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rustc: Reduce default CGUs to 16 

Rationale explained in the included comment as well as #44941
This commit is contained in:
Alex Crichton 2017-10-06 07:06:30 -07:00
parent 3ed8b69842
commit 1988447007
1 changed files with 16 additions and 2 deletions
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18
src/librustc/session/config.rs
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@ -1716,7 +1716,7 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
let codegen_units = codegen_units.unwrap_or_else(|| {
match opt_level {
// If we're compiling at `-O0` then default to 32 codegen units.
// If we're compiling at `-O0` then default to 16 codegen units.
// The number here shouldn't matter too too much as debug mode
// builds don't rely on performance at all, meaning that lost
// opportunities for inlining through multiple codegen units is
@ -1734,7 +1734,21 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
// unit takes *too* long to build we'll be guaranteed that all
// cpus will finish pretty closely to one another and we should
// make relatively optimal use of system resources
OptLevel::No => 32,
//
// Another note worth mentioning here, however, is that this number
// isn't *too* high. When codegen units are increased that means we
// currently have to codegen `#[inline]` functions into each codegen
// unit, which means the more codegen units we're using the more we
// may be generating. In other words, increasing codegen units may
// increase the overall work the compiler does. If we don't have
// enough cores to make up for this loss then increasing the number
// of codegen units could become an overall loss!
//
// As a result we choose a hopefully conservative value 16, which
// should be more than the number of cpus of most hardware compiling
// Rust but also not too much for 2-4 core machines to have too much
// loss of compile time.
OptLevel::No => 16,
// All other optimization levels default use one codegen unit,
// the historical default in Rust for a Long Time.