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Subject: Re: Is breaking strict-aliasing rules something to worry about?
From: "Johnson, Andrew N." <anj@anl.gov>
To: Michael Davidsaver <mdavidsaver@gmail.com>
Cc: "core-talk@aps.anl.gov" <core-talk@aps.anl.gov>
Date: Wed, 3 Oct 2018 20:55:25 +0000
I have no reason to doubt your statement. I am just wary of this topic given the amount of code we have that casts pointers between dbCommon, void* and/or some specific record type.

I may have been reading this blog post or some of the items it links to: https://blog.regehr.org/archives/1621

- Andrew

-- 
Sent from my iPad

On Oct 3, 2018, at 1:49 PM, Michael Davidsaver <mdavidsaver@gmail.com> wrote:

On 10/3/18 11:07 AM, Johnson, Andrew N. wrote:
Did I read recently that apparently even the use of a union (as shown by
Ben at the end of that thread) is no longer guaranteed to prevent
problems with the latest language rules? *That* would be somewhat
worrying if true...

Before we rehash this.  Is there any new information?  Specifically,
is there anything which casts doubt on my statement?

https://epics.anl.gov/core-talk/2017/msg00726.php

The construct used (std::aligned_storage) is well defined in c++11.

http://en.cppreference.com/w/cpp/types/aligned_storage
...
GCC >=4.8.4 (at latest) doesn't warn about this even with -std=c++98.  So if anything, this is only need for older compilers.

cf. definition of std::aligned_storage

https://github.com/gcc-mirror/gcc/blob/d3a3029ca7489cb168d493de3d695809e84ffb0f/libstdc%2B%2B-v3/include/std/type_traits#L1943

Please compare with struct AnyScalar::wrap_t

https://github.com/epics-base/pvDataCPP/blob/aa87a2a23dc825e090bae9271703e3d51717eca8/src/misc/pv/anyscalar.h#L70-L84

class AnyScalar is itself similar in construction to std::any of c++17

https://github.com/gcc-mirror/gcc/blob/41d6b10e96a1de98e90a7c0378437c3255814b16/libstdc%2B%2B-v3/include/std/any#L71



- Andrew

On 10/03/2018 11:06 AM, Michael Davidsaver wrote:
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? Michael Davidsaver
References:
Is breaking strict-aliasing rules something to worry about? Dirk Zimoch
Re: Is breaking strict-aliasing rules something to worry about? Michael Davidsaver
Re: Is breaking strict-aliasing rules something to worry about? Johnson, Andrew N.
Re: Is breaking strict-aliasing rules something to worry about? Michael Davidsaver

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