Re: [PATCH v2 1/2] kbuild: Require a 'make clean' if we detect gcc changed underneath us

[Guenter Roeck <linux@roeck-us.net>]

On Fri, Dec 22, 2017 at 02:14:53PM -0800, Douglas Anderson wrote:
> Several people reported that the commit 3298b690b21c ("kbuild: Add a
> cache for generated variables") caused them problems when they updated
> gcc versions.  Specifically the reports all looked something similar
> to this:
> 
> > In file included from ./include/uapi/linux/uuid.h:21:0,
> >                  from ./include/linux/uuid.h:19,
> >                  from ./include/linux/mod_devicetable.h:12,
> >                  from scripts/mod/devicetable-offsets.c:2:
> > ./include/linux/string.h:8:20: fatal error: stdarg.h: No such file or
> >                  directory
> >  #include <stdarg.h>
> 
> Masahiro Yamada determined that the problem was with:
> 
>   NOSTDINC_FLAGS += -nostdinc -isystem $(call shell-cached,$(CC)
>   -print-file-name=include)
> 
> Specifically that the stale result of -print-file-name is stored in
> the cache file.  It was determined that a "make clean" fixed the
> problems in all cases.
> 
> In this particular case we could certainly try to clean just the cache
> when we detect a gcc update, but it seems like overall it's a bad idea
> to do an incremental build when gcc changes.  We should warn the user
> and tell them that they need a 'make clean'.
> 
> Fixes: 3298b690b21c ("kbuild: Add a cache for generated variables")
> Reported-by: Yang Shi <yang.s@alibaba-inc.com>
> Reported-by: Dave Hansen <dave.hansen@intel.com>
> Reported-by: Mathieu Malaterre <malat@debian.org>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

No idea if it is correct, but this version works for me.

Tested-by: Guenter Roeck <linux@roeck-us.net>

> ---
> 
> Changes in v2:
> - Don't error if MAKECMDGOALS is blank.
> 
>  scripts/Kbuild.include | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index 065324a8046f..f7efb59d85d1 100644
> --- a/scripts/Kbuild.include
> +++ b/scripts/Kbuild.include
> @@ -222,6 +222,10 @@ cc-version = $(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-version.
>  cc-fullversion = $(call shell-cached,$(CONFIG_SHELL) \
>  	$(srctree)/scripts/gcc-version.sh -p $(CC))
>  
> +# cc-fullversion-uncached
> +cc-fullversion-uncached := $(shell $(CONFIG_SHELL) \
> +	$(srctree)/scripts/gcc-version.sh -p $(CC))
> +
>  # cc-ifversion
>  # Usage:  EXTRA_CFLAGS += $(call cc-ifversion, -lt, 0402, -O1)
>  cc-ifversion = $(shell [ $(cc-version) $(1) $(2) ] && echo $(3) || echo $(4))
> @@ -475,3 +479,16 @@ endif
>  endef
>  #
>  ###############################################################################
> +
> +# Require a 'make clean' if the compiler changed; not only does the .cache.mk
> +# need to be thrown out but we should also start with fresh object files.
> +#
> +# NOTE: it's important that we don't error out when the goal is actually to
> +# try to make clean, distclean or mrproper.
> +ifeq ($(filter %clean,$(MAKECMDGOALS))$(filter mrproper,$(MAKECMDGOALS)),)
> +  ifneq ($(MAKECMDGOALS),)
> +    ifneq ($(cc-fullversion-uncached),$(cc-fullversion))
> +      $(error Detected new CC version ($(cc-fullversion-uncached) vs $(cc-fullversion)).  Please 'make clean')
> +    endif
> +  endif
> +endif
> -- 
> 2.15.1.620.gb9897f4670-goog
>