On 2/18/23 11:26, Palmer Dabbelt wrote:
On Fri, 17 Feb 2023 06:02:40 PST (-0800), gcc-patches@gcc.gnu.org wrote:
Hi all,
If we have division and remainder calculations with the same operands:
a = b / c;
d = b % c;
We can replace the calculation of remainder with multiplication +
subtraction, using the result from the previous division:
a = b / c;
d = a * c;
d = b - d;
Which will be faster.
Do you have any benchmarks that show that performance increase? The ISA
manual specifically says the suggested sequence is div+mod, and while
those suggestions don't always pan out for real hardware it's likely
that at least some implementations will end up with the ISA-suggested
fusions.
It'll almost certainly be visible in mcf. Been there, done that. In
fact, that's why I asked the team Matevos works on to poke at this case
as I went through this issue on another processor.
It can also be run through LLVM's MCA to estimate counts if you've got a
pipeline description. THe div+rem will come out at around ~40c while a
div+mul+sub should weigh in around 25c for Veyron v1.
Jeff
