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.
Regards,
Ed.
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