[PATCH] kbuild: use -S instead of -E for precise cc-option test in Kconfig

[PATCH] kbuild: use -S instead of -E for precise cc-option test in Kconfig

[Masahiro Yamada]

Currently, -E (stop after the preprocessing stage) is used to check
whether the given compiler flag is supported.

While it is faster than -S (or -c), it can be false-positive. You need
to run the compilation proper to check the flag more precisely.

For example, when testing "--param asan-instrument-allocas=1", my gcc
gives a different result for -E vs -S.

$ gcc -Werror --param asan-instrument-allocas=1 -E -x c /dev/null -o /dev/null
$ echo $?
0

$ gcc -Werror --param asan-instrument-allocas=1 -S -x c /dev/null -o /dev/null
cc1: error: invalid --param name ‘asan-instrument-allocas’; did you mean ‘asan-instrument-writes’?
$ echo $?
1

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/Kconfig.include | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index d4adfbe42690..bfb44b265a94 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -25,7 +25,7 @@ failure = $(if-success,$(1),n,y)
 
 # $(cc-option,<flag>)
 # Return y if the compiler supports <flag>, n otherwise
-cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -E -x c /dev/null -o /dev/null)
+cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -S -x c /dev/null -o /dev/null)
 
 # $(ld-option,<flag>)
 # Return y if the linker supports <flag>, n otherwise
-- 
2.17.1

Re: [PATCH] kbuild: use -S instead of -E for precise cc-option test in Kconfig

[Masahiro Yamada]

On Sat, Jan 18, 2020 at 2:16 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> Currently, -E (stop after the preprocessing stage) is used to check
> whether the given compiler flag is supported.
>
> While it is faster than -S (or -c), it can be false-positive. You need
> to run the compilation proper to check the flag more precisely.
>
> For example, when testing "--param asan-instrument-allocas=1", my gcc
> gives a different result for -E vs -S.
>
> $ gcc -Werror --param asan-instrument-allocas=1 -E -x c /dev/null -o /dev/null
> $ echo $?
> 0
>
> $ gcc -Werror --param asan-instrument-allocas=1 -S -x c /dev/null -o /dev/null
> cc1: error: invalid --param name ‘asan-instrument-allocas’; did you mean ‘asan-instrument-writes’?
> $ echo $?
> 1
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---

Applied to linux-kbuild.


>  scripts/Kconfig.include | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
> index d4adfbe42690..bfb44b265a94 100644
> --- a/scripts/Kconfig.include
> +++ b/scripts/Kconfig.include
> @@ -25,7 +25,7 @@ failure = $(if-success,$(1),n,y)
>
>  # $(cc-option,<flag>)
>  # Return y if the compiler supports <flag>, n otherwise
> -cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -E -x c /dev/null -o /dev/null)
> +cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) -S -x c /dev/null -o /dev/null)
>
>  # $(ld-option,<flag>)
>  # Return y if the linker supports <flag>, n otherwise
> --
> 2.17.1
>


-- 
Best Regards
Masahiro Yamada