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Makefile: remove stale cc-option checks
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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 GCC only and supported since at least GCC 4.9.
Remove cc-option and cc-disable-warning tests.
* -fno-tree-loop-im
* -Wno-maybe-uninitialized
* -fno-reorder-blocks
* -fno-ipa-cp-clone
* -fno-partial-inlining
* -femit-struct-debug-baseonly
* -fno-inline-functions-called-once
* -fconserve-stack

The following flags are supported by all supported versions of GCC and
Clang. Remove their cc-option, cc-option-yn, and cc-disable-warning tests.
* -fno-delete-null-pointer-checks
* -fno-var-tracking
* -Wno-array-bounds

The following configs are made dependent on GCC, since they use GCC
specific flags.
* READABLE_ASM
* DEBUG_SECTION_MISMATCH

-mfentry was not supported by s390-linux-gnu-gcc until gcc-9+, add a
comment.

--param=allow-store-data-races=0 was renamed to -fno-allow-store-data-races
in the GCC 10 release; add a comment.

-Wmaybe-uninitialized (GCC specific) was being added for CONFIG_GCOV,
then again unconditionally; add it only once.

Also, base RETPOLINE_CFLAGS and RETPOLINE_VDSO_CFLAGS on CONFIC_CC_IS_*
then remove cc-option tests for Clang.

Link: ClangBuiltLinux/linux#1436
Acked-by: Miguel Ojeda <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
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@nickdesaulniers @masahir0y
nickdesaulniers authored and masahir0y committed Sep 2, 2021
1 parent 36f1386 commit 7d73c3e
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Showing 2 changed files with 33 additions and 19 deletions.
50 changes: 31 additions & 19 deletions Makefile
View file
Original file line number Diff line number Diff line change
Expand Up @@ -669,22 +669,25 @@ endif # KBUILD_EXTMOD
# Defaults to vmlinux, but the arch makefile usually adds further targets
all: vmlinux

CFLAGS_GCOV := -fprofile-arcs -ftest-coverage \
$(call cc-option,-fno-tree-loop-im) \
$(call cc-disable-warning,maybe-uninitialized,)
CFLAGS_GCOV := -fprofile-arcs -ftest-coverage
ifdef CONFIG_CC_IS_GCC
CFLAGS_GCOV += -fno-tree-loop-im
endif
export CFLAGS_GCOV

# The arch Makefiles can override CC_FLAGS_FTRACE. We may also append it later.
ifdef CONFIG_FUNCTION_TRACER
CC_FLAGS_FTRACE := -pg
endif

RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
RETPOLINE_VDSO_CFLAGS_GCC := -mindirect-branch=thunk-inline -mindirect-branch-register
RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk
RETPOLINE_VDSO_CFLAGS_CLANG := -mretpoline
RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG)))
RETPOLINE_VDSO_CFLAGS := $(call cc-option,$(RETPOLINE_VDSO_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_VDSO_CFLAGS_CLANG)))
ifdef CONFIG_CC_IS_GCC
RETPOLINE_CFLAGS := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
RETPOLINE_VDSO_CFLAGS := $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
endif
ifdef CONFIG_CC_IS_CLANG
RETPOLINE_CFLAGS := -mretpoline-external-thunk
RETPOLINE_VDSO_CFLAGS := -mretpoline
endif
export RETPOLINE_CFLAGS
export RETPOLINE_VDSO_CFLAGS

Expand Down Expand Up @@ -737,7 +740,7 @@ include/config/auto.conf:
endif # may-sync-config
endif # need-config

KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks,)
KBUILD_CFLAGS += -fno-delete-null-pointer-checks
KBUILD_CFLAGS += $(call cc-disable-warning,frame-address,)
KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation)
KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow)
Expand All @@ -752,17 +755,19 @@ KBUILD_CFLAGS += -Os
endif

# Tell gcc to never replace conditional load with a non-conditional one
ifdef CONFIG_CC_IS_GCC
# gcc-10 renamed --param=allow-store-data-races=0 to
# -fno-allow-store-data-races.
KBUILD_CFLAGS += $(call cc-option,--param=allow-store-data-races=0)
KBUILD_CFLAGS += $(call cc-option,-fno-allow-store-data-races)
endif

ifdef CONFIG_READABLE_ASM
# Disable optimizations that make assembler listings hard to read.
# reorder blocks reorders the control in the function
# ipa clone creates specialized cloned functions
# partial inlining inlines only parts of functions
KBUILD_CFLAGS += $(call cc-option,-fno-reorder-blocks,) \
$(call cc-option,-fno-ipa-cp-clone,) \
$(call cc-option,-fno-partial-inlining)
KBUILD_CFLAGS += -fno-reorder-blocks -fno-ipa-cp-clone -fno-partial-inlining
endif

ifneq ($(CONFIG_FRAME_WARN),0)
Expand Down Expand Up @@ -854,8 +859,10 @@ DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
endif

ifdef CONFIG_DEBUG_INFO_REDUCED
DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
$(call cc-option,-fno-var-tracking)
DEBUG_CFLAGS += -fno-var-tracking
ifdef CONFIG_CC_IS_GCC
DEBUG_CFLAGS += -femit-struct-debug-baseonly
endif
endif

ifdef CONFIG_DEBUG_INFO_COMPRESSED
Expand Down Expand Up @@ -889,6 +896,7 @@ ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
endif
endif
ifdef CONFIG_HAVE_FENTRY
# s390-linux-gnu-gcc did not support -mfentry until gcc-9.
ifeq ($(call cc-option-yn, -mfentry),y)
CC_FLAGS_FTRACE += -mfentry
CC_FLAGS_USING += -DCC_USING_FENTRY
Expand All @@ -901,7 +909,7 @@ endif

# We trigger additional mismatches with less inlining
ifdef CONFIG_DEBUG_SECTION_MISMATCH
KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
KBUILD_CFLAGS += -fno-inline-functions-called-once
endif

ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
Expand Down Expand Up @@ -980,14 +988,16 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)

# We'll want to enable this eventually, but it's not going away for 5.7 at least
KBUILD_CFLAGS += $(call cc-disable-warning, zero-length-bounds)
KBUILD_CFLAGS += $(call cc-disable-warning, array-bounds)
KBUILD_CFLAGS += -Wno-array-bounds
KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)

# Another good warning that we'll want to enable eventually
KBUILD_CFLAGS += $(call cc-disable-warning, restrict)

# Enabled with W=2, disabled by default as noisy
KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += -Wno-maybe-uninitialized
endif

# disable invalid "can't wrap" optimizations for signed / pointers
KBUILD_CFLAGS += -fno-strict-overflow
Expand All @@ -996,7 +1006,9 @@ KBUILD_CFLAGS += -fno-strict-overflow
KBUILD_CFLAGS += -fno-stack-check

# conserve stack if available
KBUILD_CFLAGS += $(call cc-option,-fconserve-stack)
ifdef CONFIG_CC_IS_GCC
KBUILD_CFLAGS += -fconserve-stack
endif

# Prohibit date/time macros, which would make the build non-deterministic
KBUILD_CFLAGS += -Werror=date-time
Expand Down
2 changes: 2 additions & 0 deletions lib/Kconfig.debug
View file
Original file line number Diff line number Diff line change
Expand Up @@ -365,6 +365,7 @@ config STRIP_ASM_SYMS
config READABLE_ASM
bool "Generate readable assembler code"
depends on DEBUG_KERNEL
depends on CC_IS_GCC
help
Disable some compiler optimizations that tend to generate human unreadable
assembler output. This may make the kernel slightly slower, but it helps
Expand All @@ -383,6 +384,7 @@ config HEADERS_INSTALL

config DEBUG_SECTION_MISMATCH
bool "Enable full Section mismatch analysis"
depends on CC_IS_GCC
help
The section mismatch analysis checks if there are illegal
references from one section to another section.
Expand Down

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