> I believe the theory is that the compiler (remember, this is
> __builtin_memset) can optimize away portions of the zeroing, or can optimize
> zeroing for small sizes.
Ahhh! I didn't consider that the compiler would be doing analysis of the larger
context, and potentially skipping zeroing parts that are set immediately after
the call.
Thanks!
-Ravi (rpokala@)
-----Original Message-----
From: "Jonathan T. Looney" <j...@freebsd.org>
Date: 2018-06-06, Wednesday at 22:58
To: Ravi Pokala <rpok...@freebsd.org>
Cc: Mateusz Guzik <mjgu...@gmail.com>, Mateusz Guzik <m...@freebsd.org>,
src-committers <src-committ...@freebsd.org>, <svn-src-all@freebsd.org>,
<svn-src-h...@freebsd.org>
Subject: Re: svn commit: r334702 - head/sys/sys
> On Wed, Jun 6, 2018 at 10:14 PM, Ravi Pokala <rpok...@freebsd.org> wrote:
>>
>> -----Original Message-----
>> From: <owner-src-committ...@freebsd.org> on behalf of Mateusz Guzik
>> <mjgu...@gmail.com>
>> Date: 2018-06-06, Wednesday at 09:01
>> To: Ravi Pokala <rpok...@freebsd.org>
>> Cc: Mateusz Guzik <m...@freebsd.org>, src-committers
>> <src-committ...@freebsd.org>, <svn-src-all@freebsd.org>,
>> <svn-src-h...@freebsd.org>
>> Subject: Re: svn commit: r334702 - head/sys/sys
>>
>>> On Wed, Jun 6, 2018 at 1:35 PM, Ravi Pokala <rpok...@freebsd.org> wrote:
>>>
>>>>> + * Passing the flag down requires malloc to blindly zero the entire
>>>>> object.
>>>>> + * In practice a lot of the zeroing can be avoided if most of the object
>>>>> + * gets explicitly initialized after the allocation. Letting the compiler
>>>>> + * zero in place gives it the opportunity to take advantage of this
>>>>> state.
>>>>
>>>> This part, I still don't understand. :-(
>>>
>>> The call to bzero() is still for the full length passed in, so how does
>>> this help?
>>>
>>> bzero is:
>>> #define bzero(buf, len) __builtin_memset((buf), 0, (len))
>>
>> I'm afraid that doesn't answer my question; you're passing the full length
>> to __builtin_memset() too.
>
> I believe the theory is that the compiler (remember, this is
> __builtin_memset) can optimize away portions of the zeroing, or can optimize
> zeroing for small sizes.
>
> For example, imagine you do this:
>
> struct foo {
> uint32_t a;
> uint32_t b;
> };
>
> struct foo *
> alloc_foo(void)
> {
> struct foo *rv;
>
> rv = malloc(sizeof(*rv), M_TMP, M_WAITOK|M_ZERO);
> rv->a = 1;
> rv->b = 2;
> return (rv);
> }
>
> In theory, the compiler can be smart enough to know that the entire structure
> is initialized, so it is not necessary to zero it.
>
> (I personally have not tested how well this works in practice. However, this
> change theoretically lets the compiler be smarter and optimize away unneeded
> work.)
>
> At minimum, it should let the compiler replace calls to memset() (and the
> loops there) with optimal instructions to zero the exact amount of memory
> that needs to be initialized. (Again, I haven't personally tested how smart
> the compilers we use are about producing optimal code in this situation.)
>
> Jonathan
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