linux-kernel - 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...

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Message-ID: <YYXNfJFjdjT9x3+D@hirez.programming.kicks-ass.net>
Date:   Sat, 6 Nov 2021 01:34:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org
Subject: 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...

On Fri, Nov 05, 2021 at 02:06:22PM -0700, Nick Desaulniers wrote:
> On Fri, Nov 5, 2021 at 5:21 AM Peter Zijlstra <peterz@...radead.org> wrote:

> > 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?

They are, for FIELD_PREP() as is the case here, _mask is u32 while _reg
is typeof(0ULL). In my reading of the code this is on purpose to
basically disable the test for this caller.