http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60486
Bug ID: 60486
Summary: [avr] missed optimization on detecting zero flag set
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: darryl.piper at gmail dot com
detection of a variable being decremented to reach zero missed optimization.
int main(uint16_t, uint16_t );
int main(uint16_t x, uint16_t y) {
uint16_t z = x;
while (x > y) {
if ( --z == 0 ) return 1;
x++;
}
return 0;
}
produces with gcc 4.8.0 and 4.8.1 and I expect 4.8.2 as well.
compiled with -Os
the code at 0x82 to 0x8a uses a compare against zero, when the subi and sbc,
leave the zero flag set on a atmega8.
7a: 9c 01 movw r18, r24
7c: 68 17 cp r22, r24
7e: 79 07 cpc r23, r25
80: 38 f4 brcc .+14 ; 0x90 <main+0x16>
82: 21 50 subi r18, 0x01 ; 1
84: 31 09 sbc r19, r1
86: 21 15 cp r18, r1
88: 31 05 cpc r19, r1
8a: 29 f0 breq .+10 ; 0x96 <main+0x1c>
8c: 01 96 adiw r24, 0x01 ; 1
8e: f6 cf rjmp .-20 ; 0x7c <main+0x2>
90: 80 e0 ldi r24, 0x00 ; 0
92: 90 e0 ldi r25, 0x00 ; 0
94: 08 95 ret
96: 81 e0 ldi r24, 0x01 ; 1
98: 90 e0 ldi r25, 0x00 ; 0
9a: 08 95 ret
in gcc/config/avr/avr.md the code for (define_insn "add<mode>3" no longer says
it alters the zero of negative flag which the 4.7.2 branch did depending on
which choice of add & adc or sub & sbc choice it used.
