Re: [Patch] [v2] Fortran: Fix Bind(C) Array-Descriptor Conversion (Move to Front-End Code)

[Tobias Burnus <tobias@codesourcery.com>]

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Re-sent with gzipped attachment as gcc-patches@ did reject it as being too large.
Alternatively, you can find it at
https://gcc.gnu.org/pipermail/fortran/attachments/20211009/dc26f92d/attachment-0001.bin
as fortran@ contrary to gcc-patches@ did accept the patch ...

Tobias

On 09.10.21 23:48, Tobias Burnus wrote:
> Hi all,
>
> attached is the updated version. Changes:
> * Handle noncontiguous arrays – with BIND(C), (g)Fortran needs to make it
>   contiguous in the caller but also handle noncontiguous in the callee.
> * Fixes/handle 'character(len=*)' with BIND(C); those always use an
>   array descriptor - also with explicit-size and assumed-size arrays
> * Fixed a bunch of bugs, found when writing extensive testcases.
> * Fixed type(*) handling - those now pass properly type and elem_len
>   on when calling a new function (bind(C) or not).
>
> Besides adding the type itself (which is rather straight forward),
> this patch only had minor modifications – and then the two big
> conversion functions.
>
> While it looks intimidating, it should be comparably simple to
> review as everything is on one place and hopefully sufficiently
> well documented.
>
> OK – for mainline?  Other comments? More PRs which are fixed?
> Issues not yet fixed (which are inside the scope of this patch)?
>
> (If this patch is too long, I also have a nine-day old pending patch
> at https://gcc.gnu.org/pipermail/gcc-patches/2021-October/580624.html )
>
> Tobias
>
> PS: The following still applies.
>
> On 06.09.21 12:52, Tobias Burnus wrote:
>> gfortran's internal array descriptor (xgfc descriptor) and
>> the descriptor used with BIND(C) (CFI descriptor, ISO_Fortran_binding.h
>> of TS29113 / Fortran 2018) are different. Thus, when calling a BIND(C)
>> procedure the gfc descriptor has to be converted to cfi – and when a
>> BIND(C) procedure is implemented in Fortran, the argument has to be
>> converted back from CFI to gfc.
>>
>> The current implementation handles part in the FE and part in
>> libgfortran,
>> but there were several issues, e.g. PR101635 failed due to alias issues,
>> debugging wasn't working well, uninitialized memory was used in some
>> cases
>> etc.
>>
>> This patch now moves descriptor conversion handling to the FE – which
>> also
>> can make use of compile-time knowledge, useful both for diagnostic
>> and to
>> optimize the code.
>>
>> Additionally:
>> - Some cases where TS29113 mandates that the array descriptor should be
>>   used now use the array descriptor, in particular character scalars
>> with
>>   'len=*' and allocatable/pointer scalars.
>> - While debugging the alias issue, I simplified 'select rank'. While
>> some
>>   special case is needed for assumed-shape arrays, those cannot
>> appear when
>>   the argument has the pointer or allocatable attribute. That's not
>> only a
>>   missed optimization, pointer/allocatable arrays can also be NULL -
>> such
>>   that accessing desc->dim.ubound[rank-1] can be uninitialized memory
>> ...
>>
>> OK?  Comments? Suggestions?
>>
>>  * * *
>>
>> For some more dumps, see the discussion about the alias issue at:
>> https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578364.html
>> ("[RFH] ME optimizes variable assignment away / Fortran bind(C)
>> descriptor conversion")
>> plus the original emails:
>> - https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578271.html
>> - and (correct dump)
>> https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578274.html
>>
>> Debugging - not ideal but not too bad either. For
>>   subroutine f(x) bind(C)
>>     integer :: x(:)
>> with an uninitialized size-4 array as argument:
>>
>> m::f (_x=...) at foo4.f90:3
>> 3       subroutine f(x) bind(C)
>> (gdb) p x
>> Cannot access memory at address 0x38
>> (gdb) p _x
>> $6 = ( base_addr = 0x7fffffffe2c0, elem_len = 4, version = 1, rank =
>> 1 '\001', attribute = 2 '\002', type = 1025, dim = (( lower_bound =
>> 0, extent = 5, sm = 4 )) )
>> (gdb) s
>> 5         x(1) = 5
>> (gdb) p x
>> $7 = (0, 0, 0, -670762413, 0)
>>
>>
>> Tobias
>>
>> PS: This patch fixes but not necessarily fully the following PRs:
>> PR fortran/102086 - [F2008][TS29113] Accepts invalid scalar TYPE(*)
>> as actual argument to assumed-rank
>> PR fortran/92189 - Fortran-written bind(C) function with allocatable
>> argument does not update C descriptor on exit
>> PR fortran/92621 - Problems with memory handling with allocatable
>> intent(out) arrays with bind(c)
>> PR fortran/101308 - Bind(C): gfortran does not create C descriptors
>> for scalar pointer/allocatable arguments
>> PR fortran/101635 - FAIL: gfortran.dg/PR93963.f90 – alias-handling
>> issue with BIND(C)'s _gfortran_cfi_desc_to_gfc_desc
>> PR fortran/92482 - BIND(C) with array-descriptor mishandled for type
>> character
>> and possibly some more.
>>
>> PPS: I should add some additional testcases – I try to do this as
>> Part 2 of this patch.
>>
>> PPPS: Once the patch is in, some audit needs to be done which parts
>> of those PRs remain
>> as follow-up work. I think some still existing issues are covered by
>> José's pending
>> patches + for those which are now fixed, the testcase might still be
>> added.
>>
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