http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54816
Bug #: 54816
Summary: [avr] shift is better than widening mul
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: g...@gcc.gnu.org
CC: eric.wedding...@atmel.com
Target: avr
The following C test case
int wmul (char a, char b)
{
return a * (char) (b << 3);
}
$ avr-gcc wmul.c -S -Os -mmcu=atmega8 -dp
produces with current avr-gcc:
wmul:
ldi r25,lo8(8) ; 25 movqi_insn/2 [length = 1]
muls r22,r25 ; 26 mulqihi3 [length = 3]
movw r22,r0
clr __zero_reg__
muls r24,r22 ; 17 mulqihi3 [length = 3]
movw r24,r0
clr __zero_reg__
ret ; 29 return [length = 1]
.ident "GCC: (GNU) 4.8.0 20121004 (experimental)"
avr-gcc-4.7 was smarter with its code:
wmul:
lsl r22 ; 10 *ashlqi3/5 [length = 3]
lsl r22
lsl r22
muls r24,r22 ; 12 mulqihi3 [length = 3]
movw r22,r0
clr __zero_reg__
movw r24,r22 ; 31 *movhi/1 [length = 1]
ret ; 30 return [length = 1]
.ident "GCC: (GNU) 4.7.2"
The 4.7 code is faster, smaller and has smaller register pressure.
