https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115084
Bug ID: 115084
Summary: Missed optimization in division for AVR target, not
using __*divmodpsi4
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: kamkaz at windowslive dot com
Target Milestone: ---
In case the width of both nominator and denominator in integer division are
known to be <=24 bits, in AVR code generation it would be much more efficient
to use psi (24-bit) routines, instead of si (32bit) routines.
avr-gcc -O2
#include <stdint.h>
uint16_t division(uint16_t den) {
return 7500000u/den;
}
The code generated will use __udivmodsi4, despite __udivmodpsi4 being suitable.
00000000 <division>:
0: 28 2f mov r18, r24
2: 39 2f mov r19, r25
4: 40 e0 ldi r20, 0x00 ; 0
6: 50 e0 ldi r21, 0x00 ; 0
8: 60 ee ldi r22, 0xE0 ; 224
a: 70 e7 ldi r23, 0x70 ; 112
c: 82 e7 ldi r24, 0x72 ; 114
e: 90 e0 ldi r25, 0x00 ; 0
10: 03 d0 rcall .+6 ; 0x18 <__udivmodsi4>
12: 82 2f mov r24, r18
14: 93 2f mov r25, r19
16: 08 95 ret
Would it be considered to add an optimization, that uses *psi routines in such
cases? I expect it to be worthwhile, since division is rather computation
heavy, It would save ~250 cycles per division where applicable, while the added
division routine occupies 54 bytes.
A workaround is to cast to (__uint24) before dividing, but the __uint24
extension is not widely known, and one would expect such an optimization to
trigger by itself.
