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Subject[PATCH] kbuild: drop support for CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
From
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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