On Tue, Feb 11, 2020 at 02:01:29PM +0100, Vasily Gorbik wrote: > On Mon, Feb 10, 2020 at 08:55:46AM +0100, Christian Borntraeger wrote: > > > > > > On 08.02.20 13:57, Nathan Chancellor wrote: > > > Hi all, > > > > > > We noticed that you all added support for building s390 with clang, > > > which is great! I have noticed a few warnings for which I will send > > > patches but this one has me stumped. > > > > > > In file included from ../lib/crypto/sha256.c:16: > > > In file included from ../include/linux/module.h:13: > > > In file included from ../include/linux/stat.h:19: > > > In file included from ../include/linux/time.h:6: > > > In file included from ../include/linux/seqlock.h:36: > > > In file included from ../include/linux/spinlock.h:51: > > > In file included from ../include/linux/preempt.h:78: > > > In file included from ../arch/s390/include/asm/preempt.h:6: > > > In file included from ../include/linux/thread_info.h:38: > > > In file included from ../arch/s390/include/asm/thread_info.h:26: > > > ../arch/s390/include/asm/page.h:45:6: warning: converting the result of '<<' to a boolean always evaluates to false [-Wtautological-constant-compare] > > > if (PAGE_DEFAULT_KEY) > > > ^ > > > ../arch/s390/include/asm/page.h:23:44: note: expanded from macro 'PAGE_DEFAULT_KEY' > > > #define PAGE_DEFAULT_KEY (PAGE_DEFAULT_ACC << 4) > > > ^ > > > 1 warning generated. > > This warning only shows up for the decompressor code and purgatory which > have separate set of build flags not derived from top level KBUILD_CFLAGS. > For the rest of the code this warning is suppressed by: > Makefile: > 740 ifdef CONFIG_CC_IS_CLANG > ... > 744 # Quiet clang warning: comparison of unsigned expression < 0 is always false > 745 KBUILD_CFLAGS += -Wno-tautological-compare > > At the same time both decompressor and purgatory Makefiles include > CLANG_FLAGS into their CFLAGS. And this -Wno-tautological-compare is > clang specific. So I believe this option belongs to CLANG_FLAGS > rather than being included into KBUILD_CFLAGS under ifdef > CONFIG_CC_IS_CLANG. But this raises question about other clang > specific options inside that ifdef CONFIG_CC_IS_CLANG. Should they all > be made part of CLANG_FLAGS? Hi Vasily, I am trying to turn on -Wtautological-compare for the kernel as a whole, hence me trying to deal with this one now :) That flag controls a bunch of useful subwarnings that can point out potentially problematic code. I think that it would be worth adding warnings that we want disabled in all code to CLANG_FLAGS but as of right now, this is the only instance of this warning that I see within the s390 code so it is probably just worth silencing with an explicit comparison (!= 0). I will send a patch for this later. Cheers, Nathan
