https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70676
Bug ID: 70676
Summary: suboptimal code generation on AVR
Product: gcc
Version: 5.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: night_ghost at ykoctpa dot ru
Target Milestone: ---
gcc -Os -fno-optimize-sibling-calls
don't optimizes out tail recursion:
return SPI::transfer(0xff);
f54:8f ef ldi r24, 0xFF; 255
f56:0e 94 d9 2e call 0x5db2 ; 0x5db2 <_ZN3SPI8transferEh>
}
f5a:08 95 ret
where it should be
ldi r24, 0xFF; 255
jmp 0x5db2 ; 0x5db2 <_ZN3SPI8transferEh>
Yes I can remove -fno-optimize-sibling-calls but then size of compiled project
will be much more:
with: 30566 bytes (93.3% Full)
without: 30772 bytes and binary don't fit to flash (30720 is maximum)
Reason is that even with -Os optimize-sibling-calls tries to make epilogue for
each tail recursion:
2080:<----->92 e0 <-->ldi<--->r25, 0x02<----->; 2
2082:<----->df 91 <-->pop<--->r29
2084:<----->cf 91 <-->pop<--->r28
2086:<----->1f 91 <-->pop<--->r17
2088:<----->0f 91 <-->pop<--->r16
208a:<----->ff 90 <-->pop<--->r15
208c:<----->ef 90 <-->pop<--->r14
208e:<----->bf 90 <-->pop<--->r11
2090:<----->af 90 <-->pop<--->r10
2092:<----->9f 90 <-->pop<--->r9
2094:<----->8f 90 <-->pop<--->r8
2096:<----->0c 94 2a 0f <-->jmp<--->0x1e54<>; 0x1e54
<_ZL12osd_printf_1PKcf>
(rjmp here)
209a:<----->df 91 <-->pop<--->r29
209c:<----->cf 91 <-->pop<--->r28
209e:<----->1f 91 <-->pop<--->r17
20a0:<----->0f 91 <-->pop<--->r16
20a2:<----->ff 90 <-->pop<--->r15
20a4:<----->ef 90 <-->pop<--->r14
20a6:<----->bf 90 <-->pop<--->r11
20a8:<----->af 90 <-->pop<--->r10
20aa:<----->9f 90 <-->pop<--->r9
20ac:<----->8f 90 <-->pop<--->r8
20ae:<----->08 95 <-->ret
Can GCC in -Os really optimize only size and rollback optimizations if size of
"optimized" code is more than size of non-optimized ?
