forked from lix-project/lix
Fix performance of builtins.substring for empty substrings
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.
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@ -3712,9 +3712,6 @@ static RegisterPrimOp primop_toString({
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static void prim_substring(EvalState & state, const PosIdx pos, Value * * args, Value & v)
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{
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int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
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int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
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NixStringContext context;
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auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
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if (start < 0)
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state.debugThrowLastTrace(EvalError({
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@ -3722,6 +3719,22 @@ static void prim_substring(EvalState & state, const PosIdx pos, Value * * args,
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.errPos = state.positions[pos]
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}));
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int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
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// Special-case on empty substring to avoid O(n) strlen
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// This allows for the use of empty substrings to efficently capture string context
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if (len == 0) {
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state.forceValue(*args[2], pos);
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if (args[2]->type() == nString) {
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v.mkString("", args[2]->context());
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return;
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}
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}
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NixStringContext context;
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auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
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v.mkString((unsigned int) start >= s->size() ? "" : s->substr(start, len), context);
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}
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