https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123506
Bug ID: 123506
Summary: Some unnecessary assembly generated in simple function
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tobi at gcc dot gnu.org
Target Milestone: ---
This is a small offshot from PR123505
https://godbolt.org/z/xx54ejr1G
=================================
#include <stdint.h>
#include <string.h>
struct Result { int dec_exp; char *buffer; };
Result bar(char *buffer, int dec_exp) {
uint16_t e_sign = dec_exp >= 0 ? ('-' << 8 | 'e') : ('+' << 8 | 'e');
memcpy(buffer, &e_sign, 2);
int mask = (dec_exp >= 0) - 1;
dec_exp = ((dec_exp + mask) ^ mask); // absolute value
return { dec_exp, buffer + 2 };
}
==================================
yields:
bar(char*, int):
mov eax, 11621
test esi, esi
jns .L2
neg esi
mov eax, 11109
.L2:
mov WORD PTR [rdi], ax
mov esi, esi <---- NOP
add rdi, 2 <--\
mov rdx, rdi <----- lea [rdx, rdi+2] seems shorter
mov eax, esi
ret
Where the assembly contains a seemingly unnecessary no-op instruction and the
addition appears to be executed in a suboptimal way.
