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

[Masahiro Yamada <masahiroy@kernel.org>]

On Wed, Aug 11, 2021 at 5:42 AM Nick Desaulniers
<ndesaulniers@google.com> 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@google.com>
> ---
> 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
>  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
>  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


Can you insert a comment here?

# GCC 10 renamed --param=allow-store-data-races=0 to --allow-store-data-races


It will remind us of dropping this conditional
in the (long long distant) future.




> +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
>

-- 
Best Regards
Masahiro Yamada