Re: linux-next: build warning after merge of the kbuild tree

[Michal Marek <mmarek@suse.cz>]

On Mon, May 02, 2011 at 02:53:38PM +1000, Stephen Rothwell wrote:
> Hi Dave,
> 
> hmmm,
> 
> On my build machine:
> $ /opt/cross/gcc-4.4.5-nolibc/x86_64-linux/bin/x86_64-linux-gcc -Wno-unused-but-set-variable -c -xc /dev/null -o /tmp/xx.o; echo $?
> 0
> $ /opt/cross/gcc-4.4.5-nolibc/x86_64-linux/bin/x86_64-linux-gcc -Wunused-but-set-variable -c -xc /dev/null -o /tmp/xx.o; echo $?
> cc1: error: unrecognized command line option "-Wunused-but-set-variable"
> 1
> $ gcc -Wno-unused-but-set-variable -c -xc /dev/null -o /tmp/xx.o; echo $?
> 0
> $ gcc -Wunused-but-set-variable -c -xc /dev/null -o /tmp/xx.o; echo $?
> cc1: error: unrecognized command line option "-Wunused-but-set-variable"
> 1

I reproduced this myself with 4.5.1 and 4.6.0 and was about to file a
bug at gcc.gnu.org, but I learned that this is a feature :-(, see here:
http://gcc.gnu.org/PR28322 . It treats invalid -Wno-* options as
warnings and it only issues them when there is another warning or error
in the source.

I have a patch below, but after learning the above, I'm going to rewrite
it to some more generic cc-disable-warning fuction, because it can hit
us anytime again.

Michal

Subject: [PATCH] kbuild: Fix passing -Wno-unused-but-set-variable to some gcc versions

Some versions of gcc will happily accept -Wno-<anything> in the
cc-option test and complain later when compiling some random file that
-Wno-unused-but-set-variable is not recognized. Work around this by
testing if gcc supports the -Wunused-but-set-variable option insteaed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michal Marek <mmarek@suse.cz>

diff --git a/Makefile b/Makefile
index 4527dc2..db6b6df 100644
--- a/Makefile
+++ b/Makefile
@@ -569,7 +569,12 @@ endif
 
 # This warning generated too much noise in a regular build.
 # Use make W=1 to enable this warning (see scripts/Makefile.build)
-KBUILD_CFLAGS += $(call cc-option, -Wno-unused-but-set-variable)
+# We test for support of the -Wunused-but-set-variable option, because
+# some version of gcc will happily accept any -Wno-* switch and complain
+# later during build.
+ifeq ($(call cc-option-yn, -Wunused-but-set-variable), y)
+KBUILD_CFLAGS += -Wno-unused-but-set-variable
+endif
 
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls