1963571 - Do not use fat lto for default opt build

Status: ASSIGNED

(Firefox Build System :: General, task)

Description

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As showcased by https://treeherder.mozilla.org/jobs?repo=try&revision=7ba2abd2b69b3102c631a30e287f52e9c6c4befb&selectedTaskRun=P9I07KH9SEGLbE8DDZub1g.0 the default opt build (without any kind of lto asked for by the user) is cpu bound by gkrust, probably because of single-threaded full lto being used. We should rationalize our use of lto there.

Comment 1

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Attachment: Bug 1963571 - Default to thin LTO for rust crates r=glandium!

Rust has four LTO modes:

1. off, 2. local-thin, 3. thin, 4. fat

With the extra possibility to pass a linker plugin for cross language optimization.

When passing -Clto, fat is assumed. When passing no lto option, thin is used

moz.configure controls LTO through --enable-lto=[cross,thin,full]*. The
possible setups are:

a. off, b. cross,thin, c. cross,full, d. thin, e. full

Current matching (including the rust.mk changes) is:

--enable-lto=off => -Clto => fat # bad!
--enable-lto=cross,thin => -Clto => fat # bad!
--enable-lto=cross,full => -Clto=fat => fat
--enable-lto=thin => -Clto => fat # bad!
--enable-lto=full => -Clto => fat

plus in developer mode or rust debug mode, we where using the default,
aka thin.

This patch implies the more legit:

--enable-lto=off => -Clto=thin => thin
--enable-lto=cross,thin => -Clto=thin => thin
--enable-lto=cross,full => -Clto=fat => fat
--enable-lto=thin => -Clto => thin
--enable-lto=full => -Clto=fat => fat

The impact is a more predictable behavior based on options being passed
to configure, a single place to handle the option (less rust.mk hooks),
and faster opt build (because we don't use fat lto there).

And in debug mode, we now use either thin-local lto or no lto,
respectively if --enable-lto is used or not.

Comment 2

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Attachment: WIP: Bug 1963571 - Configurable rust release lto mode through --with-rust-lto-mode