Re: movmem pattern and missed alignment

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Oct 8, 2018 at 3:57 PM Paul Koning <paulkoning@comcast.net> wrote:
>
> I have a movmem pattern in my target that pays attention to the alignment argument.
>
> GCC isn't passing in the expected alignment part of the time.  I have this test case:
>
> extern int *i, *j;
> extern int iv[40], jv[40];
>
> void f1(void)
> {
>     __builtin_memcpy (i, j, 32);
> }
>
> void f2(void)
> {
>     __builtin_memcpy (iv, jv, 32);
> }
>
> When the movmem pattern is called for f1, alignment is 1.  In f2, it is 2 (int is 2 bytes in pdp11) as expected.
>
> The compiler clearly knows that int* points to aligned data, since it generates instructions that assume alignment (this is a strict-alignment target) when I dereference the pointer.  But somehow it gets it wrong for block move.
>
> I also see this for the individual move operations that are generated for very short memcpy operations; if the count is 4, I get four move byte operations for f1, but two move word operations for f2.
>
> This seems like a bug.  Am I missing something?

Yes, memcpy doesn't require anything bigger than byte alignment and
GCC infers alignemnt
only from actual memory references or from declarations (like iv /
jv).  For i and j there
are no dereferences and thus you get alignment of 1.

Richard.

>
>         paul
>