Hi!

Anyone have any input here, especially whether something like the WIP
patch attached to generally "Fold 'NON_LVALUE_EXPR' some more" is
preferable over local 'STRIP_NOPS'?

On 2020-03-26T20:53:19+0100, I wrote:
> On 2020-03-26T09:09:01-0600, Sandra Loosemore <sandra@codesourcery.com> wrote:
>> On 3/26/20 8:27 AM, Thomas Schwinge wrote:
>>> Note that as this code is shared between OpenACC/OpenMP, this might
>>> affect OpenMP, too, as far as I can tell.  (Subject updated.)  Jakub, can
>>> you please have a look, too?
>>>
>>> On 2020-03-25T23:02:38-0600, Sandra Loosemore <sandra@codesourcery.com> wrote:
>>>> The attached patch fixes a bug I found in the C++ front end's handling
>>>> of OpenACC data clauses.  The problem here is that the C++ front end
>>>> wraps the array bounds expressions in NON_LVALUE_EXPR tree nodes, and
>>>> the code here (which appears to have been copied from similar code in
>>>> the C front end) was failing to strip those before checking to see if
>>>> they were INTEGER_CST nodes, etc.
>>>
>>> So, I had a quick look.  I'm confirming the 'NON_LVALUE_EXPR' (C++ only,
>>> not seen for C) difference, and that 'STRIP_NOPS' gets rid of these.
>>> However, I also in some code paths see, for example, 'integer_nonzerop'
>>> calls, which internally do 'STRIP_ANY_LOCATION_WRAPPER'.
>>>
>>> I don't know if 'STRIP_NOPS' is the correct "hammer" to use here, I don't
>>> know what the usual convention is: explicitly remove (via 'STRIP_NOPS' as
>>> you suggested, or something else), or have the enquiry functions do it
>>> ('STRIP_ANY_LOCATION_WRAPPER' as 'integer_nonzerop' is doing, for
>>> example).
>>>
>>> Empirical data doesn't mean too much here, of course, I'm not seeing a
>>> lot of explicit 'STRIP_*' calls in 'gcc/cp/semantics.c'.  ;-)
>>
>> I saw that STRIP_NOPS seem to be the preferred way to do things in e.g.
>> fold-const.c.  I don't know if there's a reason to use some less general
>> form of STRIP_* here?

>>>> Sadly, I have no test case for this because it was only triggering an
>>>> error in conjunction with some other OpenACC patches that are not yet on
>>>> trunk

>> In the current code [we have]
>> checks like
>>
>> TREE_CODE (low_bound) == INTEGER_CST
>>
>> etc.  So when they're wrapped in NON_LVALUE_EXPRs, it's basically
>> failing to detect constants in array dimension specifiers and falling
>> through to other code.
>
> Eh, indeed...  ;-\ (So we should be able to deduce some misbehavior from
> that, to construct a test case.  I'll have a look.)

(Have not yet been able to look into constructing any test cases.)


> So.  I'm not objecting to handling 'NON_LVALUE_EXPR's locally here via
> any kind of 'STRIP_*', but it somehow doesn't seem the general solution.
> Another option seems to be to teach 'fold_simple' to handle
> 'NON_LVALUE_EXPR's, so that the existing code:
>
>     /* We need to reduce to real constant-values for checks below.  */
>     if (length)
>       length = fold_simple (length);
>     if (low_bound)
>       low_bound = fold_simple (low_bound);
>     if (low_bound
>         && TREE_CODE (low_bound) == INTEGER_CST
>         && [...])
>       low_bound = fold_convert (sizetype, low_bound);
>     if (length
>         && TREE_CODE (length) == INTEGER_CST
>         && [...])
>       length = fold_convert (sizetype, length);
>
> ... would then just work.  But: I don't know if 'fold_simple' (and
> others?) should handle 'NON_LVALUE_EXPR's, or if there's a reason why it
> doesn't.  (Have not yet tried to figure that out.)  What I can tell is
> that the attached patch does solve the issue in the C++ OMP array section
> handling, and survives a powerpc64le-unknown-linux-gnu
> '--enable-checking=yes' (will now re-run with 'fold' checking) bootstrap,
> with no testsuite regressions.
>
> Hmm.


Grüße
 Thomas


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