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Tweak thread names for CGU processing.
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For non-incremental builds on Unix, currently all the thread names look
like `opt regex.f10ba03eb5ec7975-cgu.0`. But they are truncated by
`pthread_setname` to `opt regex.f10ba`, hiding the numeric suffix that
distinguishes them. This is really annoying when using a profiler like
Samply.

This commit changes these thread names to a form like `opt cgu.0`, which
is much better.
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nnethercote committed Jun 25, 2023
1 parent 487bdeb commit 666b1b6
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Showing 2 changed files with 45 additions and 21 deletions.
63 changes: 42 additions & 21 deletions compiler/rustc_codegen_ssa/src/back/write.rs
Original file line number Diff line number Diff line change
Expand Up @@ -698,28 +698,49 @@ impl<B: WriteBackendMethods> WorkItem<B> {

/// Generate a short description of this work item suitable for use as a thread name.
fn short_description(&self) -> String {
// `pthread_setname()` on *nix is limited to 15 characters and longer names are ignored.
// Use very short descriptions in this case to maximize the space available for the module name.
// Windows does not have that limitation so use slightly more descriptive names there.
// `pthread_setname()` on *nix ignores anything beyond the first 15
// bytes. Use short descriptions to maximize the space available for
// the module name.
#[cfg(not(windows))]
fn desc(short: &str, _long: &str, name: &str) -> String {
// The short label is three bytes, and is followed by a space. That
// leaves 11 bytes for the CGU name. How we obtain those 11 bytes
// depends on the the CGU name form.
//
// - Non-incremental, e.g. `regex.f10ba03eb5ec7975-cgu.0`: the part
// before the `-cgu.0` is the same for every CGU, so use the
// `cgu.0` part. The number suffix will be different for each
// CGU.
//
// - Incremental (normal), e.g. `2i52vvl2hco29us0`: use the whole
// name because each CGU will have a unique ASCII hash, and the
// first 11 bytes will be enough to identify it.
//
// - Incremental (with `-Zhuman-readable-cgu-names`), e.g.
// `regex.f10ba03eb5ec7975-re_builder.volatile`: use the whole
// name. The first 11 bytes won't be enough to uniquely identify
// it, but no obvious substring will, and this is a rarely used
// option so it doesn't matter much.
//
assert_eq!(short.len(), 3);
let name = if let Some(index) = name.find("-cgu.") {
&name[index + 1..] // +1 skips the leading '-'.
} else {
name
};
format!("{short} {name}")
}

// Windows has no thread name length limit, so use more descriptive names.
#[cfg(windows)]
fn desc(_short: &str, long: &str, name: &str) -> String {
format!("{long} {name}")
}

match self {
WorkItem::Optimize(m) => {
#[cfg(windows)]
return format!("optimize module {}", m.name);
#[cfg(not(windows))]
return format!("opt {}", m.name);
}
WorkItem::CopyPostLtoArtifacts(m) => {
#[cfg(windows)]
return format!("copy LTO artifacts for {}", m.name);
#[cfg(not(windows))]
return format!("copy {}", m.name);
}
WorkItem::LTO(m) => {
#[cfg(windows)]
return format!("LTO module {}", m.name());
#[cfg(not(windows))]
return format!("LTO {}", m.name());
}
WorkItem::Optimize(m) => desc("opt", "optimize module {}", &m.name),
WorkItem::CopyPostLtoArtifacts(m) => desc("cpy", "copy LTO artifacts for {}", &m.name),
WorkItem::LTO(m) => desc("lto", "LTO module {}", m.name()),
}
}
}
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3 changes: 3 additions & 0 deletions compiler/rustc_monomorphize/src/partitioning.rs
Original file line number Diff line number Diff line change
Expand Up @@ -431,6 +431,9 @@ fn merge_codegen_units<'tcx>(
codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
let num_digits = codegen_units.len().ilog10() as usize + 1;
for (index, cgu) in codegen_units.iter_mut().enumerate() {
// Note: `WorkItem::short_description` depends on this name ending
// with `-cgu.` followed by a numeric suffix. Please keep it in
// sync with this code.
let suffix = format!("{index:0num_digits$}");
let numbered_codegen_unit_name =
cgu_name_builder.build_cgu_name_no_mangle(LOCAL_CRATE, &["cgu"], Some(suffix));
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