https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124471
Bug ID: 124471
Summary: [AVR] Counterproductive byte copy optimization by
multiplication
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: dimich.dmb at gmail dot com
Target Milestone: ---
Simple function to return 16-bit word containing the same low and high byte:
uint16_t clone(uint8_t n)
{
return n | (n << 8);
}
with "-Wall -Os -mmcu=atmega328p" compiles to 9 instructions:
<clone>:
21 e0 ldi r18, 0x01 ; 1
31 e0 ldi r19, 0x01 ; 1
48 2f mov r20, r24
42 9f mul r20, r18
c0 01 movw r24, r0
43 9f mul r20, r19
90 0d add r25, r0
11 24 eor r1, r1
08 95 ret
Moreover, for targets without hardware multiplier software multiplication
invoked. E.g. with "-mmcu=attiny13a":
<clone>:
90 e0 ldi r25, 0x00 ; 0
61 e0 ldi r22, 0x01 ; 1
71 e0 ldi r23, 0x01 ; 1
09 d0 rcall .+18 ; 0x<__mulhi3>
08 95 ret
5 instructions + 17 instructions of __mulhi3.
Older GCC <= 11.1.0 compile the same function to
<clone>:
98 2f mov r25, r24
08 95 ret
The issue observed at least since 12.1.0.
$ avr-gcc -v
Using built-in specs.
Reading specs from /usr/lib/gcc/avr/15.2.0/device-specs/specs-avr2
COLLECT_GCC=avr-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/avr/15.2.0/lto-wrapper
Target: avr
Configured with: /home/dimich/work/src/avr/avr-gcc.pkg/src/gcc-15.2.0/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 15.2.0 (GCC)
System: Arch Linux x86_64
