3%, system builds by a bit more than 1%

 # before

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     574.7 ms ±   2.8 ms    [User: 566.3 ms, System: 8.0 ms]
  Range (min … max):   569.2 ms … 580.7 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     394.4 ms ±   0.8 ms    [User: 361.8 ms, System: 32.3 ms]
  Range (min … max):   392.7 ms … 395.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.976 s ±  0.005 s    [User: 2.757 s, System: 0.218 s]
  Range (min … max):    2.966 s …  2.990 s    50 runs

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     572.4 ms ±   2.3 ms    [User: 563.4 ms, System: 8.6 ms]
  Range (min … max):   566.9 ms … 579.1 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     381.7 ms ±   1.0 ms    [User: 348.3 ms, System: 33.1 ms]
  Range (min … max):   380.2 ms … 387.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.936 s ±  0.005 s    [User: 2.715 s, System: 0.221 s]
  Range (min … max):    2.923 s …  2.946 s    50 runs
This commit is contained in:
pennae 2021-12-20 11:29:14 +01:00
parent 61a9d16d5c
commit eee0bcee22
2 changed files with 15 additions and 8 deletions
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2
src/libexpr/nixexpr.cc
View file

@ -473,7 +473,7 @@ string ExprLambda::showNamePos() const
size_t SymbolTable::totalSize() const size_t SymbolTable::totalSize() const
{ {
size_t n = 0; size_t n = 0;
for (auto & i : symbols) for (auto & i : store)
n += i.size(); n += i.size();
return n; return n;
} }

21
src/libexpr/symbol-table.hh
View file

@ -1,7 +1,8 @@
#pragma once #pragma once
#include <list>
#include <map> #include <map>
#include <unordered_set> #include <unordered_map>
#include "types.hh" #include "types.hh"
@ -70,15 +71,21 @@ public:
class SymbolTable class SymbolTable
{ {
private: private:
typedef std::unordered_set<string> Symbols; std::unordered_map<std::string_view, Symbol> symbols;
Symbols symbols; std::list<string> store;
public: public:
Symbol create(std::string_view s) Symbol create(std::string_view s)
{ {
// FIXME: avoid allocation if 's' already exists in the symbol table. // Most symbols are looked up more than once, so we trade off insertion performance
std::pair<Symbols::iterator, bool> res = symbols.emplace(std::string(s)); // for lookup performance.
return Symbol(&*res.first); // TODO: could probably be done more efficiently with transparent Hash and Equals
// on the original implementation using unordered_set
auto it = symbols.find(s);
if (it != symbols.end()) return it->second;
const string & rawSym = store.emplace_back(s);
return symbols.emplace(rawSym, Symbol(&rawSym)).first->second;
} }
size_t size() const size_t size() const
@ -91,7 +98,7 @@ public:
template<typename T> template<typename T>
void dump(T callback) void dump(T callback)
{ {
for (auto & s : symbols) for (auto & s : store)
callback(s); callback(s);
} }
}; };