https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90706
Bug ID: 90706
Summary: Useless code generated for stack / register operations
on AVR
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: bseifert at gmx dot at
Target Milestone: ---
given the following function:
unsigned char check(float x)
{
return (0.0 < x);
}
in avr-gcc 8.3.0 the following code is generated:
00000098 <_Z5checkf>:
98: cf 93 push r28
9a: c1 e0 ldi r28, 0x01 ; 1
9c: 20 e0 ldi r18, 0x00 ; 0
9e: 30 e0 ldi r19, 0x00 ; 0
a0: a9 01 movw r20, r18
a2: 0e 94 a8 00 call 0x150 ; 0x150 <__gesf2>
a6: 18 16 cp r1, r24
a8: 0c f0 brlt .+2 ; 0xac <_Z5checkf+0x14>
aa: c0 e0 ldi r28, 0x00 ; 0
ac: 8c 2f mov r24, r28
ae: cf 91 pop r28
b0: 08 95 ret
I don't see any room for improvements here. avr-gcc 9.1.0 compiles to the
following. I've marked the lines that don't make sense to me.
00000098 <_Z5checkf>:
98: cf 93 push r28
9a: df 93 push r29
* 9c: 00 d0 rcall .+0 ; 0x9e <_Z5checkf+0x6>
* 9e: 00 d0 rcall .+0 ; 0xa0 <_Z5checkf+0x8>
* a0: 0f 92 push r0
* a2: cd b7 in r28, 0x3d ; 61
* a4: de b7 in r29, 0x3e ; 62
a6: 21 e0 ldi r18, 0x01 ; 1
a8: 2d 83 std Y+5, r18 ; 0x05
aa: 20 e0 ldi r18, 0x00 ; 0
ac: 30 e0 ldi r19, 0x00 ; 0
ae: a9 01 movw r20, r18
* b0: 69 83 std Y+1, r22 ; 0x01
* b2: 7a 83 std Y+2, r23 ; 0x02
* b4: 8b 83 std Y+3, r24 ; 0x03
* b6: 9c 83 std Y+4, r25 ; 0x04
* b8: 69 81 ldd r22, Y+1 ; 0x01
* ba: 7a 81 ldd r23, Y+2 ; 0x02
* bc: 8b 81 ldd r24, Y+3 ; 0x03
* be: 9c 81 ldd r25, Y+4 ; 0x04
c0: 0e 94 dc 00 call 0x1b8 ; 0x1b8 <__gesf2>
c4: 18 16 cp r1, r24
c6: 0c f0 brlt .+2 ; 0xca <_Z5checkf+0x32>
c8: 1d 82 std Y+5, r1 ; 0x05
ca: 8d 81 ldd r24, Y+5 ; 0x05
cc: 0f 90 pop r0
ce: 0f 90 pop r0
d0: 0f 90 pop r0
d2: 0f 90 pop r0
d4: 0f 90 pop r0
* d6: df 91 pop r29
* d8: cf 91 pop r28
da: 08 95 ret
The value is put to Y+1 to Y+4 and then immediately read back. why?
pushing r0 does not make sense at all since it is by definition a temporary
register that can freely be modified. Maybe it's just pushed to make room for
the stack operations?
compilation:
"D:\AVR\Toolchain\9.1.0\bin\avr-g++.exe" -funsigned-char -funsigned-bitfields
-DNDEBUG -I"C:\Program Files
(x86)\Atmel\Studio\7.0\Packs\atmel\ATmega_DFP\1.2.272\include" -Os
-ffunction-sections -fdata-sections -fpack-struct -fshort-enums -Wall
-mmcu=atmega644 -c -MD -MP -MF "main.d" -MT"main.d" -MT"main.o" -o "main.o"
".././main.cpp"
linker:
"D:\AVR\Toolchain\9.1.0\bin\avr-g++.exe" -o BugTest.elf main.o
-Wl,-Map="BugTest.map" -Wl,--start-group -Wl,-lm -Wl,--end-group
-Wl,--gc-sections -mmcu=atmega644
