On 4 July 2014 18:13, Edward Nevill <edward.nev...@linaro.org> wrote:
> Hi,
>
> Does anyone know of a nice way to detect 64 x 64 -> 64 multiply overflow on
> aarch64?
>
> On x86 you can just use the V flag, but aarch64 seems to have no option to
> set the flags after a MUL instruction which I find very odd.
>
> The only solution I can think of is to do the equivalent of
>
> r = a * b;
> if (r/a != b) overflow();
>
> There is some short circuiting you can do to avoid the div if you know there
> is no possibility it could have overflowed.
>
> Eg.
>
> if (clz(labs(a)) + clz(labs(b)) < 32) {
> r = a * b;
> if (r/a != b) overflow();
> }
>
> Then there is the special case of a = 0x8000000000000000 and b == -1.
>
> All in all its a mess just to detect overflow.
>
If you are saying AArch64, I am assuming you can use inline asm and umulh, no?
--
Ard.
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