On Mon, May 23, 2022 at 2:34 PM Changbin Du <changbin.du@xxxxxxxxx> wrote: > On Sun, May 22, 2022 at 12:38:17AM +0900, Masahiro Yamada wrote: > > (+CC: Arnd, Changbin Du) > > > > > > If -O1 support does not require additional effort, > > I have no objection to this patch. > > > > (but I do not have enough insight about > > the compiler's inlining heuristic) > > > > > > > > BTW, when we attempted to add the -Og support, > > we fixed various parts, and Linus rejected it. > > > > https://lore.kernel.org/linux-kbuild/CAK7LNARQggM3aKEPRKJqa4tunFAfmfErMZuS-rrnRv6UB1VpPQ@xxxxxxxxxxxxxx/ > > > > > > > > > > > > > > > I am afraid that '-O1' has the same situation with '-Og'. As described in GCC > mannual: > > Like -O0, -Og completely disables a number of optimization passes so that > individual options controlling them have no effect. Otherwise -Og enables all > -O1 optimization flags except for those that may interfere with debugging: > -fbranch-count-reg -fdelayed-branch > -fdse -fif-conversion -fif-conversion2 > -finline-functions-called-once > -fmove-loop-invariants -fmove-loop-stores -fssa-phiopt > -ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra Correct, I'm fairly sure this never worked. It may be possible to narrow down the individual optimization flags to have something that works inbetween -O1 and -O2, but that requires a lot of testing work, and it is questionable whether there are any practical upsides. Note that we already disable some optimizations for certain configurations, e.g. CONFIG_READABLE_ASM, CONFIG_FRAME_POINTER or CONFIG_DEBUG_SECTION_MISMATCH. The gcc manual suggests listing all optimization flags with $ gcc -c -Q -O1 --help=optimizers > O1-opts $ gcc -c -Q -O2 --help=optimizers > O2-opts $ diff -u O1-opts O2-opts If the goal is to speed up compilation, there may be value in trying which of the various options make the most difference here and still produce a working kernel when disabled. I know we need the inlining to happen or things go badly wrong, but I don't know if e.g. -fno-expensive-optimizations would work. Arnd
