mbox series

[0/7] lto: Incremental LTO.

Message ID 18cc1c3980551ac1881eea6e78811a629c7baa82.1700222403.git.mjires@suse.cz
Headers
Series lto: Incremental LTO. |

Message

Michal Jires Nov. 17, 2023, 8:16 p.m. UTC 
  Hi,
these patches implement Incremental LTO, specifically by caching results of
ltrans phase. Secondarily these patches contain changes to reduce divergence of
ltrans partitions so that they can be cached.

The aim is to reduce compile times for quick edit-compile cycles while using
LTO. Even with these minimal changes to the rest of GCC it works surprisingly
well. Currently testing by self compiling cc1, with individual commits used as
incremental changes, on average only ~1/3 of partitions need to be recompiled
with `-O2 -g0` and ~1/2 with `-O2 -g`. Which directly reduces time spent in
ltrans phase of LTO.

Unfortunately larger gains are a bit fragile. You may remember that during my
Cauldron talk I claimed reduction to ~1/6 and ~1/3 recompilations. That was
achieved with branch from March. Since then there were at least two commits
which introduced new divergence of partitions, though they seem fixable in
future.