https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65410
Bug ID: 65410
Summary: "Short local string array" optimization doesn't happen
if string has NULs
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vda.linux at googlemail dot com
void f(char *);
void g() {
char buf[12] = "1234567890";
f(buf);
}
In the above example, "gcc -O2" creates buf[12] with immediate stores:
subq $24, %rsp
movabsq $4050765991979987505, %rax
movq %rsp, %rdi
movq %rax, (%rsp)
movl $12345, 8(%rsp)
call f
addq $24, %rsp
ret
But if buf[] definition has \0 anywhere (for example, at the end where it does
not even change the semantics of the code), optimization is not happening, gcc
allocates a constant string and copies it into buf[]:
void f(char *);
void g() {
char buf[12] = "1234567890\0";
f(buf);
}
.section .rodata
.LC0:
.string "1234567890"
.string ""
.text
g:
subq $24, %rsp
movq .LC0(%rip), %rax
movq %rsp, %rdi
movq %rax, (%rsp)
movl .LC0+8(%rip), %eax
movl %eax, 8(%rsp)
call f
addq $24, %rsp
ret
