https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80929
Bug ID: 80929
Summary: [7/8 Regression] Division with constant no more
optimized to mult highpart
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
Since PR79665, division with known denominator are no more optimized to
__umulhisi3 but transfomed to an expensive signed division instead.
unsigned scale255 (unsigned val)
{
return val / 255;
}
$ avr-gcc -O2 -mmcu=atmega328 -S ...
Reason is that PR79655 uses rtlanal.c::seq_cost() to compute the cost of
(un)signed division, and seq_cost assumes anything that's not a single_set has
the very low cost of 1.
However avr BE, represents division as a PARALLEL, not as a single_set, i.e.
something like:
(insn 14 13 0 (parallel [
(set (reg:HI 52)
(div:HI (reg:HI 47)
(reg:HI 54)))
(set (reg:HI 53)
(mod:HI (reg:HI 47)
(reg:HI 54)))
(clobber (reg:QI 21 r21))
(clobber (reg:HI 22 r22))
(clobber (reg:HI 24 r24))
(clobber (reg:HI 26 r26))
]) "scale.c":7 -1
(nil))
