Re: Does malloc tell us anything about alignment modulo k?

[Richard Henderson <rth at redhat dot com>]

On Thu, Jul 15, 2004 at 07:35:10PM +0000, Joseph S. Myers wrote:
> As you say, this probably isn't particularly useful.

Alpha, pre-bwx instructions, can make use of this.

	struct S
	{
	  unsigned char x, y;
	};
	 
	int foo()
	{
	  struct S *s = malloc(sizeof (struct S));
	  fill(s);
	  return s->x + s->y;
	}

The return line compiles to, with alignment 1,

        ldq_u $2,0($9)
        ldq_u $3,1($9)
        lda $1,1($9)
        extbl $2,$9,$2
        ldq $9,8($30)
        extbl $3,$1,$1
        addl $2,$1,$0

with alignment 4,

        ldl $2,0($9)
        extbl $2,1,$1
        and $2,0xff,$2
        addl $2,$1,$0

That we can assume alignment 2 based on the object size of 2 isn't good
enough, since pre-bwx only has 4 and 8 byte load instructions.


r~