[Bug libc/14455] roundup macro bug

["johan.bolmsjo at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=14455

Johan Bolmsjö <johan.bolmsjo at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |johan.bolmsjo at gmail dot com

--- Comment #2 from Johan Bolmsjö <johan.bolmsjo at gmail dot com> ---
// The roundup macro from /usr/include/sys/param.h (misc/sys/param.h in
// git repository) produce different results in the GCC and non-GCC
// versions for some input. This is why I would consider this a bug.
// Also only the fast path of the macro contains the "bug" so it
// will produce different results based on if y is a constant or not.

// Compile this program using:
//    gcc -std=c99 -O2 -Wall -g -o roundup_glibc_bug roundup_glibc_bug.c

/*
Code block in sys/param.h looks like this:

#ifdef __GNUC__
# define roundup(x, y)  (__builtin_constant_p (y) && powerof2 (y)     \
                         ? (((x) + (y) - 1) & ~((y) - 1))             \
                         : ((((x) + ((y) - 1)) / (y)) * (y)))
#else
# define roundup(x, y)  ((((x) + ((y) - 1)) / (y)) * (y))
#endif
*/

// Both macros has been made available with _gcc and _non_gcc postfix to
// be able to illustrate the issue.

# define roundup_gcc(x, y)  (__builtin_constant_p (y) && powerof2 (y) \
                         ? (((x) + (y) - 1) & ~((y) - 1))             \
                         : ((((x) + ((y) - 1)) / (y)) * (y)))

# define roundup_non_gcc(x, y)  ((((x) + ((y) - 1)) / (y)) * (y))

#define powerof2(x) ((((x) - 1) & (x)) == 0)

#include <inttypes.h>
#include <stdio.h>

int main()
{
    uint64_t       value    = 0x100000003;
    const unsigned multiple = 8;

    printf("gcc      roundup(0x%"PRIx64", %u) => %"PRIx64"\n",
           value, multiple, roundup_gcc(value, multiple));

    printf("non-gcc  roundup(0x%"PRIx64", %u) => %"PRIx64"\n",
           value, multiple, roundup_non_gcc(value, multiple));

    uint32_t       value2    = 0x000000003;
    const uint64_t multiple2 = 0x100000000;

    printf("gcc      roundup(%u, 0x%"PRIx64") => %"PRIx64"\n",
           value2, multiple2, roundup_gcc(value2, multiple2));

    printf("non-gcc  roundup(%u, 0x%"PRIx64") => %"PRIx64"\n",
           value2, multiple2, roundup_non_gcc(value2, multiple2));

    // Output:
    // gcc      roundup(0x100000003, 8) => 8
    // non-gcc  roundup(0x100000003, 8) => 100000008
    // gcc      roundup(3, 0x100000000) => 100000000
    // non-gcc  roundup(3, 0x100000000) => 100000000

    return 0;
}

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