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Fix performance of builtins.substring for empty substrings

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When returning a 0-length substring, avoid calling coerceToString,
since it returns a string_view with the string's length, which is
expensive to compute for large strings.
This commit is contained in:
Mel Zuser 2024-01-11 14:40:54 -08:00
parent 84502674f8
commit 25c889baac
1 changed files with 16 additions and 3 deletions
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19
src/libexpr/primops.cc
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@ -3712,9 +3712,6 @@ static RegisterPrimOp primop_toString({
static void prim_substring(EvalState & state, const PosIdx pos, Value * * args, Value & v)
{
int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
NixStringContext context;
auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
if (start < 0)
state.debugThrowLastTrace(EvalError({
@ -3722,6 +3719,22 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
.errPos = state.positions[pos]
}));
int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
// Special-case on empty substring to avoid O(n) strlen
// This allows for the use of empty substrings to efficently capture string context
if (len == 0) {
state.forceValue(*args[2], pos);
if (args[2]->type() == nString) {
v.mkString("", args[2]->context());
return;
}
}
NixStringContext context;
auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
v.mkString((unsigned int) start >= s->size() ? "" : s->substr(start, len), context);
}