[PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

[PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

[Nick Desaulniers]

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 15f5db60a137 ("kbuild,arc: add
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
ARCH_CFLAGS

Ever since
commit cfdbc2e16e65 ("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>
---
 Makefile                                   | 2 --
 arch/arc/configs/axs101_defconfig          | 1 -
 arch/arc/configs/axs103_defconfig          | 1 -
 arch/arc/configs/axs103_smp_defconfig      | 1 -
 arch/arc/configs/haps_hs_defconfig         | 1 -
 arch/arc/configs/haps_hs_smp_defconfig     | 1 -
 arch/arc/configs/hsdk_defconfig            | 1 -
 arch/arc/configs/nsim_700_defconfig        | 1 -
 arch/arc/configs/nsimosci_defconfig        | 1 -
 arch/arc/configs/nsimosci_hs_defconfig     | 1 -
 arch/arc/configs/nsimosci_hs_smp_defconfig | 1 -
 arch/arc/configs/tb10x_defconfig           | 1 -
 arch/arc/configs/vdk_hs38_defconfig        | 1 -
 arch/arc/configs/vdk_hs38_smp_defconfig    | 1 -
 init/Kconfig                               | 7 -------
 15 files changed, 22 deletions(-)

diff --git a/Makefile b/Makefile
index 8973b285ce6c..b69f6cd7f2e2 100644
--- a/Makefile
+++ b/Makefile
@@ -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
diff --git a/arch/arc/configs/axs101_defconfig b/arch/arc/configs/axs101_defconfig
index 0016149f9583..e31a8ebc3ecc 100644
--- a/arch/arc/configs/axs101_defconfig
+++ b/arch/arc/configs/axs101_defconfig
@@ -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
diff --git a/arch/arc/configs/axs103_defconfig b/arch/arc/configs/axs103_defconfig
index 5b031582a1cf..e0e8567f0d75 100644
--- a/arch/arc/configs/axs103_defconfig
+++ b/arch/arc/configs/axs103_defconfig
@@ -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
diff --git a/arch/arc/configs/axs103_smp_defconfig b/arch/arc/configs/axs103_smp_defconfig
index d4eec39e0112..fcbc952bc75b 100644
--- a/arch/arc/configs/axs103_smp_defconfig
+++ b/arch/arc/configs/axs103_smp_defconfig
@@ -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
diff --git a/arch/arc/configs/haps_hs_defconfig b/arch/arc/configs/haps_hs_defconfig
index 7337cdf4ffdd..d87ad7e88d62 100644
--- a/arch/arc/configs/haps_hs_defconfig
+++ b/arch/arc/configs/haps_hs_defconfig
@@ -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
diff --git a/arch/arc/configs/haps_hs_smp_defconfig b/arch/arc/configs/haps_hs_smp_defconfig
index bc927221afc0..8d82cdb7f86a 100644
--- a/arch/arc/configs/haps_hs_smp_defconfig
+++ b/arch/arc/configs/haps_hs_smp_defconfig
@@ -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
diff --git a/arch/arc/configs/hsdk_defconfig b/arch/arc/configs/hsdk_defconfig
index aa000075a575..f856b03e0fb5 100644
--- a/arch/arc/configs/hsdk_defconfig
+++ b/arch/arc/configs/hsdk_defconfig
@@ -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
diff --git a/arch/arc/configs/nsim_700_defconfig b/arch/arc/configs/nsim_700_defconfig
index 326f6cde7826..a1ce12bf5b16 100644
--- a/arch/arc/configs/nsim_700_defconfig
+++ b/arch/arc/configs/nsim_700_defconfig
@@ -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
diff --git a/arch/arc/configs/nsimosci_defconfig b/arch/arc/configs/nsimosci_defconfig
index bf39a0091679..ca10f4a2c823 100644
--- a/arch/arc/configs/nsimosci_defconfig
+++ b/arch/arc/configs/nsimosci_defconfig
@@ -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
diff --git a/arch/arc/configs/nsimosci_hs_defconfig b/arch/arc/configs/nsimosci_hs_defconfig
index 7121bd71c543..31b6ec3683c6 100644
--- a/arch/arc/configs/nsimosci_hs_defconfig
+++ b/arch/arc/configs/nsimosci_hs_defconfig
@@ -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
diff --git a/arch/arc/configs/nsimosci_hs_smp_defconfig b/arch/arc/configs/nsimosci_hs_smp_defconfig
index f9863b294a70..41a0037f48a5 100644
--- a/arch/arc/configs/nsimosci_hs_smp_defconfig
+++ b/arch/arc/configs/nsimosci_hs_smp_defconfig
@@ -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
diff --git a/arch/arc/configs/tb10x_defconfig b/arch/arc/configs/tb10x_defconfig
index a12656ec0072..d93b65008d4a 100644
--- a/arch/arc/configs/tb10x_defconfig
+++ b/arch/arc/configs/tb10x_defconfig
@@ -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
diff --git a/arch/arc/configs/vdk_hs38_defconfig b/arch/arc/configs/vdk_hs38_defconfig
index d7c858df520c..0c3b21416819 100644
--- a/arch/arc/configs/vdk_hs38_defconfig
+++ b/arch/arc/configs/vdk_hs38_defconfig
@@ -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
diff --git a/arch/arc/configs/vdk_hs38_smp_defconfig b/arch/arc/configs/vdk_hs38_smp_defconfig
index 015c1d43889e..f9ad9d3ee702 100644
--- a/arch/arc/configs/vdk_hs38_smp_defconfig
+++ b/arch/arc/configs/vdk_hs38_smp_defconfig
@@ -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
diff --git a/init/Kconfig b/init/Kconfig
index c7900e8975f1..1b4d8acc3def 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -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
-- 
2.37.0.rc0.161.g10f37bed90-goog


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Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

[Miko Larsson]

On Tuesday, 28 June 2022 23:04:07 CEST Nick Desaulniers wrote:
> 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 15f5db60a137 ("kbuild,arc: add
> CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
> ARCH_CFLAGS
> 
> Ever since
> commit cfdbc2e16e65 ("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.
> 

Might be worth cleaning up the rest of the kernel of instances of -O3,
too. -O3 used to build lz4 and mips vdso, for instance. Might be a bit
of a digression, though

--
~miko



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Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

[Masahiro Yamada]

On Wed, Jun 29, 2022 at 6:06 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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 15f5db60a137 ("kbuild,arc: add
> CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
> ARCH_CFLAGS
>
> Ever since
> commit cfdbc2e16e65 ("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>
> ---

Applied to linux-kbuild.
Thanks.


>  Makefile                                   | 2 --
>  arch/arc/configs/axs101_defconfig          | 1 -
>  arch/arc/configs/axs103_defconfig          | 1 -
>  arch/arc/configs/axs103_smp_defconfig      | 1 -
>  arch/arc/configs/haps_hs_defconfig         | 1 -
>  arch/arc/configs/haps_hs_smp_defconfig     | 1 -
>  arch/arc/configs/hsdk_defconfig            | 1 -
>  arch/arc/configs/nsim_700_defconfig        | 1 -
>  arch/arc/configs/nsimosci_defconfig        | 1 -
>  arch/arc/configs/nsimosci_hs_defconfig     | 1 -
>  arch/arc/configs/nsimosci_hs_smp_defconfig | 1 -
>  arch/arc/configs/tb10x_defconfig           | 1 -
>  arch/arc/configs/vdk_hs38_defconfig        | 1 -
>  arch/arc/configs/vdk_hs38_smp_defconfig    | 1 -
>  init/Kconfig                               | 7 -------
>  15 files changed, 22 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 8973b285ce6c..b69f6cd7f2e2 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -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
> diff --git a/arch/arc/configs/axs101_defconfig b/arch/arc/configs/axs101_defconfig
> index 0016149f9583..e31a8ebc3ecc 100644
> --- a/arch/arc/configs/axs101_defconfig
> +++ b/arch/arc/configs/axs101_defconfig
> @@ -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
> diff --git a/arch/arc/configs/axs103_defconfig b/arch/arc/configs/axs103_defconfig
> index 5b031582a1cf..e0e8567f0d75 100644
> --- a/arch/arc/configs/axs103_defconfig
> +++ b/arch/arc/configs/axs103_defconfig
> @@ -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
> diff --git a/arch/arc/configs/axs103_smp_defconfig b/arch/arc/configs/axs103_smp_defconfig
> index d4eec39e0112..fcbc952bc75b 100644
> --- a/arch/arc/configs/axs103_smp_defconfig
> +++ b/arch/arc/configs/axs103_smp_defconfig
> @@ -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
> diff --git a/arch/arc/configs/haps_hs_defconfig b/arch/arc/configs/haps_hs_defconfig
> index 7337cdf4ffdd..d87ad7e88d62 100644
> --- a/arch/arc/configs/haps_hs_defconfig
> +++ b/arch/arc/configs/haps_hs_defconfig
> @@ -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
> diff --git a/arch/arc/configs/haps_hs_smp_defconfig b/arch/arc/configs/haps_hs_smp_defconfig
> index bc927221afc0..8d82cdb7f86a 100644
> --- a/arch/arc/configs/haps_hs_smp_defconfig
> +++ b/arch/arc/configs/haps_hs_smp_defconfig
> @@ -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
> diff --git a/arch/arc/configs/hsdk_defconfig b/arch/arc/configs/hsdk_defconfig
> index aa000075a575..f856b03e0fb5 100644
> --- a/arch/arc/configs/hsdk_defconfig
> +++ b/arch/arc/configs/hsdk_defconfig
> @@ -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
> diff --git a/arch/arc/configs/nsim_700_defconfig b/arch/arc/configs/nsim_700_defconfig
> index 326f6cde7826..a1ce12bf5b16 100644
> --- a/arch/arc/configs/nsim_700_defconfig
> +++ b/arch/arc/configs/nsim_700_defconfig
> @@ -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
> diff --git a/arch/arc/configs/nsimosci_defconfig b/arch/arc/configs/nsimosci_defconfig
> index bf39a0091679..ca10f4a2c823 100644
> --- a/arch/arc/configs/nsimosci_defconfig
> +++ b/arch/arc/configs/nsimosci_defconfig
> @@ -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
> diff --git a/arch/arc/configs/nsimosci_hs_defconfig b/arch/arc/configs/nsimosci_hs_defconfig
> index 7121bd71c543..31b6ec3683c6 100644
> --- a/arch/arc/configs/nsimosci_hs_defconfig
> +++ b/arch/arc/configs/nsimosci_hs_defconfig
> @@ -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
> diff --git a/arch/arc/configs/nsimosci_hs_smp_defconfig b/arch/arc/configs/nsimosci_hs_smp_defconfig
> index f9863b294a70..41a0037f48a5 100644
> --- a/arch/arc/configs/nsimosci_hs_smp_defconfig
> +++ b/arch/arc/configs/nsimosci_hs_smp_defconfig
> @@ -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
> diff --git a/arch/arc/configs/tb10x_defconfig b/arch/arc/configs/tb10x_defconfig
> index a12656ec0072..d93b65008d4a 100644
> --- a/arch/arc/configs/tb10x_defconfig
> +++ b/arch/arc/configs/tb10x_defconfig
> @@ -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
> diff --git a/arch/arc/configs/vdk_hs38_defconfig b/arch/arc/configs/vdk_hs38_defconfig
> index d7c858df520c..0c3b21416819 100644
> --- a/arch/arc/configs/vdk_hs38_defconfig
> +++ b/arch/arc/configs/vdk_hs38_defconfig
> @@ -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
> diff --git a/arch/arc/configs/vdk_hs38_smp_defconfig b/arch/arc/configs/vdk_hs38_smp_defconfig
> index 015c1d43889e..f9ad9d3ee702 100644
> --- a/arch/arc/configs/vdk_hs38_smp_defconfig
> +++ b/arch/arc/configs/vdk_hs38_smp_defconfig
> @@ -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
> diff --git a/init/Kconfig b/init/Kconfig
> index c7900e8975f1..1b4d8acc3def 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -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
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>


-- 
Best Regards
Masahiro Yamada

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Re: [PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3

[Masahiro Yamada]

On Wed, Jun 29, 2022 at 7:48 PM Miko Larsson <mikoxyzzz@gmail.com> wrote:
>
> On Tuesday, 28 June 2022 23:04:07 CEST Nick Desaulniers wrote:
> > 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 15f5db60a137 ("kbuild,arc: add
> > CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3 for ARC") without re-adding
> > ARCH_CFLAGS
> >
> > Ever since
> > commit cfdbc2e16e65 ("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.
> >
>
> Might be worth cleaning up the rest of the kernel of instances of -O3,
> too. -O3 used to build lz4 and mips vdso, for instance. Might be a bit
> of a digression, though


This patch focuses on the removal of CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3,
so I think it is OK as-is.

The rest of cleanups, if needed,
should be submitted separately.




>
> --
> ~miko
>
>


-- 
Best Regards
Masahiro Yamada

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