https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97492
Bug ID: 97492
Summary: arm cortex-m0+ or constant value can use adds
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: kjetilos at gmail dot com
Target Milestone: ---
Given the following C code the compiler generates code that can be further
optimized.
void write_byte(void)
{
unsigned *regs = (unsigned *) 0x4210000;
*regs = (*regs & ~0xff) | (0x9 & 0xff);
}
In this code the first byte is cleared and a constant 9 is or'ed into the word.
This is compiled down to these instructions for cortex-m0+.
write_byte:
movs r3, #255
ldr r1, .L2
ldr r2, [r1]
bics r2, r3
subs r3, r3, #246
orrs r3, r2
str r3, [r1]
bx lr
.L3:
.align 2
.L2:
.word 69271552
In this case where a constant less than 255 is or'ed into the last byte of a
word then the subs + orrs instruction (or sometimes movs + orrs) sequence can
be replaced by a single adds instruction to save 2 bytes.
