On Wed, 2024-03-27 at 08:54 +0100, Richard Biener wrote:
> On Tue, Mar 26, 2024 at 10:52 AM Xi Ruoyao <xry...@xry111.site> wrote:
> >
> > The latency of LA464 and LA664 division instructions depends on the
> > input. When I updated the costs in r14-6642, I unintentionally set the
> > division costs to the best-case latency (when the first operand is 0).
> > Per a recent discussion [1] we should use "something sensible" instead
> > of it.
> >
> > Use the average of the minimum and maximum latency observed instead.
> > This enables multiplication to reciprocal sequence reduction and speeds
> > up the following test case for about 30%:
> >
> > int
> > main (void)
> > {
> > unsigned long stat = 0xdeadbeef;
> > for (int i = 0; i < 100000000; i++)
> > stat = (stat * stat + stat * 114514 + 1919810) % 1000000007;
> > asm(""::"r"(stat));
> > }
>
> I think you should be able to see a constant divisor and thus could do
> better than return the same latency for everything. For non-constant
> divisors using the best-case latency shouldn't be a problem.
Hmm, it seems not really possible as at now. expand_divmod does
something like:
max_cost = (unsignedp
? udiv_cost (speed, compute_mode)
: sdiv_cost (speed, compute_mode));
which is reading the pre-calculated costs from a table. Thus we don't
really know the denominator and cannot estimate the cost based on it :(.
CSE really invokes the cost hook with the actual (mod (a, (const_int
1000000007)) RTX but it's less important.
--
Xi Ruoyao <xry...@xry111.site>
School of Aerospace Science and Technology, Xidian University
