> Am 03.02.2024 um 09:46 schrieb Jakub Jelinek <ja...@redhat.com>:
>
> Hi!
>
> Since bswap has been converted from a method to a function we miscompile
> the following testcase. The problem is the assumption that the passed in
> len argument (number of limbs in the xval array) is the upper bound for the
> bswap result, which is true only if precision is <= 64. If precision is
> larger than that, e.g. 128 as in the testcase, if the argument has only
> one limb (i.e. 0 to ~(unsigned HOST_WIDE_INT) 0), the result can still
> need 2 limbs for that precision, or generally BLOCKS_NEEDED (precision)
> limbs, it all depends on how many least significant limbs of the operand
> are zero. bswap_large as implemented only cleared len limbs of result,
> then swapped the bytes (invoking UB when oring something in all the limbs
> above it) and finally passed len to canonize, saying that more limbs
> aren't needed.
>
> The following patch fixes it by renaming len to xlen (so that it is clear
> it is X's length), using it solely for safe_uhwi argument when we attempt
> to read from X, and using new len = BLOCKS_NEEDED (precision) instead in
> the other two spots (i.e. when clearing the val array, turned it also
> into memset, and in canonize argument). wi::bswap asserts it isn't invoked
> on widest_int, so we are always invoked on wide_int or similar and those
> have preallocated result sized for the corresponding precision (i.e.
> BLOCKS_NEEDED (precision)).
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Ok
Richard
> 2024-02-03 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/113722
> * wide-int.cc (wi::bswap_large): Rename third argument from
> len to xlen and adjust use in safe_uhwi. Add len variable, set
> it to BLOCKS_NEEDED (precision) and use it for clearing of val
> and as canonize argument. Clear val using memset instead of
> a loop.
>
> * gcc.dg/pr113722.c: New test.
>
> --- gcc/wide-int.cc.jj 2024-01-03 11:51:42.077584823 +0100
> +++ gcc/wide-int.cc 2024-02-02 18:13:34.993332159 +0100
> @@ -729,20 +729,19 @@ wi::set_bit_large (HOST_WIDE_INT *val, c
> }
> }
>
> -/* Byte swap the integer represented by XVAL and LEN into VAL. Return
> +/* Byte swap the integer represented by XVAL and XLEN into VAL. Return
> the number of blocks in VAL. Both XVAL and VAL have PRECISION bits. */
> unsigned int
> wi::bswap_large (HOST_WIDE_INT *val, const HOST_WIDE_INT *xval,
> - unsigned int len, unsigned int precision)
> + unsigned int xlen, unsigned int precision)
> {
> - unsigned int i, s;
> + unsigned int s, len = BLOCKS_NEEDED (precision);
>
> /* This is not a well defined operation if the precision is not a
> multiple of 8. */
> gcc_assert ((precision & 0x7) == 0);
>
> - for (i = 0; i < len; i++)
> - val[i] = 0;
> + memset (val, 0, sizeof (unsigned HOST_WIDE_INT) * len);
>
> /* Only swap the bytes that are not the padding. */
> for (s = 0; s < precision; s += 8)
> @@ -753,7 +752,7 @@ wi::bswap_large (HOST_WIDE_INT *val, con
> unsigned int block = s / HOST_BITS_PER_WIDE_INT;
> unsigned int offset = s & (HOST_BITS_PER_WIDE_INT - 1);
>
> - byte = (safe_uhwi (xval, len, block) >> offset) & 0xff;
> + byte = (safe_uhwi (xval, xlen, block) >> offset) & 0xff;
>
> block = d / HOST_BITS_PER_WIDE_INT;
> offset = d & (HOST_BITS_PER_WIDE_INT - 1);
> --- gcc/testsuite/gcc.dg/pr113722.c.jj 2024-02-02 18:25:22.702561427 +0100
> +++ gcc/testsuite/gcc.dg/pr113722.c 2024-02-02 18:21:00.109186858 +0100
> @@ -0,0 +1,22 @@
> +/* PR middle-end/113722 */
> +/* { dg-do run { target int128 } } */
> +/* { dg-options "-O2" } */
> +
> +int
> +main ()
> +{
> + unsigned __int128 a = __builtin_bswap128 ((unsigned __int128) 2);
> + if (a != ((unsigned __int128) 2) << 120)
> + __builtin_abort ();
> + a = __builtin_bswap128 ((unsigned __int128) 0xdeadbeefULL);
> + if (a != ((unsigned __int128) 0xefbeaddeULL) << 96)
> + __builtin_abort ();
> + a = __builtin_bswap128 (((unsigned __int128) 0xdeadbeefULL) << 64);
> + if (a != ((unsigned __int128) 0xefbeaddeULL) << 32)
> + __builtin_abort ();
> + a = __builtin_bswap128 ((((unsigned __int128) 0xdeadbeefULL) << 64)
> + | 0xcafed00dfeedbac1ULL);
> + if (a != ((((unsigned __int128) 0xc1baedfe0dd0fecaULL) << 64)
> + | (((unsigned __int128) 0xefbeaddeULL) << 32)))
> + __builtin_abort ();
> +}
>
> Jakub
>
