Re: [Patch] Fortran: Avoid SAVE_EXPR for deferred-len char types

[Tobias Burnus <tobias@codesourcery.com>]

On 17.02.23 17:27, Steve Kargl wrote:
> On Fri, Feb 17, 2023 at 12:13:52PM +0100, Tobias Burnus wrote:
>> OK for mainline?
> Short version: no.

Would you mind to write a reasoning beyond only a single word?

>> subroutine foo(n)
>>    integer :: n
>>    integer :: array(n*5)
>>    integer :: my_len
>>    ...
>>    my_len = 5
>>    block
>>      character(len=my_len, kind=4) :: str
>>
>>      my_len = 99
>>      print *, len(str)  ! still shows 5 - not 99
>>    end block
>> end
> Are you sure about the above comment?

Yes - for three reasons:
* On the what-feels-right side: It does not make any sense to print
   any other value than 5 given that 'str' has been declared with len = 5.
* On the GCC side, the SAVE_EXPR ensures that the length is evaluated
   early and then "saved" to ensure its original value is available
* The quoted text from the standard implies that this is what
   should happen.

Why do you think that printing "5" is wrong? GCC does so since
years; it still does so with my patch.

Hence, can you elaborate? And also state which value you did expect instead?

* * *

The patch itself is about *deferred* length parameters, i.e.
'len=:', and thus for code like:

character(len=:), pointer :: str
...
allocate(character(len=4) :: str)
print *, len(str)  ! should print 4
...
allocate(character(len=99) :: str)
print *, len(str)  ! should now print 99
...

Currently, the SAVE_EXPR causes that the original value might
get used, which is often 0 (by chance 0 initialized) or some
random value like 57385973, depending what on what was on the
stack before. - There are more issues with deferred strings,
but at least one is solved by not having a SAVE_EXPR for
deferred-length character strings.

Tobias

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