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

[Douglas Anderson <dianders@chromium.org>]

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>
---

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