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Nick Desaulniers a6036a41bf kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 
The difference in most compilers between `-O3` and `-O2` is mostly down
to whether loops with statically determinable trip counts are fully
unrolled vs unrolled to a multiple of SIMD width.

This patch is effectively a revert of
commit 15f5db60a1 ("kbuild,arc: add
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
ARCH_CFLAGS

Ever since
commit cfdbc2e16e ("ARC: Build system: Makefiles, Kconfig, Linker
script")
ARC has been built with -O3, though the reason for doing so was not
specified in inline comments or the commit message. This commit does not
re-add -O3 to arch/arc/Makefile.

Folks looking to experiment with `-O3` (or any compiler flag for that
matter) may pass them along to the command line invocation of make:

$ make KCFLAGS=-O3

Code that looks to re-add an explicit Kconfig option for `-O3` should
provide:
1. A rigorous and reproducible performance profile of a reasonable
   userspace workload that demonstrates a hot loop in the kernel that
   would benefit from `-O3` over `-O2`.
2. Disassembly of said loop body before and after.
3. Provides stats on terms of increase in file size.

Link: https://lore.kernel.org/linux-kbuild/CA+55aFz2sNBbZyg-_i8_Ldr2e8o9dfvdSfHHuRzVtP2VMAUWPg@mail.gmail.com/
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
2022-07-27 21:17:59 +09:00
..
calibrate.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
do_mounts.c block: remove genhd.h 2022-02-02 07:49:59 -07:00
do_mounts.h init: add an init_mknod helper 2020-07-31 08:17:54 +02:00
do_mounts_initrd.c kernel/do_mount_initrd: move real_root_dev sysctls to its own file 2022-04-06 13:43:44 -07:00
do_mounts_rd.c init: add an init_unlink helper 2020-07-31 08:17:52 +02:00
init_task.c kcsan: Remove redundant zero-initialization of globals 2021-12-09 16:42:26 -08:00
initramfs.c This set of changes updates init and user mode helper tasks to be 2022-06-03 16:03:05 -07:00
Kconfig kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 2022-07-27 21:17:59 +09:00
main.c This set of changes updates init and user mode helper tasks to be 2022-06-03 16:03:05 -07:00
Makefile sched/preempt: Tell about PREEMPT_DYNAMIC on kernel headers 2022-03-11 15:36:35 +01:00
noinitramfs.c init: move usermodehelper_enable() to populate_rootfs() 2021-09-08 11:50:27 -07:00
version.c kbuild: add an elfnote for whether vmlinux is built with lto 2021-04-25 05:25:42 +09:00