From: Paul Koning <paulkoning@comcast.net>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: "Markus Fröschle" <markus@mubf.de>, gcc@gcc.gnu.org
Subject: Re: asking for __attribute__((aligned()) clarification
Date: Wed, 21 Aug 2019 14:32:00 -0000 [thread overview]
Message-ID: <0A5DAB14-F153-486B-BA04-6AD500C85E71@comcast.net> (raw)
In-Reply-To: <alpine.LNX.2.20.13.1908211714420.32047@monopod.intra.ispras.ru>
> On Aug 21, 2019, at 10:28 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Tue, 20 Aug 2019, "Markus Fröschle" wrote:
>
>> Thank you (and others) for your answers. Now I'm just as smart as before, however.
>>
>> Is it a supported, documented, 'long term' feature we can rely on or not?
>>
>> If yes, I would expect it to be properly documented. If not, never mind.
>
> I think it's properly documented in gcc-9:
>
> https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Common-Type-Attributes.html
>
> (the "old" behavior where the compiler would neither honor reduced alignment
> nor issue a warning seems questionable, the new documentation promises a more
> sensible approach)
I agree, but if the new approach generates a warning for code that was written to the old rules, that would be unfortunate.
> In portable code one can also use memcpy to move unaligned data, the compiler
> should translate it like an unaligned load/store when size is a suitable
> constant:
>
> int val;
> memcpy(&val, ptr, sizeof val);
>
> (or __builtin_memcpy when -ffreestanding is in effect)
Yes. But last I tried, optimizing that for > 1 alignment is problematic because that information often doesn't make it down to the target code even though it is documented to do so.
paul
next prev parent reply other threads:[~2019-08-21 14:32 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-19 12:46 "Markus Fröschle"
2019-08-19 13:57 ` Paul Koning
2019-08-19 14:01 ` Richard Earnshaw (lists)
2019-08-19 14:08 ` Alexander Monakov
2019-08-19 14:12 ` Paul Koning
2019-08-20 5:46 ` Aw: " "Markus Fröschle"
2019-08-21 14:29 ` Alexander Monakov
2019-08-21 14:32 ` Paul Koning [this message]
2019-08-21 14:58 ` Alexander Monakov
2019-08-21 16:50 ` Paul Koning
2019-08-21 17:35 ` Jonathan Wakely
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