Re: CVS commit: src/sys/compat/sys

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

On Fri, Jun 15, 2018 at 19:55:28 +0700, Robert Elz wrote:

>     Date:        Fri, 15 Jun 2018 12:41:56 +0300
>     From:        Valery Ushakov <uwe%stderr.spb.ru@localhost>
>     Message-ID:  <20180615094156.GD3351%pony.stderr.spb.ru@localhost>
> 
>   | Re memset - I now wonder if
>   | compiler is even allowed to be smart here b/c strict aliasing
> 
> Another way would be
> 
> 	static const struct timespec50 zts = { 0 };
> and
> 	*ts50 = zts;
> 
> which is unlikely to have that problem, and which the compiler might
> be smart enough to optimise into a single assignment where there is
> padding, and perhaps even nothing where there is none.

The compiler does field-by-field assignment in that case and uses
"narrow" movl that doesn't scrub the pad.

With memset you seem to also need to add "restrict" so that the
compiler can elide the dead moves.

The example I'm playing with is:

#include <string.h>

struct s   {  int i; long l; };
struct s64 { long i; long l; };

//#define restrict
void foo(struct s * restrict l, const struct s64 * restrict r)
{
#if 0
    static const struct s s0 = { 0, 0 };
    *l = s0;
#else
    memset(l, 0, sizeof(*l));
#endif
    l->i = r->i;
    l->l = r->l;
}


> I'm also not really a fan of the #if that tests the number of bits in
> int and long (by testing their max values) - that is assuming that
> when long is wider than int, it also has wider alignment, which is
> not necessarily so - a system with 4 byte ints and 8 byte long,
> could have long with 4 byte alignment (and that struct be 12 bytes
> long, alogned 4).     The code as it is isn't wrong, ijust the attempt
> to avoid the memset() when it isn't needed might fail, and do the
> memset() when there is nothing to clear - and if the compiler doesn't
> or even can't, optimise it away then it is wasteful.

Yes, the #if test feels icky...

It seems that with restrict we can just memset the whole struct and
the compiler will elide the memset completely when there's no pad to
scrub.

-uwe