fmt: Skip calling write_str for empty strings #31966
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`format_args!` uses one string literal per used `{}`, in many cases, the
separating string pieces are empty.
For example, `write!(w, "{}", x);` will attempt to write one empty
string before the `{}` format, and `write!(w, "{}{}{}", x, y, z);" will
write three empty string pieces between formats.
Simply skip writing if the string is empty. It is a cheap branch
compared to the virtual Write::write_str call that it makes possible to
skip.
It does not solve issue rust-lang#10761 in any way, yet that's where I noticed
this. The testcase in the issue shows a performance difference between
`write!(w, "abc")` and `write!(w, "{}", "abc")`, and this change halves
the size of the difference.
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(rust_highfive has picked a reviewer for you, use r? to override) |
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I wonder if it may make sense to optimize away these at compile time instead of runtime? They're taking up space in rodata and it may make sense to get rid of them? That may be a bit difficult, however, as we'd have to describe what piece is used when |
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Yes, that's even better. I don't know how all the pieces work, but maybe there's a way to do it. |
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☔ The latest upstream changes (presumably #32390) made this pull request unmergeable. Please resolve the merge conflicts. |
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Closing for now. Benchmarks on this weren't that radical anyway (less than 10% savings or so). |
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This means that `writeln!(something, "{}{}", s1, s2)` no longer calls
`write_str` two times with empty strings (before each substitution).
See rust-lang#31966 for an earlier attempt.
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fmt: Skip calling write_str for empty strings
format_args!uses one string literal per used{}, in many cases, theseparating string pieces are empty.
For example,
write!(w, "{}", x);will attempt to write one emptystring before the
{}format, andwrite!(w, "{}{}{}", x, y, z);"willwrite one empty string piece between each format.
Simply skip writing if the string is empty. It is a cheap branch
compared to the virtual Write::write_str call that it makes possible to
skip.
It does not solve issue #10761 in any way, yet that's where I noticed
this. The testcase in the issue shows a performance difference between
write!(w, "abc")andwrite!(w, "{}", "abc"), and this change halvesthe size of the difference.