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linux-stable/arch/loongarch/Makefile
WANG Xuerui 38b10b269d LoongArch: Tweak CFLAGS for Clang compatibility 
Now the arch code is mostly ready for LLVM/Clang consumption, it is time
to re-organize the CFLAGS a little to actually enable the LLVM build.
Namely, all -G0 switches from CFLAGS are removed, and -mexplicit-relocs
and -mdirect-extern-access are now wrapped with cc-option (with the
related asm/percpu.h definition guarded against toolchain combos that
are known to not work).

A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU
environment; support for the two features are currently blocked on
LLVM/Clang, and will come later.

Why -G0 can be removed:

In GCC, -G stands for "small data threshold", that instructs the
compiler to put data smaller than the specified threshold in a dedicated
"small data" section (called .sdata on LoongArch and several other
arches).

However, benefiting from this would require ABI cooperation, which is
not the case for LoongArch; and current GCC behave the same whether -G0
(equal to disabling this optimization) is given or not. So, remove -G0
from CFLAGS altogether for one less thing to care about. This also
benefits LLVM/Clang compatibility where the -G switch is not supported.

Why -mexplicit-relocs can now be conditionally applied without
regressions:

Originally -mexplicit-relocs is unconditionally added to CFLAGS in case
of CONFIG_AS_HAS_EXPLICIT_RELOCS, because not having it (i.e. old GCC +
new binutils) would not work: modules will have R_LARCH_ABS_* relocs
inside, but given the rarity of such toolchain combo in the wild, it may
not be worthwhile to support it, so support for such relocs in modules
were not added back when explicit relocs support was upstreamed, and
-mexplicit-relocs is unconditionally added to fail the build early.

Now that Clang compatibility is desired, given Clang is behaving like
-mexplicit-relocs from day one but without support for the CLI flag, we
must ensure the flag is not passed in case of Clang. However, explicit
compiler flavor checks can be more brittle than feature detection: in
this case what actually matters is support for __attribute__((model))
when building modules. Given neither older GCC nor current Clang support
this attribute, probing for the attribute support and #error'ing out
would allow proper UX without checking for Clang, and also automatically
work when Clang support for the attribute is to be added in the future.

Why -mdirect-extern-access is now conditionally applied:

This is actually a nice-to-have optimization that can reduce GOT
accesses, but not having it is harmless either. Because Clang does not
support the option currently, but might do so in the future, conditional
application via cc-option ensures compatibility with both current and
future Clang versions.

Suggested-by: Xi Ruoyao <xry111@xry111.site> # cc-option changes
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
2023-06-29 20:58:43 +08:00

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Makefile
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# SPDX-License-Identifier: GPL-2.0
#
# Author: Huacai Chen <chenhuacai@loongson.cn>
# Copyright (C) 2020-2022 Loongson Technology Corporation Limited
boot := arch/loongarch/boot
KBUILD_DEFCONFIG := loongson3_defconfig
image-name-y := vmlinux
image-name-$(CONFIG_EFI_ZBOOT) := vmlinuz
ifndef CONFIG_EFI_STUB
KBUILD_IMAGE := $(boot)/vmlinux.elf
else
KBUILD_IMAGE := $(boot)/$(image-name-y).efi
endif
#
# Select the object file format to substitute into the linker script.
#
64bit-tool-archpref = loongarch64
32bit-bfd = elf32-loongarch
64bit-bfd = elf64-loongarch
32bit-emul = elf32loongarch
64bit-emul = elf64loongarch
ifdef CONFIG_DYNAMIC_FTRACE
KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
CC_FLAGS_FTRACE := -fpatchable-function-entry=2
endif
ifdef CONFIG_64BIT
tool-archpref = $(64bit-tool-archpref)
UTS_MACHINE := loongarch64
endif
ifneq ($(SUBARCH),$(ARCH))
ifeq ($(CROSS_COMPILE),)
CROSS_COMPILE := $(call cc-cross-prefix, $(tool-archpref)-linux- $(tool-archpref)-linux-gnu- $(tool-archpref)-unknown-linux-gnu-)
endif
endif
ifdef CONFIG_64BIT
ld-emul = $(64bit-emul)
cflags-y += -mabi=lp64s
endif
cflags-y += -pipe -msoft-float
LDFLAGS_vmlinux += -static -n -nostdlib
# When the assembler supports explicit relocation hint, we must use it.
# GCC may have -mexplicit-relocs off by default if it was built with an old
# assembler, so we force it via an option.
#
# When the assembler does not supports explicit relocation hint, we can't use
# it. Disable it if the compiler supports it.
#
# The combination of a "new" assembler and "old" GCC is not supported, given
# the rarity of this combo and the extra complexity needed to make it work.
# Either upgrade the compiler or downgrade the assembler; the build will error
# out if it is the case (by probing for the model attribute; all supported
# compilers in this case would have support).
#
# Also, -mdirect-extern-access is useful in case of building with explicit
# relocs, for avoiding unnecessary GOT accesses. It is harmless to not have
# support though.
ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
cflags-y += $(call cc-option,-mexplicit-relocs)
KBUILD_CFLAGS_KERNEL += $(call cc-option,-mdirect-extern-access)
else
cflags-y += $(call cc-option,-mno-explicit-relocs)
KBUILD_AFLAGS_KERNEL += -Wa,-mla-global-with-pcrel
KBUILD_CFLAGS_KERNEL += -Wa,-mla-global-with-pcrel
KBUILD_AFLAGS_MODULE += -Wa,-mla-global-with-abs
KBUILD_CFLAGS_MODULE += -fplt -Wa,-mla-global-with-abs,-mla-local-with-abs
endif
ifeq ($(CONFIG_RELOCATABLE),y)
KBUILD_CFLAGS_KERNEL += -fPIE
LDFLAGS_vmlinux += -static -pie --no-dynamic-linker -z notext
endif
cflags-y += -ffreestanding
cflags-y += $(call cc-option, -mno-check-zero-division)
load-y = 0x9000000000200000
bootvars-y = VMLINUX_LOAD_ADDRESS=$(load-y)
drivers-$(CONFIG_PCI) += arch/loongarch/pci/
KBUILD_AFLAGS += $(cflags-y)
KBUILD_CFLAGS += $(cflags-y)
KBUILD_CPPFLAGS += -DVMLINUX_LOAD_ADDRESS=$(load-y)
# This is required to get dwarf unwinding tables into .debug_frame
# instead of .eh_frame so we don't discard them.
KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
ifdef CONFIG_ARCH_STRICT_ALIGN
# Don't emit unaligned accesses.
# Not all LoongArch cores support unaligned access, and as kernel we can't
# rely on others to provide emulation for these accesses.
KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
else
# Optimise for performance on hardware supports unaligned access.
KBUILD_CFLAGS += $(call cc-option,-mno-strict-align)
endif
KBUILD_CFLAGS += -isystem $(shell $(CC) -print-file-name=include)
KBUILD_LDFLAGS += -m $(ld-emul)
ifdef CONFIG_LOONGARCH
CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
grep -E -vw '__GNUC_(MINOR_|PATCHLEVEL_)?_' | \
sed -e "s/^\#define /-D'/" -e "s/ /'='/" -e "s/$$/'/" -e 's/\$$/&&/g')
endif
libs-y += arch/loongarch/lib/
libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
drivers-y += arch/loongarch/crypto/
# suspend and hibernation support
drivers-$(CONFIG_PM) += arch/loongarch/power/
ifeq ($(KBUILD_EXTMOD),)
prepare: vdso_prepare
vdso_prepare: prepare0
$(Q)$(MAKE) $(build)=arch/loongarch/vdso include/generated/vdso-offsets.h
endif
PHONY += vdso_install
vdso_install:
$(Q)$(MAKE) $(build)=arch/loongarch/vdso $@
all: $(notdir $(KBUILD_IMAGE))
vmlinux.elf vmlinux.efi vmlinuz.efi: vmlinux
$(Q)$(MAKE) $(build)=$(boot) $(bootvars-y) $(boot)/$@
install:
$(Q)install -D -m 755 $(KBUILD_IMAGE) $(INSTALL_PATH)/$(image-name-y)-$(KERNELRELEASE)
$(Q)install -D -m 644 .config $(INSTALL_PATH)/config-$(KERNELRELEASE)
$(Q)install -D -m 644 System.map $(INSTALL_PATH)/System.map-$(KERNELRELEASE)
define archhelp
echo ' install - install kernel into $(INSTALL_PATH)'
echo
endef
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