Re: CONFIG_CC_IS_CLANG

[Nathan Chancellor <natechancellor@gmail.com>]

On Thu, Feb 07, 2019 at 01:03:01PM -0800, Nick Desaulniers wrote:
> Masahiro,
> I was looking into sprucing up Kbuild for improved LLD support.  One
> thing I was curious about was the different checks for clang in the
> top level Makefile.
> 
> How come a change like:
> diff --git a/Makefile b/Makefile
> index 3142e67d03f1..525b3fef7d27 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -489,7 +489,7 @@ ifneq ($(KBUILD_SRC),)
>   $(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile $(srctree)
>  endif
> 
> -ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
> +ifdef CONFIG_CC_IS_CLANG
>  ifneq ($(CROSS_COMPILE),)
>  CLANG_FLAGS := --target=$(notdir $(CROSS_COMPILE:%-=%))
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(LD)))
> 
> 
> doesn't work?  It's odd that later on in the top level Makefile, we
> have that kind of check.  I was hoping to do something similar for
> LLD, but it seems that the config options are not defined for that
> particular make target?  Does this have to do with config-targets?
> 
> -- 
> Thanks,
> ~Nick Desaulniers

According to commit 995167420797 ("kbuild: remove cc-name variable"), it
says that this section of the Makefile runs before Kconfig. Masahiro can
obviously give more context and explanation.

Nathan