[PATCH] Overflow-trapping integer arithmetic routines7code

[Stefan Kanthak]

Jeff Law wrote Wednesday, November 25, 2020 7:11 PM:

> On 11/25/20 6:18 AM, Stefan Kanthak wrote:
>> Jeff Law <law@redhat.com> wrote:

[...]

>>> My inclination is to leave the overflow checking double-word multiplier
>>> as-is.
>> See but <https://gcc.gnu.org/pipermail/gcc/2020-October/234048.html> ff.
> Already read and considered it. 
>>
>>> Though I guess you could keep the same structure as the existing
>>> implementation which tries to avoid unnecessary multiplies and still use
>>> the __builtin_{add,mul}_overflow to simplify the code a bit less
>>> aggressively.

Only relying on GCC to generate efficient code for __builtin_mul_overflow,
this should be either

DWtype
__mulvDI3 (DWtype u, DWtype v)
{
    DWtype w;
    if (__builtin_mul_overflow (u, v, &w))
        abort ();
    return w;
}

or

DWtype
__mulvDI3 (DWtype u, DWtype v)
{
    UDWtype w, s = 0 - (u < 0), t = 0 - (v < 0);
    if (__builtin_mul_overflow ((u ^ s) - s, (v ^ t) - t, &w))
        abort ();
    s ^= t;
    w = (w ^ s) - s;
    if ((DWtype) (w ^ s) < 0)
        abort ();
    return w;
}

if the latter produces better machine code (which it does at least for
i386 and AMD64).

> Instead keep the tests that detect the special cases that don't need as
> many multiplies and use the overflow builtins within that implementation
> framework. In cases where we can use the operands directly, that's
> helpful as going through the struct/union likely leads to unnecessary
> register shuffling.

regards
Stefan