Re: [committed] Fix -Werror=class-memaccess failures in jit testsuite (PR jit/81144)

[David Malcolm <dmalcolm at redhat dot com>]

On Tue, 2017-06-20 at 17:15 -0600, Martin Sebor wrote:
> On 06/20/2017 03:25 PM, David Malcolm wrote:
> > This patch fixes a couple of failures of the form:
> > 
> >   error: 'void* memset(void*, int, size_t)' clearing an object of
> > non-trivial
> >     type 'struct quadratic_test'; use assignment or value
> > -initialization
> >     instead [-Werror=class-memaccess]
> >   note: 'struct quadratic_test' declared here
> >   cc1plus: all warnings being treated as errors
> > 
> > seen within the jit testsuite, by using zero-initialization instead
> > of memset.
> > 
> > (presumably introduced by r249234 aka
> > a324786b4ded9047d05463b4bce9d238b6c6b3ef)
> > 
> > Successfully tested on x86_64-pc-linux-gnu; takes jit.sum from:
> >   # of expected passes            9211
> >   # of unexpected failures        2
> > to:
> >   # of expected passes            9349
> > 
> > Martin: it's unclear to me what the benefit of the warning is for
> > these
> > cases.  AIUI, it's complaining because the code is calling
> > the default ctor for struct quadratic_test, and then that object is
> > being clobbered by the memset.
> > But if I'm reading things right, the default ctor for this struct
> > zero-initializes all fields.  Can't the compiler simply optimize
> > away
> > the redundant memset, and not issue a warning?

Thanks for the info.

> -Wclass-memaccess is issued because struct quadratic_test contains
> members of classes that define a default ctor to initialize their
> private members.  
> The premise behind the warning is that objects
> of types with user-defined default and copy ctors should be
> initialized by making use of their ctors, and those with private
> data members manipulated via member functions rather than by
> directly modifying their raw representation.  Using memset to
> bypass the default ctor doesn't begin the lifetime of an object,
> can violate invariants set up by it, and using it to overwrite
> private members breaks encapsulation.  Examples of especially
> insidious errors include overwriting const data, references, or
> pointer to data members for which zero-initialization isn't
> the same as clearing their bytes.

If I'm reading my code correctly, all of the default ctors of all of
the members of this struct are "merely" initializing the pointer they
wrap to NULL.

So the ctors are initializing everything to NULL, and then the memset
redundant re-init's everything to 0 bits (I guess I was going for a
"belt and braces" approach to ensure that things are initialized).

> The warning runs early on in the C++ front end and has no knowledge
> of either the effects of the type's ctors, dtor, and copy assignment
> operator, or whether the raw memory function is called in lieu of
> initializing an object (e.g., in storage obtained from malloc or
> operator new), or as a shortcut to zero out its members, or when
> zeroing them out happens to be safe and doesn't actually do any
> of those bad things I mentioned above.

Aha: so at the place where the warning runs it's not possible to access
the ctors and tell that they're assigning NULL everywhere?

Might it be possible to convert the warning to work in a two-phase way
where it first gathers up a vec of suspicious-looking modifications,
and then flushes them later, filtering against ctor information when it
has the latter?  (so that we don't have to warn for this case at
-Wall?)

Alternatively maybe this is PEBCAK at my end; if so, maybe a case for adding this to the changes.html page?  (and maybe adding some notes on workarounds there, and/or to invoke.texi?)

> 
> That said, I'm sorry (and a little surprised) that I missed these
> errors in my tests.  I thought I had all the languages covered by
> using
> 
>    --enable-languages=all,ada,c,c++,fortran,go,lto,objc,obj-c++
> 
> but I guess jit still isn't implied by all, even after Nathan's
> recent change to it.  Let me add jit to my script (IIRC, I once
> had it there but it was causing some trouble and I took it out.)

Reading r248454 (aka 01b4453cde8f1871495955298043d9fb589e4a36), it
looks like "jit" is only included in "all" if you also pass
  --enable-host-shared
Presumably that's what happened.  Bother.

Thanks; hope this is constructive.
Dave