From: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Date: Tue, 21 Jun 2022 21:15:53 +0200
> Add a function to the bitmap test suite, which will ensure that
> compilers are able to evaluate operations performed by the
> bitops/bitmap helpers to compile-time constants when all of the
> arguments are compile-time constants as well, or trigger a build
> bug otherwise. This should work on all architectures and all the
> optimization levels supported by Kbuild.
> The function doesn't perform any runtime tests and gets optimized
> out to nothing after passing the build assertions.
>
> Suggested-by: Yury Norov <yury.norov@xxxxxxxxx>
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
> lib/test_bitmap.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 45 insertions(+)
>
> diff --git a/lib/test_bitmap.c b/lib/test_bitmap.c
> index d5923a640457..3a7b09b82794 100644
> --- a/lib/test_bitmap.c
> +++ b/lib/test_bitmap.c
> @@ -869,6 +869,50 @@ static void __init test_bitmap_print_buf(void)
> }
> }
>
> +static void __init test_bitmap_const_eval(void)
> +{
> + DECLARE_BITMAP(bitmap, BITS_PER_LONG);
> + unsigned long initvar = BIT(2);
> + unsigned long bitopvar = 0;
> + unsigned long var = 0;
> + int res;
> +
> + /*
> + * Compilers must be able to optimize all of those to compile-time
> + * constants on any supported optimization level (-O2, -Os) and any
> + * architecture. Otherwise, trigger a build bug.
> + * The whole function gets optimized out then, there's nothing to do
> + * in runtime.
> + */
> +
> + /* Equals to `unsigned long bitmap[1] = { BIT(5), }` */
> + bitmap_clear(bitmap, 0, BITS_PER_LONG);
> + if (!test_bit(7, bitmap))
> + bitmap_set(bitmap, 5, 1);
So for now, when building for s390, Clang (up to the latest Git
snapshot) generates some incorrect code here.
It does expand both test_bit() and bitmap_set() to const_test_bit()
and const___set_bit(), but at the same time thinks that starting
from this point, @bitmap and @bitopvar (???) are *not* constants
and fails the assertions below, which is not true and weird.
Any other architecture + compiler couples work fine, including
s390 on GCC.
So would it be acceptable for now to do:
/* Equals to `unsigned long bitmap[1] = { BIT(5), }` */
bitmap_clear(bitmap, 0, BITS_PER_LONG);
/*
* Some comment saying that this is currently broken
* on s390 + Clang
*/
#if !(defined(__s390__) && defined(__clang__))
if (!test_bit(7, bitmap))
bitmap_set(bitmap, 5, 1);
#endif
/* Equals to `unsigned long bitopvar = BIT(20)` */
__change_bit(31, &bitopvar);
bitmap_shift_right(&bitopvar, &bitopvar, 11, BITS_PER_LONG);
[...]
or there could be any better solutions?
(+Cc LLVM folks)
> +
> + /* Equals to `unsigned long bitopvar = BIT(20)` */
> + __change_bit(31, &bitopvar);
> + bitmap_shift_right(&bitopvar, &bitopvar, 11, BITS_PER_LONG);
> +
> + /* Equals to `unsigned long var = BIT(25)` */
> + var |= BIT(25);
> + if (var & BIT(0))
> + var ^= GENMASK(9, 6);
> +
> + /* __const_hweight<32|64>(BIT(5)) == 1 */
> + res = bitmap_weight(bitmap, 20);
> + BUILD_BUG_ON(!__builtin_constant_p(res));
> +
> + /* !(BIT(31) & BIT(18)) == 1 */
> + res = !test_bit(18, &bitopvar);
> + BUILD_BUG_ON(!__builtin_constant_p(res));
> +
> + /* BIT(2) & GENMASK(14, 8) == 0 */
> + BUILD_BUG_ON(!__builtin_constant_p(initvar & GENMASK(14, 8)));
> + /* ~BIT(25) */
> + BUILD_BUG_ON(!__builtin_constant_p(~var));
> +}
> +
> static void __init selftest(void)
> {
> test_zero_clear();
> @@ -884,6 +928,7 @@ static void __init selftest(void)
> test_for_each_set_clump8();
> test_bitmap_cut();
> test_bitmap_print_buf();
> + test_bitmap_const_eval();
> }
>
> KSTM_MODULE_LOADERS(test_bitmap);
> --
> 2.36.1
Thanks,
Olek
