__builtin_memcpy and alignment assumptions

[Steve Ellcey]

I have a question about __builtin_memcpy and alignment.  While working on
MIPS I noticed that this program:

void foo(int *a, int *b) 
{
	__builtin_memcpy (a, b, 8);
}

Is generating the following MIPS code (allowing for any alignment):

	lwl	$3,3($5)
	lwl	$2,7($5)
	lwr	$3,0($5)
	lwr	$2,4($5)
	swl	$3,3($4)
	swr	$3,0($4)
	swl	$2,7($4)
	swr	$2,4($4)

But it seems like it should generate this code which assumes 32 bit alignment,
since int types need to be 32 bit aligned:

	lw	$3,0($5)
	lw	$2,4($5)
	sw	$3,0($4)
	sw	$2,4($4)

When I look at expand_builtin_memcpy, and I look at the trees for the
source and destination, they both show pointers pointing to something
with 32 bit aligned data, but src_align and dest_align are getting set
to 8 and not to 32, in trying to track this down I get into
get_pointer_alignment_1 with this tree:

 <ssa_name 0x7f13ac4669d8
    type <pointer_type 0x7f13ac47c738
        type <integer_type 0x7f13ac46f690 int public SI
            size <integer_cst 0x7f13ac46e4c0 constant 32>
            unit size <integer_cst 0x7f13ac46e4e0 constant 4>
            align 32 symtab 0 alias set -1 canonical type 0x7f13ac46f690 precision 32 min <integer_cst 0x7f13ac46e7e0 -2147483648> max <integer_cst 0x7f13ac46e800 2147483647>
            pointer_to_this <pointer_type 0x7f13ac47c738>>
        unsigned SI size <integer_cst 0x7f13ac46e4c0 32> unit size <integer_cst 0x7f13ac46e4e0 4>
        align 32 symtab 0 alias set 2 canonical type 0x7f13ac47c738>
    var <parm_decl 0x7f13ac568080 b>def_stmt GIMPLE_NOP

    version 3
    ptr-info 0x7f13ac55a1e0>

And when I look at ptr-info (returned by SSA_NAME_PTR_INFO), I see that its
align value is zero and that is why I get an 8 instead of a 32 for alignment.

The problem is, who/where/when should the ptr-info information get set,
or am I off base and can I not actually assume 32 bit alignment instead
of 8 bit alignment in this code?

Steve Ellcey
sellcey@imgtec.com