Re: [PATCH] x86/build: remove stale cc-option checks

From: Nathan Chancellor
Date: Wed Aug 11 2021 - 15:22:57 EST

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On 8/11/2021 10:56 AM, Nick Desaulniers wrote:

cc-option, __cc-option, cc-option-yn, and cc-disable-warning all invoke
the compiler during build time, and can slow down the build when these
checks become stale for our supported compilers, whose minimally
supported versions increases over time. See
Documentation/process/changes.rst for the current supported minimal
versions (GCC 4.9+, clang 10.0.1+). Compiler version support for these
flags may be verified on godbolt.org.

The following flags are supported by all supported versions of GCC and
Clang. Remove their cc-option, __cc-option, and cc-option-yn tests.
* -Wno-address-of-packed-member
* -mno-avx
* -m32
* -mno-80387
* -march=k8
* -march=nocona
* -march=core2
* -march=atom
* -mtune=generic
* -mfentry

-mpreferred-stack-boundary= is specific to GCC, while
-mstack-alignment= is specific to Clang. Rather than test for this three
times via cc-option and __cc-option, rely on CONFIG_CC_IS_* from
Kconfig.

Link: https://github.com/ClangBuiltLinux/linux/issues/1436
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

A few comments below.

---
arch/x86/Makefile | 51 +++++++++++++++++------------------------------
1 file changed, 18 insertions(+), 33 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 0fa7dc73b5d8..6de2985ba1a5 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -14,10 +14,13 @@ endif
# For gcc stack alignment is specified with -mpreferred-stack-boundary,
# clang has the option -mstack-alignment for that purpose.
+ifdef CONFIG_CC_IS_GCC
ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)

Is this cc-option call still needed? It looks like it was added by commit d77698df39a5 ("x86/build: Specify stack alignment for clang"), which at the time claimed to support back to gcc 3.2, but that flag appears to be supported at least back to gcc 4.1.2 according to godbolt.

cc_stack_align4 := -mpreferred-stack-boundary=2
cc_stack_align8 := -mpreferred-stack-boundary=3
-else ifneq ($(call cc-option, -mstack-alignment=16),)
+endif
+endif
+ifdef CONFIG_CC_IS_CLANG
cc_stack_align4 := -mstack-alignment=4
cc_stack_align8 := -mstack-alignment=8
endif
@@ -31,8 +34,8 @@ REALMODE_CFLAGS := -m16 -g -Os -DDISABLE_BRANCH_PROFILING \
REALMODE_CFLAGS += -ffreestanding
REALMODE_CFLAGS += -fno-stack-protector
-REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
-REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
+REALMODE_CFLAGS += -Wno-address-of-packed-member
+REALMODE_CFLAGS += $(cc_stack_align4)
REALMODE_CFLAGS += $(CLANG_FLAGS)
export REALMODE_CFLAGS
@@ -48,8 +51,7 @@ export BITS
#
# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
#
-KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
-KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
+KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
# Intel CET isn't enabled in the kernel
KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
@@ -59,9 +61,8 @@ ifeq ($(CONFIG_X86_32),y)
UTS_MACHINE := i386
CHECKFLAGS += -D__i386__
- biarch := $(call cc-option,-m32)
- KBUILD_AFLAGS += $(biarch)
- KBUILD_CFLAGS += $(biarch)
+ KBUILD_AFLAGS += -m32
+ KBUILD_CFLAGS += -m32
KBUILD_CFLAGS += -msoft-float -mregparm=3 -freg-struct-return
@@ -72,7 +73,7 @@ ifeq ($(CONFIG_X86_32),y)
# Align the stack to the register width instead of using the default
# alignment of 16 bytes. This reduces stack usage and the number of
# alignment instructions.
- KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align4))
+ KBUILD_CFLAGS += $(cc_stack_align4)
# CPU-specific tuning. Anything which can be shared with UML should go here.
include $(srctree)/arch/x86/Makefile_32.cpu

What tree were you intending to target with this? If it is -tip, this is going to cause it to fail to apply because the '$(srctree)' comes from Masahiro:

https://git.kernel.org/masahiroy/linux-kbuild/c/d8285639550578a1bf2d102391d1a9e08e0586ca

@@ -93,7 +94,6 @@ else
UTS_MACHINE := x86_64
CHECKFLAGS += -D__x86_64__
- biarch := -m64
KBUILD_AFLAGS += -m64
KBUILD_CFLAGS += -m64
@@ -104,7 +104,7 @@ else
KBUILD_CFLAGS += $(call cc-option,-falign-loops=1)
# Don't autogenerate traditional x87 instructions
- KBUILD_CFLAGS += $(call cc-option,-mno-80387)
+ KBUILD_CFLAGS += -mno-80387
KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
# By default gcc and clang use a stack alignment of 16 bytes for x86.
@@ -114,20 +114,17 @@ else
# default alignment which keep the stack *mis*aligned.
# Furthermore an alignment to the register width reduces stack usage
# and the number of alignment instructions.
- KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align8))
+ KBUILD_CFLAGS += $(cc_stack_align8)
# Use -mskip-rax-setup if supported.
KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
# FIXME - should be integrated in Makefile.cpu (Makefile_32.cpu)
- cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
- cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)
-
- cflags-$(CONFIG_MCORE2) += \
- $(call cc-option,-march=core2,$(call cc-option,-mtune=generic))
- cflags-$(CONFIG_MATOM) += $(call cc-option,-march=atom) \
- $(call cc-option,-mtune=atom,$(call cc-option,-mtune=generic))
- cflags-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=generic)
+ cflags-$(CONFIG_MK8) += -march=k8
+ cflags-$(CONFIG_MPSC) += -march=nocona
+ cflags-$(CONFIG_MCORE2) += -march=core2
+ cflags-$(CONFIG_MATOM) += -march=atom
+ cflags-$(CONFIG_GENERIC_CPU) += -mtune=generic
KBUILD_CFLAGS += $(cflags-y)
KBUILD_CFLAGS += -mno-red-zone
@@ -158,18 +155,6 @@ export CONFIG_X86_X32_ABI
ifdef CONFIG_FUNCTION_GRAPH_TRACER
ifndef CONFIG_HAVE_FENTRY
ACCUMULATE_OUTGOING_ARGS := 1
- else
- ifeq ($(call cc-option-yn, -mfentry), n)
- ACCUMULATE_OUTGOING_ARGS := 1
-
- # GCC ignores '-maccumulate-outgoing-args' when used with '-Os'.
- # If '-Os' is enabled, disable it and print a warning.
- ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
- undefine CONFIG_CC_OPTIMIZE_FOR_SIZE
- $(warning Disabling CONFIG_CC_OPTIMIZE_FOR_SIZE. Your compiler does not have -mfentry so you cannot optimize for size with CONFIG_FUNCTION_GRAPH_TRACER.)
- endif
-
- endif
endif
endif
@@ -193,7 +178,7 @@ ifdef CONFIG_RETPOLINE
# only been fixed starting from gcc stable version 8.4.0 and
# onwards, but not for older ones. See gcc bug #86952.
ifndef CONFIG_CC_IS_CLANG
- KBUILD_CFLAGS += $(call cc-option,-fno-jump-tables)
+ KBUILD_CFLAGS += -fno-jump-tables
endif
endif

Next message: Jiri Olsa: "Re: [PATCH v4 1/2] perf pmu: Add PMU alias support"
Previous message: andrey . konovalov: "[PATCH 5/8] kasan: test: only do kmalloc_uaf_memset for generic mode"
In reply to: Nick Desaulniers: "[PATCH] x86/build: remove stale cc-option checks"
Next in thread: Nick Desaulniers: "Re: [PATCH] x86/build: remove stale cc-option checks"
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