Re: [peterz-queue:x86/wip.extable 1/22] drivers/crypto/ccree/cc_driver.c:117:18: warning: result of comparison of constant 18446744073709551615 with expression of type 'typeof (_Generic((mask), char: (unsigned char)0, unsigned char: (unsigned char)0, signed char: (unsigned char)0, unsigned shor...

From: Nick Desaulniers
Date: Fri Nov 05 2021 - 17:06:39 EST

On Fri, Nov 5, 2021 at 5:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Nov 05, 2021 at 06:17:44PM +0800, kernel test robot wrote:
> > tree: https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/wip.extable
> > head: a4d149db054a77aa35b04bb088f749cbb9a2edea
> > commit: 4b9e954553a4137f7504e068438851c27edc6b22 [1/22] bitfield.h: Fix "type of reg too small for mask" test
> > config: riscv-randconfig-r032-20211105 (attached as .config)
> > compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project 847a6807332b13f43704327c2d30103ec0347c77)
> > reproduce (this is a W=1 build):
> > wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > chmod +x ~/bin/make.cross
> > # install riscv cross compiling tool for clang build
> > # apt-get install binutils-riscv64-linux-gnu
> > # https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?id=4b9e954553a4137f7504e068438851c27edc6b22
> > git remote add peterz-queue https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git
> > git fetch --no-tags peterz-queue x86/wip.extable
> > git checkout 4b9e954553a4137f7504e068438851c27edc6b22
> > # save the attached .config to linux build tree
> > COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 ARCH=riscv
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> >
> > All warnings (new ones prefixed by >>):
> >
> > >> drivers/crypto/ccree/cc_driver.c:117:18: warning: result of comparison of constant 18446744073709551615 with expression of type 'typeof (_Generic((mask), char: (unsigned char)0, unsigned char: (unsigned char)0, signed char: (unsigned char)0, unsigned short: (unsigned short)0, short: (unsigned short)0, unsigned int: (unsigned int)0, int: (unsigned int)0, unsigned long: (unsigned long)0, long: (unsigned long)0, unsigned long long: (unsigned long long)0, long long: (unsigned long long)0, default: (mask)))' (aka 'unsigned int') is always false [-Wtautological-constant-out-of-range-compare]
> > cache_params |= FIELD_PREP(mask, val);
> > ^~~~~~~~~~~~~~~~~~~~~
> > include/linux/bitfield.h:111:3: note: expanded from macro 'FIELD_PREP'
> > __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/bitfield.h:68:53: note: expanded from macro '__BF_FIELD_CHECK'
> > BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> > include/linux/build_bug.h:39:58: note: expanded from macro 'BUILD_BUG_ON_MSG'
> > #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> > ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
> > include/linux/compiler_types.h:322:22: note: expanded from macro 'compiletime_assert'
> > _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> > ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/compiler_types.h:310:23: note: expanded from macro '_compiletime_assert'
> > __compiletime_assert(condition, msg, prefix, suffix)
> > ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > include/linux/compiler_types.h:302:9: note: expanded from macro '__compiletime_assert'
> > if (!(condition)) \
> > ^~~~~~~~~
>
> I'm confused here.. the code reads:
>
>
> #define __scalar_type_to_unsigned_cases(type) \
> unsigned type: (unsigned type)0, \
> signed type: (unsigned type)0
>
> #define __unsigned_scalar_typeof(x) typeof( \
> _Generic((x), \
> char: (unsigned char)0, \
> __scalar_type_to_unsigned_cases(char), \
> __scalar_type_to_unsigned_cases(short), \
> __scalar_type_to_unsigned_cases(int), \
> __scalar_type_to_unsigned_cases(long), \
> __scalar_type_to_unsigned_cases(long long), \
> default: (x)))
>
> #define __bf_cast_unsigned(type, x) ((__unsigned_scalar_typeof(type))(x))
>
> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
> ({ \
> BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
> _pfx "mask is not constant"); \
> BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
> BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
> ~((_mask) >> __bf_shf(_mask)) & (_val) : 0, \
> _pfx "value too large for the field"); \
> BUILD_BUG_ON_MSG(__bf_cast_unsigned(_mask, _mask) > \
> __bf_cast_unsigned(_reg, ~0ull), \
> _pfx "type of reg too small for mask"); \
> __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \
> (1ULL << __bf_shf(_mask))); \
> })
>
> #define FIELD_PREP(_mask, _val) \
> ({ \
> __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
> ((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask); \
> })
>
>
> static void init_cc_cache_params(struct cc_drvdata *drvdata)
> {
> struct device *dev = drvdata_to_dev(drvdata);
> u32 cache_params, ace_const, val, mask;
>
> ...
>
> mask = CC_GENMASK(CC_AXIM_CACHE_PARAMS_AWCACHE); <-- *BOOM*
> cache_params &= ~mask;
> cache_params |= FIELD_PREP(mask, val);
>
> ...
> }
>
> So we're having "(unsigned int)mask > (unsigned long long)~0ull" as
> a compile time constant input to the BUILD_BUG_ON_MSG(), and clang-14 is
> now complaining it's a constant ?!?

No, the error message (trimmed for clarity):
>> warning: result of comparison of constant 18446744073709551615 (ie ~0ull / ULLONG_MAX) with expression of type ... 'unsigned int') is always false [-Wtautological-constant-out-of-range-compare]

Is `val` an int, but should be a long long? We're comparing if an
unsigned int is greater than ULLONG_MAX, which is impossible (or
rather a tautology; something that's always either true or false, in
this case false). Or maybe mask and reg are different widths?

>
> It is that by design.
>


--
Thanks,
~Nick Desaulniers