From: haynberg@yahoo.com
To: Andrew Haley <aph@redhat.com>,
"gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
Subject: Re: is portable aliasing possible in C++?
Date: Thu, 11 Sep 2014 23:25:00 -0000 [thread overview]
Message-ID: <1410477938.56522.YahooMailNeo@web140205.mail.bf1.yahoo.com> (raw)
In-Reply-To: <54115917.1040602@redhat.com>
>> msg *p = reinterpret_cast<msg*>(get_bytes());
>
> Why are you doing this?
For efficiency, by preventing a copy (imagine get_bytes() is getting bytes out of a socket buffer).
Putting alignment/padding concerns aside, it would be nice if there was a way to explicitly tell the compiler, I want to do this and, please don’t reorder stores and loads, or perform other strict-aliasing optimizations, on the memory pointed to by this pointer (similar to the effect of a memcpy). I believe the only way to do this is with the GCC may_alias attribute, or a more heavy-handed memory clobber. I think the OP wanted to ask the GCC folks if there was another, possibly more portable, way; for example, placement new, but that turned out not to be an option.
Another related, maybe more important, question is if GCC sees a reinterpet_cast like this (without a may_alias type), is it free to discard code or otherwise drastically change it due to the fact that it’s undefined by the standard? Like some of the cases shown in:
“any undefined behavior in C gives license to the implementation (the compiler and runtime) to produce code that ... does completely unexpected things, or worse”
http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html
Jay Haynberg
next prev parent reply other threads:[~2014-09-11 23:25 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-10 23:03 haynberg
2014-09-11 8:11 ` Andrew Haley
2014-09-11 23:25 ` haynberg [this message]
2014-09-12 8:32 ` Andrew Haley
2014-09-12 22:58 ` haynberg
2014-09-13 7:23 ` Andrew Haley
2014-09-13 11:45 ` Oleg Endo
2014-09-15 2:37 ` Hei Chan
2014-09-15 8:35 ` Andrew Haley
2014-09-15 11:07 ` Hei Chan
2014-09-15 11:21 ` Andrew Haley
2014-09-15 11:29 ` Hei Chan
2014-09-15 11:32 ` Andrew Haley
2014-09-15 11:57 ` Hei Chan
2014-09-15 13:21 ` Andrew Haley
2014-09-15 13:31 ` Hei Chan
2014-09-15 14:11 ` Andrew Haley
2014-09-15 11:27 ` Jonathan Wakely
2014-09-15 12:09 ` Paul Smith
2014-11-02 23:55 ` Hei Chan
2014-11-03 9:34 ` Andrew Haley
-- strict thread matches above, loose matches on Subject: below --
2014-09-09 23:13 haynberg
2014-09-10 8:17 ` Andrew Haley
[not found] <A76FB9DDEDFA994BAF6B77704A4AF465BC2464@xchmbbal502.ds.susq.com>
2014-09-04 16:11 ` Andy Webber
2014-09-04 16:51 ` Andrew Haley
2014-09-04 17:18 ` Andy Webber
2014-09-04 17:23 ` Andrew Haley
2014-09-04 17:44 ` Andy Webber
2014-09-04 17:47 ` Andy Webber
2014-09-04 17:48 ` Andrew Haley
2014-09-04 23:11 ` Jonathan Wakely
2014-09-05 7:16 ` Andrew Haley
2014-09-05 14:19 ` Jason Merrill
2014-09-08 9:33 ` Richard Biener
2014-09-10 14:31 ` Jason Merrill
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