https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115817
Bug ID: 115817
Summary: [AVR] Suboptimal code for zeroing SRAM byte from ISR
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: dimich.dmb at gmail dot com
Target Milestone: ---
There are two almost identical ISRs, the difference is only an immediate value
to be stored in SRAM.
```
static volatile unsigned char flag;
__attribute__ ((__signal__,__used__, __externally_visible__))
void __vector_set_one(void)
{
flag = 1;
}
__attribute__ ((__signal__,__used__, __externally_visible__))
void __vector_set_zero(void)
{
flag = 0;
}
```
(The real code is reduced to this MRE to avoid ISR macro from avr-libc header)
First function compiles to the following code:
0000000c <__vector_set_zero>:
c: 1f 92 push r1
e: 1f b6 in r1, 0x3f ; 63
10: 1f 92 push r1
12: 11 24 eor r1, r1
14: 10 92 00 00 sts 0x0000, r1 ; 0x800000 <__SREG__+0x7fffc1>
18: 1f 90 pop r1
1a: 1f be out 0x3f, r1 ; 63
1c: 1f 90 pop r1
1e: 18 95 reti
which looks overcomplicated comparing to the first one:
00000000 <__vector_set_one>:
0: 8f 93 push r24
2: 81 e0 ldi r24, 0x01 ; 1
4: 80 93 00 00 sts 0x0000, r24 ; 0x800000 <__SREG__+0x7fffc1>
8: 8f 91 pop r24
a: 18 95 reti
For other non-zero values the code is the same as for `__vector_set_one` except
the value itself.
The same difference is observed in final ELF binary, regardless of whether
compiled and linked with or without `-flto`.
Preprocessed file:
```
# 0 "example.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "example.c"
static volatile unsigned char flag;
__attribute__ ((__signal__,__used__, __externally_visible__))
void __vector_set_one(void)
{
flag = 1;
}
__attribute__ ((__signal__,__used__, __externally_visible__))
void __vector_set_zero(void)
{
flag = 0;
}
```
GCC version and build flags:
$ avr-gcc -v
Using built-in specs.
Reading specs from /usr/lib/gcc/avr/14.1.0/device-specs/specs-avr2
COLLECT_GCC=avr-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/avr/14.1.0/lto-wrapper
Target: avr
Configured with: /build/avr-gcc/src/gcc-14.1.0/configure
--disable-install-libiberty --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-linker-build-id --disable-nls
--disable-werror --disable-__cxa_atexit --enable-checking=release
--enable-clocale=gnu --enable-gnu-unique-object --enable-gold
--enable-languages=c,c++ --enable-ld=default --enable-lto --enable-plugin
--enable-shared --infodir=/usr/share/info --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --prefix=/usr --target=avr
--with-as=/usr/bin/avr-as --with-gnu-as --with-gnu-ld --with-ld=/usr/bin/avr-ld
--with-plugin-ld=ld.gold --with-system-zlib --with-isl
--enable-gnu-indirect-function
Thread model: single
Supported LTO compression algorithms: zlib zstd
gcc version 14.1.0 (GCC)
System: Arch Linux, x86_64 GNU/Linux
Compilation:
$ avr-gcc -Wall -pedantic -Os -mmcu=attiny13a --save-temps -c -o example.o
example.c
Expected code:
__vector_set_zero:
push r24
ldi r24, 0x00
sts flag, r24
pop r24
reti
