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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>
This commit is contained in:
Nick Desaulniers 2022-06-28 14:04:07 -07:00 committed by Masahiro Yamada
parent 5439d4d4dc
commit a6036a41bf
15 changed files with 0 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
Makefile
View File

@ -755,8 +755,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
KBUILD_CFLAGS += -O2
else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
KBUILD_CFLAGS += -O3
else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
KBUILD_CFLAGS += -Os
endif

1
arch/arc/configs/axs101_defconfig
View File

@ -9,7 +9,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/axs103_defconfig
View File

@ -9,7 +9,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/axs103_smp_defconfig
View File

@ -9,7 +9,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/haps_hs_defconfig
View File

@ -11,7 +11,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EXPERT=y
CONFIG_PERF_EVENTS=y
# CONFIG_COMPAT_BRK is not set

1
arch/arc/configs/haps_hs_smp_defconfig
View File

@ -11,7 +11,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/hsdk_defconfig
View File

@ -9,7 +9,6 @@ CONFIG_NAMESPACES=y
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_BLK_DEV_RAM=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/nsim_700_defconfig
View File

@ -11,7 +11,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_KALLSYMS_ALL=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y

1
arch/arc/configs/nsimosci_defconfig
View File

@ -10,7 +10,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_KALLSYMS_ALL=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y

1
arch/arc/configs/nsimosci_hs_defconfig
View File

@ -10,7 +10,6 @@ CONFIG_NAMESPACES=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_KALLSYMS_ALL=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y

1
arch/arc/configs/nsimosci_hs_smp_defconfig
View File

@ -8,7 +8,6 @@ CONFIG_IKCONFIG_PROC=y
# CONFIG_UTS_NS is not set
# CONFIG_PID_NS is not set
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_PERF_EVENTS=y
# CONFIG_COMPAT_BRK is not set
CONFIG_KPROBES=y

1
arch/arc/configs/tb10x_defconfig
View File

@ -14,7 +14,6 @@ CONFIG_INITRAMFS_SOURCE="../tb10x-rootfs.cpio"
CONFIG_INITRAMFS_ROOT_UID=2100
CONFIG_INITRAMFS_ROOT_GID=501
# CONFIG_RD_GZIP is not set
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_KALLSYMS_ALL=y
# CONFIG_AIO is not set
CONFIG_EMBEDDED=y

1
arch/arc/configs/vdk_hs38_defconfig
View File

@ -4,7 +4,6 @@ CONFIG_HIGH_RES_TIMERS=y
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

1
arch/arc/configs/vdk_hs38_smp_defconfig
View File

@ -4,7 +4,6 @@ CONFIG_HIGH_RES_TIMERS=y
CONFIG_IKCONFIG=y
CONFIG_IKCONFIG_PROC=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3=y
CONFIG_EMBEDDED=y
CONFIG_PERF_EVENTS=y
# CONFIG_VM_EVENT_COUNTERS is not set

7
init/Kconfig
View File

@ -1401,13 +1401,6 @@ config CC_OPTIMIZE_FOR_PERFORMANCE
with the "-O2" compiler flag for best performance and most
helpful compile-time warnings.
config CC_OPTIMIZE_FOR_PERFORMANCE_O3
bool "Optimize more for performance (-O3)"
depends on ARC
help
Choosing this option will pass "-O3" to your compiler to optimize
the kernel yet more for performance.
config CC_OPTIMIZE_FOR_SIZE
bool "Optimize for size (-Os)"
help