On Thu, Jan 7, 2016 at 9:05 PM, Steve Ellcey <sell...@imgtec.com> wrote:
> 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?
See some existing PR. The GCC middle-end cannot assume that pointers
are aligned
according to their type (while at least the C language would support
that notion).
In theory we could lessen the middle-end restriction for function
parameters or frontends
could insert proper __builtin_assume_aligned calls.
Richard.
> Steve Ellcey
> sell...@imgtec.com
