[avr-gcc-list] Re: AVR byte swap optimization

[Rask Ingemann Lambertsen]

On Fri, Nov 17, 2006 at 04:30:53PM -0700, Shaun Jackman wrote:
> Is there anything I can do to help GCC along here? I'm using GCC 4.1.0
> with -O2.
> 
> I won't bother to show bswap_32 here, which produces a real disaster!
> Think 47 instructions, for what should be 6.

   You may get better code if you write it something like this:

#include <string.h>

uint32_t bswap_32 (uint32_t x)
{
        unsigned char c[4], temp;

        memcpy (c, &x, 4);
        temp = c[0];
        c[0] = c[3];
        c[3] = temp;
        temp = c[1];
        c[1] = c[2];
        c[2] = temp;
        memcpy (&x, c, 4);

        return (x);
}

   It isn't only on the AVR that bswap_32() is nontrivial to get right.
These two versions would rule on the i386 if GCC would be just a little bit
smarter:

#include <string.h>
#define BSWAP_16(x) \
        ((((x) >> 8) & 0xff) | (((x) & 0xff) << 8))

uint32_t bswap_32_a (uint32_t y)
{
        uint16_t d[2];
        uint32_t x = y;

        memcpy (d, &x, sizeof (d));
        d[0] = BSWAP_16 (d[0]);
        memcpy (&x, d, sizeof (x));
        x = ((x >> 16) & 0xffff) | ((x & 0xffff) << 16);
        memcpy (d, &x, sizeof (d));
        d[0] = BSWAP_16 (d[0]);
        memcpy (&x, d, sizeof (x));

        return (x);
}
/*
bswap_32_a:
        subl    $16, %esp
        movl    20(%esp), %eax
        movl    %eax, 12(%esp)
        rolw    $8, 12(%esp)
        roll    $16, 12(%esp)
        rolw    $8, 12(%esp)
        movl    12(%esp), %eax
        addl    $16, %esp
        ret
*/
uint32_t bswap_32_b (uint32_t y)
{
        union { uint16_t d[2]; uint32_t x; } t;
        t.x = y;

        t.d[0] = BSWAP_16 (t.d[0]);
        t.x = ((t.x >> 16) & 0xffff) | ((t.x & 0xffff) << 16);
        t.d[0] = BSWAP_16 (t.d[0]);

        return (t.x);
}
/*
bswap_32_b:
        movl    4(%esp), %edx
        movl    %edx, %eax
        rolw    $8, %ax
        movw    %ax, %dx
        movl    %edx, %eax
        roll    $16, %eax
        movl    %eax, %edx
        rolw    $8, %ax
        movw    %ax, %dx
        movl    %edx, %eax
        ret
*/

-- 
Rask Ingemann Lambertsen