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Subject: Re: Is breaking strict-aliasing rules something to worry about?
From: Michael Davidsaver <mdavidsaver@gmail.com>
To: Dirk Zimoch <dirk.zimoch@psi.ch>
Cc: EPICS Core Talk <core-talk@aps.anl.gov>
Date: Wed, 3 Oct 2018 09:06:29 -0700
This question sounds familiar.

https://epics.anl.gov/core-talk/2017/msg00722.php

On 10/3/18 7:12 AM, Dirk Zimoch wrote:
> I had removed all the -fno-strict-aliasing flags from my configurations because I thought it was not necessary any more with EPICS 7, but the warnings are back:
> 
> /usr/local/epics/base-7.0.1/include/pv/anyscalar.h:111: warning: dereferencing pointer ‘<anonymous>’ does break strict-aliasing rules
> 
> As I understand, strict aliasing rules allow the compiler to do some aggressive optimization by assuming an assignment to a pointer of type A* can never change a variable of type B, given A and B are sufficiently different types. Thus a variable of type B can be safely kept in a register and does not need to be reloaded from memory only because a pointer of type A* has been written to.
> 
> The option -fno-strict-aliasing switches off that optimization and thus the warning as well.
> 
> But I don't know if the warning means that the optimizer will not touch the expressions that break strict-aliasing rules or if the optimizer will do its work anyway, potentially leading to very subtle and hard to debug run-time errors.
> 
> I propose to either
> a) Fix the code and be careful to never break strict-aliasing rules, i.e fix it immediately when such warnings pop up.
> b) Use -fno-strict-aliasing for all compilers that support it (gcc 3+ ?)
> 
> Dirk


Replies:
Re: Is breaking strict-aliasing rules something to worry about? Johnson, Andrew N.
References:
Is breaking strict-aliasing rules something to worry about? Dirk Zimoch

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