Re: Warn when returning the address of a temporary (middle-end) v2

[Marc Glisse <marc.glisse@inria.fr>]

On Tue, 29 Jul 2014, David Malcolm wrote:

> On Tue, 2014-07-29 at 19:36 +0200, Marc Glisse wrote:
>> On Sun, 27 Jul 2014, Richard Sandiford wrote:
>>
>>> Marc Glisse <marc.glisse@inria.fr> writes:
>>>> Hello,
>>>>
>>>> I followed the advice in this discussion:
>>>> https://gcc.gnu.org/ml/gcc-patches/2014-04/msg00269.html
>>>>
>>>> and here is a new patch. I made an effort to isolate a path in at least
>>>> one subcase so it doesn't look too strange that the warning is in this
>>>> file. Computing the dominance info just to tweak the warning message may
>>>> be a bit excessive.
>>>
>>> How about only calculating it once you've decided to issue a message?
>>>
>>>> +		      if (always_executed)
>>>> +			msg = "function returns address of local variable";
>>>> +		      else
>>>> +			msg = "function may return address of local variable";
>>>
>>> I think you need _(...) here, unless some magic makes that unnecessary now.
>>
>> Current version, which passed bootstrap+testsuite on x86_64-linux-gnu.
>> (the original discussion is at
>> https://gcc.gnu.org/ml/gcc-patches/2014-06/msg01692.html )
>
> This is possibly a dumb question, but what happens for a static local,
> rather than an auto local? e.g.
>
> int *f (void)
> {
>    static int i;
>    return &i;
> }

is_global_var returns true and we don't warn.

> (e.g. should the test case cover this?)

I can add it if you want. I already test a global variable (function h) 
and I expect this is likely already covered elsewhere, but it would make 
the testcase more complete, so I'll do that.


I just noticed an issue with my patch: since I am using a compound 
expression in the front-ends to preserve side-effects, I am now sometimes 
getting this extra warning:

warning: left-hand operand of comma expression has no effect [-Wunused-value]

(I am surprised the testsuite didn't tell me about it)

I will probably have to use the compound expr only if there are side 
effects.

-- 
Marc Glisse