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Reserve HashSet capacity before inserting cfgs/check-cfgs #137069

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Reserve HashSet capacity before inserting cfgs/check-cfgs #137069

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26 changes: 23 additions & 3 deletions compiler/rustc_session/src/config/cfg.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -151,7 +151,13 @@ pub(crate) fn disallow_cfgs(sess: &Session, user_cfgs: &Cfg) {

/// Generate the default configs for a given session
pub(crate) fn default_configuration(sess: &Session) -> Cfg {
// As of 2025-02-14 a default `x86_64-unknown-linux-gnu` has ~20 cfgs.
//
// So let's round that up to 32 to avoid allocating unnecessarely and to
// give a bit a wigle room for the various options and cfgs that might
// affect the list of cfgs.
let mut ret = Cfg::default();
ret.reserve(32);

macro_rules! ins_none {
($key:expr) => {
Expand Down Expand Up @@ -310,6 +316,11 @@ impl CheckCfg {
return;
}

// As of 2025-02-14 there are 30 well known cfg, so pre-allocate
// at least that much.
self.well_known_names.reserve(30);
self.expecteds.reserve(30);

// for `#[cfg(foo)]` (ie. cfg value is none)
let no_values = || {
let mut values = FxHashSet::default();
Expand Down Expand Up @@ -400,18 +411,27 @@ impl CheckCfg {
&sym::target_vendor,
];

// As of 2025-02-14 the maximum number of values is 41 in `target_os`
// so allocate at least that much.
// FIXME: Be more granular as not all `taregt_*` cfg have that much values.
let target_values = || {
let mut values = FxHashSet::default();
values.reserve(41);
ExpectedValues::Some(values)
};

// Initialize (if not already initialized)
for &e in VALUES {
if !self.exhaustive_values {
ins!(e, || ExpectedValues::Any);
} else {
ins!(e, empty_values);
ins!(e, target_values);
}
}

if self.exhaustive_values {
// Get all values map at once otherwise it would be costly.
// (8 values * 220 targets ~= 1760 times, at the time of writing this comment).
// Get all values map at once otherwise it would be _very_ costly.
// (8 values * 287 targets ~= 2300 times, at the time of writing this comment).
let [
Some(values_target_abi),
Some(values_target_arch),
Expand Down
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