Re: [PATCH] wide-int: Fix up wi::bswap_large [PR113722]

[Richard Biener <richard.guenther@gmail.com>]

> Am 03.02.2024 um 09:46 schrieb Jakub Jelinek <jakub@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  <jakub@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
>