On 05/23/2017 09:42 AM, Stefan Koch wrote:
On Tuesday, 23 May 2017 at 13:27:42 UTC, Andrei Alexandrescu wrote:
On 5/22/17 4:51 PM, Johan Engelen wrote:
On Monday, 22 May 2017 at 15:05:24 UTC, Andrei Alexandrescu wrote:
[...]

A fun read!

"(Late at night, I double checked. Mozilla’s CheckedInt is just as bad as I remembered. They do a division to test for multiplication overflow. Come on, put a line of assembler in there! Portability is worth a price, just not any price.)"

Shocked: do you use assembly in Checked and cripple the optimizer?!?!
Luckily, no. But LDC and GDC do create the `seto` instruction I think you were hinting at:
https://godbolt.org/g/0jUhgs

(LDC doesn't do as good as it could, https://github.com/ldc-developers/ldc/issues/2131)

Thanks! Yes, seto is what I thought of - one way or another, it gets down to using a bit of machine-specific code to get there. I'll note that dmd does not generate seto (why?): https://goo.gl/nRjNMy. -- Andrei

it does this
overflow_flag = 0
op
if (overflowed)
{
   overflow_flag = 1;
}

Where did you see this pattern? Couldn't find it anywhere in core.checkedint. And how is "overflowed" tested?

this can in some circumstances be faster then using seto!
If the inliner does a good enough job :)

The code in core.checkedint is conservative:

pragma(inline, true)
ulong mulu(ulong x, ulong y, ref bool overflow)
{
    ulong r = x * y;
    if (x && (r / x) != y)
        overflow = true;
    return r;
}

The compiler is supposed to detect the pattern and generate optimal code.


Andrei

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