Hello. I have a simple question about restrict pointers. Consider the
following code:
salmin@salmin:~$ cat restrict0.c
void f(int *a, const int *b) {
*a++ = *b + 1;
*a++ = *b + 1;
}
salmin@salmin:~$ ./systemroot/bin/gcc-trunk-158628 -S -O4 -std=gnu99 restrict0.c
salmin@salmin:~$ grep -v '[.:]' restrict0.s
movl (%rsi), %eax
addl $1, %eax
movl %eax, (%rdi)
movl (%rsi), %eax
addl $1, %eax
movl %eax, 4(%rdi)
ret
We have mov-add-mov twice here because if (a==b) then "*b" will be
modified by the first statement. It's clear.
However if we add a "restrict" keyword to the definition of b like
that it affects nothing:
salmin@salmin:~$ cat restrict1.c
void f(int *a, const int *restrict b) {
*a++ = *b + 1;
*a++ = *b + 1;
}
salmin@salmin:~$ ./systemroot/bin/gcc-trunk-158628 -S -O4 -std=gnu99 restrict1.c
salmin@salmin:~$ grep -v '[.:]' restrict1.s
movl (%rsi), %eax
addl $1, %eax
movl %eax, (%rdi)
movl (%rsi), %eax
addl $1, %eax
movl %eax, 4(%rdi)
ret
As far as I understand the "restrict" concept "const int *restrict b"
guarantee that "*b" will not be modified by "*a++ = *b + 1;".
Another thing I don't understand is why adding the "restrict" keyword
to the definition of "a" helps:
salmin@salmin:~$ cat restrict2.c
void f(int *restrict a, const int *restrict b) {
*a++ = *b + 1;
*a++ = *b + 1;
}
salmin@salmin:~$ ./systemroot/bin/gcc-trunk-158628 -S -O4 -std=gnu99 restrict2.c
salmin@salmin:~$ grep -v '[.:]' restrict2.s
movl (%rsi), %eax
addl $1, %eax
movl %eax, (%rdi)
movl %eax, 4(%rdi)
ret
Is that an unimplemented optimization or I just don't understand the
restrict concept?
Alexey
