Re: [PATCH] Makefile: remove stale cc-option checks

From: Nathan Chancellor
Date: Tue Aug 10 2021 - 19:01:42 EST


On Tue, Aug 10, 2021 at 01:42:37PM -0700, Nick Desaulniers wrote:
> 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
> * -mfentry
> * -Wno-array-bounds
>
> The following configs are made dependent on GCC, since they use GCC
> specific flags.
> * READABLE_ASM
> * DEBUG_SECTION_MISMATCH
>
> --param=allow-store-data-races=0 was renamed to --allow-store-data-races
> in the GCC 10 release.
>
> Also, base RETPOLINE_CFLAGS and RETPOLINE_VDSO_CFLAGS on CONFIC_CC_IS_*
> then remove cc-option tests for Clang.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1436
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Fewer pointless calls to the compiler is always a good thing :)

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Small comments inline.

> ---
> Note: It may be preferred to move the test for
> -fno-inline-functions-called-once for DEBUG_SECTION_MISMATCH into
> Kconfig. That one does seem relatively more reasonable to implement in
> Clang.
>
> Makefile | 55 ++++++++++++++++++++++++++---------------------
> lib/Kconfig.debug | 2 ++
> 2 files changed, 33 insertions(+), 24 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 027fdf2a14fe..3e3fb4affba1 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -730,9 +730,10 @@ 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

Eliminating -Wno-maybe-uninitialized might warrant a comment in the
commit message as I was initially confused then I realized that it is
unconditionally added later.

> export CFLAGS_GCOV
>
> # The arch Makefiles can override CC_FLAGS_FTRACE. We may also append it later.
> @@ -740,12 +741,14 @@ 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
>
> @@ -798,7 +801,7 @@ 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)
> @@ -844,17 +847,17 @@ KBUILD_RUSTFLAGS += -Copt-level=z

Looks like this was generated against -next as it includes the rust
patchset. I was able to apply it to mainline without any complaints but
any follow ups should probably be based on Masahiro's tree.

> endif
>
> # Tell gcc to never replace conditional load with a non-conditional one
> -KBUILD_CFLAGS += $(call cc-option,--param=allow-store-data-races=0)
> +ifdef CONFIG_CC_IS_GCC
> +KBUILD_CFLAGS += $(call cc-option,--allow-store-data-races,--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)
> @@ -959,8 +962,10 @@ DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
> endif
>
> ifdef CONFIG_DEBUG_INFO_REDUCED
> -DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
> - $(call cc-option,-fno-var-tracking)
> +DEBUG_CFLAGS += -fno-var-tracking
> +ifdef CONFIG_CC_IS_GCC
> +DEBUG_CFLAGS += -femit-struct-debug-baseonly
> +endif
> endif
>
> ifdef CONFIG_DEBUG_INFO_COMPRESSED
> @@ -997,10 +1002,8 @@ ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
> endif
> endif
> ifdef CONFIG_HAVE_FENTRY
> - ifeq ($(call cc-option-yn, -mfentry),y)
> - CC_FLAGS_FTRACE += -mfentry
> - CC_FLAGS_USING += -DCC_USING_FENTRY
> - endif
> + CC_FLAGS_FTRACE += -mfentry
> + CC_FLAGS_USING += -DCC_USING_FENTRY
> endif
> export CC_FLAGS_FTRACE
> KBUILD_CFLAGS += $(CC_FLAGS_FTRACE) $(CC_FLAGS_USING)
> @@ -1009,7 +1012,7 @@ 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
> @@ -1088,14 +1091,16 @@ 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
> @@ -1104,7 +1109,9 @@ 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
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index b6b951b0ed46..a4a431606be2 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -364,6 +364,7 @@ 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
> @@ -382,6 +383,7 @@ 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.
> --
> 2.32.0.605.g8dce9f2422-goog