linux-kernel - Re: [PATCH next v2 5/5] minmax: Relax check to allow comparison between int and small unsigned constants.

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Message-ID: <CAKwvOdm6ZVSMpzSJ0tFMR_rVb1+=REk8ETLS2SBe3s8QrbYUQw@mail.gmail.com>
Date:   Tue, 1 Aug 2023 09:02:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [PATCH next v2 5/5] minmax: Relax check to allow comparison
 between int and small unsigned constants.

On Mon, Jul 31, 2023 at 6:27 AM David Laight <David.Laight@...lab.com> wrote:
>
> By the time I've done:
>
> #define __is_noneg_int(x)                                       \
>         __builtin_choose_expr(!__is_constexpr(x), false,        \
>                 ((x) >= (typeof(x))0 && (x) <= (typeof((x) + 0))(long)__INT_MAX__))
>
> #define __is_signed(x)                                                         \
>        __builtin_choose_expr(__is_constexpr(is_signed_type(typeof(x))),        \
>                is_signed_type(typeof(x)), 0)
>
> #define __types_ok(x, y)                                        \
>         (__is_signed(x) == __is_signed(y) ||                    \
>                 __is_signed((x) + 0) == __is_signed((y) + 0) || \
>                 __is_noneg_int(x) || __is_noneg_int(y))
>
> the error message for
>
> >       static_assert(__types_ok(x, y),                 \
> >               #op "(" #x ", " #y ") signedness error, fix types or consider " #op "_unsigned() before " #op "_t()"); \
>
> generated by clang 8.0.0 and later is similar to (see https://godbolt.org/z/jq613Gnsa):
>
> <source>:49:12: error: static assertion failed due to requirement '__builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((int)(-1)) < (int)1)) * 0L)) : (int *)8))), (((int)(-1)) < (int)1), 0) == __builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((unsigned int)(-1)) < (unsigned int)1)) * 0L)) : (int *)8))), (((unsigned int)(-1)) < (unsigned int)1), 0) || __builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((int)(-1)) < (int)1)) * 0L)) : (int *)8))), (((int)(-1)) < (int)1), 0) == __builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((unsigned int)(-1)) < (unsigned int)1)) * 0L)) : (int *)8))), (((unsigned int)(-1)) < (unsigned int)1), 0) || (__builtin_choose_expr(!(sizeof(int) == sizeof (*(8 ? ((void *)((long)(a) * 0L)) : (int *)8))), 0, __builtin_choose_expr(__builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((int)(-1)) < (int)1)) * 0L)) : (int *)8))), (((int)(-1)) < (int)1), 0), a, 0) >= 0 && (a) <= (int)(long)2147483647)) || (__builtin_choose_expr(!(sizeof(int) == sizeof (*(8 ? ((void *)((long)(2147483648U - 0) * 0L)) : (int *)8))), 0, __builtin_choose_expr(__builtin_choose_expr((sizeof(int) == sizeof (*(8 ? ((void *)((long)((((unsigned int)(-1)) < (unsigned int)1)) * 0L)) : (int *)8))), (((unsigned int)(-1)) < (unsigned int)1), 0), 2147483648U - 0, 0) >= 0 && (2147483648U - 0) <= (unsigned int)(long)2147483647))': min(a, 0x80000000u - 0) signedness error, fix types or consider min_unsigned() before min_t()
>
> Repeating the expression seems somewhat sub-optimal!
> Surely it shouldn't be outputting the expansion of the
> input when an error message is supplied?
>
> Is there any (sane) way to stop it being that verbose?

No, but we can probably change that in clang. Filed:
https://github.com/llvm/llvm-project/issues/64310

>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)



-- 
Thanks,
~Nick Desaulniers