Zeev Tarantov <zeev.tarantov@xxxxxxxxx> writes:
> When compiling this code:
>
> unsigned int get_le32(unsigned char *p)
> {
> return p[0] | p[1] << 8 | p[2] << 16 | p[3] << 24;
> }
>
> On gcc 4.6.0 rev. 172266 for x86-64, I get:
>
> movzbl 1(%rdi), %eax
> movzbl 2(%rdi), %edx
> sall $8, %eax
> sall $16, %edx
> orl %edx, %eax
> movzbl (%rdi), %edx
> orl %edx, %eax
> movzbl 3(%rdi), %edx
> sall $24, %edx
> orl %edx, %eax
> ret
>
> I hoped for much better code. I hoped to avoid ifdef's depending on
> endianess, but this means I can't.
> Am I missing something obvious that precludes the compiler from
> optimizing the expression?
> This is not a regression and other compilers didn't do any better, so
> I hope I'm just missing something.
The compiler could in principle do better. It basically requires a
large pattern match, essentially a new compiler pass looking for this
specific type of code, which it currently does not do.
For example, there is such a special optimization pass which looks for
cases which can use the x86 bswap instruction. This code
unsigned int f(unsigned int *p) {
unsigned x = *p;
return (((x >> 24) & 0xff)
| ((x >> 8) & 0xff00)
| ((x << 8) & 0xff0000)
| ((x << 24) & 0xff000000));
}
on x86_64 compiles into
movl (%rdi), %eax
bswap %eax
ret
So something similar could be done for your test case. Actually,
though, your test case is harder, because p might be misaligned. If it
is misaligned, the byte loads might actually be faster. But even when
using attribute ((aligned (N))), a new optimization pass would be
required to look for this case.
Ian
