On Wed, Oct 25, 2017 at 10:34 AM, Gregory Szorc <g...@mozilla.com> wrote:
> Compiling Rust code with optimizations is significantly slower than > compiling without optimizations. As was measured in bug 1411081, the > difference between rustc's -Copt-level 1 and 2 on an i7-6700K (4+4 cores) > for a recent revision of mozilla-central was 325s/802s wall/CPU versus > 625s/1282s. This made Rust compilation during Firefox builds stand out as a > long pole and significantly slowed down builds. > > Because we couldn't justify the benefits of level 2 for the build time > overhead it added, we've changed the build system default so Rust is > compiled with -Copt-level=1 (instead of 2). > > Adding --enable-release to your mozconfig (the configuration for builds we > ship to users) enables -Copt-level=2. (i.e. we didn't change optimization > settings for builds we ship to users.) > > Adding --disable-optimize sets to -Copt-level=0. (This behavior is > unchanged.) > > If you want explicit control over -Copt-level, you can `export > RUSTC_OPT_LEVEL=<value>` in your mozconfig and that value will always be > used. --enable-release implies a number of other changes. So if you just > want to restore the old build system behavior, set this variable in your > mozconfig. > > Also, due to ongoing work around Rust integration in the build system, it > is dangerous to rely on manual `cargo` invocations to compile Rust because > bypassing the build system (not using `mach build`) may not use the same > set of RUSTFLAGS that direct `cargo` invocations do. Things were mostly in > sync before. But this change and anticipated future changes will cause more > drift. If you want the correct behavior, use `mach`. > Heads up: the code change uncovered a subtle bug in sccache's argument parser. If you see "error: codegen option `opt-level` requires a string (C opt-level=<value>)", you've hit it. Until the fix merges around, the workaround is to disable sccache or graft https://hg.mozilla.org/integration/autoland/rev/bc103ec9d0b3. Sorry for any disruption. Caching and argument parsing are hard :/ _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
