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auto merge of #18213 : pcwalton/rust/pcg-default, r=aturon

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Enable parallel codegen (2 units) by default when --opt-level is 0 or 1.  This
gives a minor speedup on large crates (~10%), with only a tiny slowdown (~2%)
for small ones (which usually build in under a second regardless).  The current
default (no parallelization) is used when the user requests optimization
(--opt-level 2 or 3), and when the user has enabled LTO (which is incompatible
with parallel codegen).

This commit also changes the rust build system to use parallel codegen
when appropriate.  This means codegen-units=4 for stage0 always, and
also for stage1 and stage2 when configured with --disable-optimize.
(Other settings use codegen-units=1 for stage1 and stage2, to get
maximum performance for release binaries.)  The build system also sets
codegen-units=1 for compiletest tests (compiletest does its own
parallelization) and uses the same setting as stage2 for crate tests.

r? @aturon
This commit is contained in:
bors 2014-10-22 04:42:11 +00:00
commit c29a7520e7
3 changed files with 25 additions and 1 deletions
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15
src/librustc/driver/config.rs
View file

@ -780,7 +780,20 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
early_warn("the --crate-file-name argument has been renamed to \
--print-file-name");
}
let cg = build_codegen_options(matches);
let mut cg = build_codegen_options(matches);
if cg.codegen_units == 0 {
match opt_level {
// `-C lto` doesn't work with multiple codegen units.
_ if cg.lto => cg.codegen_units = 1,
No | Less => cg.codegen_units = 2,
Default | Aggressive => cg.codegen_units = 1,
}
}
let cg = cg;
if !cg.remark.is_empty() && debuginfo == NoDebugInfo {
early_warn("-C remark will not show source locations without --debuginfo");