On Tuesday, 25 January 2022 at 22:33:37 UTC, H. S. Teoh wrote:
interesting because idivl is known to be one of the slower
instructions, but gdc nevertheless considered it not worthwhile
to replace it, whereas ldc seems obsessed about avoid idivl at
all costs.
Interesting indeed. Two remarks:
1. Actual performance cost of div depends a lot on hardware.
IIRC on my old intel laptop it's like 40-60 cycles; on my newer
amd chip it's more like 20; on my mac it's ~10. GCC may be
assuming newer hardware than llvm. Could be worth popping on a
-march=native -mtune=native. Also could depend on how many ports
can do divs; i.e. how many of them you can have running at a time.
2. LLVM is more aggressive wrt certain optimizations than gcc, by
default. Though I don't know how relevant that is at -O3.
