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avoid copies of parser input data

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when given a string yacc will copy the entire input to a newly allocated
location so that it can add a second terminating NUL byte. since the
parser is a very internal thing to EvalState we can ensure that having
two terminating NUL bytes is always possible without copying, and have
the parser itself merely check that the expected NULs are present.

 # before

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

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     571.7 ms ±   2.4 ms    [User: 563.3 ms, System: 8.0 ms]
  Range (min … max):   566.7 ms … 579.7 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     376.6 ms ±   1.0 ms    [User: 345.8 ms, System: 30.5 ms]
  Range (min … max):   374.5 ms … 379.1 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.922 s ±  0.006 s    [User: 2.707 s, System: 0.215 s]
  Range (min … max):    2.906 s …  2.934 s    50 runs
This commit is contained in:
pennae 2021-12-21 13:56:57 +01:00
parent eee0bcee22
commit 34e3bd10e3
6 changed files with 29 additions and 17 deletions
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6
src/libexpr/eval.hh
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@ -181,8 +181,8 @@ public:
Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);
/* Parse a Nix expression from the specified string. */
Expr * parseExprFromString(std::string_view s, const Path & basePath, StaticEnv & staticEnv);
Expr * parseExprFromString(std::string_view s, const Path & basePath);
Expr * parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv);
Expr * parseExprFromString(std::string s, const Path & basePath);
Expr * parseStdin();
@ -310,7 +310,7 @@ private:
friend struct ExprAttrs;
friend struct ExprLet;
Expr * parse(const char * text, FileOrigin origin, const Path & path,
Expr * parse(char * text, size_t length, FileOrigin origin, const Path & path,
const Path & basePath, StaticEnv & staticEnv);
public: