[PATCH v2 09/18] RISC-V: The ABI implementation for 32-bit

[Adhemerval Zanella]

On 10/08/2020 18:29, Alistair Francis via Libc-alpha wrote:
> On Fri, Jul 10, 2020 at 6:24 PM Maciej W. Rozycki <macro@wdc.com> wrote:
>>
>> On Fri, 10 Jul 2020, Alistair Francis wrote:
>>
>>>>  I note there is an extensive discussion on the way to move forward here:
>>>> <https://sourceware.org/glibc/wiki/Y2038ProofnessDesign#utmp_types_and_APIs>
>>>> We might as well try to implement it right away, so as to avoid being
>>>> limited to 32-bit time records here.
>>>
>>> Is there an advantage of doing it now or can we put this off for the
>>> next release?
>>
>>  The change is major enough it'll have to wait for the next development
>> cycle anyway.  It shouldn't matter that much for RV32 glibc deployments
>> though, given the amount of suitable hardware available.
>>
>>>>  NB some existing ports do have __WORDSIZE_TIME64_COMPAT32 set and cleared
>>>> for their 64-bit and 32-bit ABIs respectively, as per the note in our
>>>> top-level bits/wordsize.h, however this reflects the state as before we
>>>> introduced the possibility for `__time_t' to be a 64-bit type with
>>>> `__WORDSIZE == 32' ABIs.  Given the turn of events I think the note ought
>>>> to be updated accordingly; I gather it was missed with commit 07fe93cd9850
>>>> ("generic/typesizes.h: Add support for 32-bit arches with 64-bit types").
>>
>>  Will you be able to look into it?
>>
>>  The context here is before Y2038 changes __WORDSIZE_TIME64_COMPAT32 would
>> only be clear for 64-bit ABIs with those 64-bit systems that do not have
>> any 32-bit ABI (compatibility mode) to support, such as the DEC Alpha.
>> And it would always be clear for 32-bit ABIs, so as to use the proper
>> `__time_t' type without changing the width of actual data held there in
>> the structure.
>>
>>  I'm not sure what the story is behind the S/390 port though; perhaps it
>> does not support ABI coexistence in a single run-time environment.
> 
> I was just about to start this but Adhemerval has very kindly sent a
> patch series already. I'm going to leave this as is (defining it as 1
> for RV32 and RV64).

Don't forget to help on review ;).

> 
> 
>>
>>>>>    int __cur_writer;
>>>>>    int __shared;
>>>>>    unsigned long int __pad1;
>>>>>    unsigned long int __pad2;
>>>>>    unsigned int __flags;
>>>>> +#else
>>>>> +# if __BYTE_ORDER == __BIG_ENDIAN
>>>>> +  unsigned char __pad1;
>>>>> +  unsigned char __pad2;
>>>>> +  unsigned char __shared;
>>>>> +  unsigned char __flags;
>>>>> +# else
>>>>> +  unsigned char __flags;
>>>>> +  unsigned char __shared;
>>>>> +  unsigned char __pad1;
>>>>> +  unsigned char __pad2;
>>>>> +# endif
>>>>> +  int __cur_writer;
>>>>> +#endif
>>>>>  };
>>>>
>>>>  I note with this change the RV32 structure will use the generic layout as
>>>> per sysdeps/nptl/bits/struct_rwlock.h, however regrettably RV64 does not.
>>>> Would it make sense to instead have the layout the same between RV64 and
>>>> RV32, perhaps by redefining `__pad1' and `__pad2' in terms of `unsigned
>>>> long long' (which would have alignment implications though) or otherwise?
>>>
>>> I'm not sure which one is better. On one hand it seems better to be
>>> more generic and therefore RV32 should use the generic interface. On
>>> the other hand the more similar they are the better. I'm still leaning
>>> towards we should be generic where possible.
>>
>>  It would be good to get a second opinion here.
> 
> The RV64 __pthread_rwlock_arch_t is exactly the same as the AArch64
> version. My guess is that it was just copied to be the same.
> 
> The generic version also has this comment
> 
> /* Generic struct for both POSIX read-write lock.  New ports are expected
>    to use the default layout, however archictetures can redefine it to add
>    arch-specific extensions (such as lock-elision).  The struct have a size
>    of 32 bytes on both LP32 and LP64 architectures.  */
> 
> As we aren't adding any arch-specific extensions I think we should
> keep the generic one. It's unfortunate we didn't do that for RV64, but
> too late now.
> 
> The only downside I see in using the generic version is what happens
> if the value of _shared or flags overflows a char. There are currently
> only 4 flags though, so I think it's ok.

I come up with the generic interface by using the already one in place for
mostly 64-bit architecture and the internal layout is mainly to avoid duplicate
the internal type on multiple architectures (so we can use the generic 
implementation).

The only usage I can think of where same structure for RV64 and RV32 may
pose a role is to use a shared rwlock between a 32-bit and a 64-bit process
(pthread_rwlockattr_setpshared with PTHREAD_PROCESS_SHARED).  But afaik this
is not supported on other architectures.

> 
>>
>>>>  Is there any benefit from having `__flags' and `__shared' (and the
>>>> reserve) grouped within a single 32-bit word?  I gather there is, given
>>>> the lengths gone to to match the bit lanes across the word regardless of
>>>> the endianness.  But what is it?
>>>
>>> I have no idea.
>>
>>  Especially given this.

The internal member that are most concurrently accessed by the common operations
(pthread_rwlock_{clock,timed}{wr,rd}lock) are the __readers, __writers,
__wrphase_futex, __writers_futex, and __cur_writer.  The __shared and __flag
are either accessed with non-atomic operation or at initialization,  so I don't
think there is much different in packing or not in a 32-bit word.  My 
understanding is architectures usually does that to avoid possible internal
padding, since the usual types for both member are uint8_t.

>>
>>>>> +# define _FP_DIV_MEAT_S(R, X, Y)     _FP_DIV_MEAT_1_udiv_norm (S, R, X, Y)
>>>>> +# define _FP_DIV_MEAT_D(R, X, Y)     _FP_DIV_MEAT_2_udiv (D, R, X, Y)
>>>>> +# define _FP_DIV_MEAT_Q(R, X, Y)     _FP_DIV_MEAT_4_udiv (Q, R, X, Y)
>>>>> +
>>>>> +# define _FP_NANFRAC_S               _FP_QNANBIT_S
>>>>> +# define _FP_NANFRAC_D               _FP_QNANBIT_D, 0
>>>>> +# define _FP_NANFRAC_Q               _FP_QNANBIT_Q, 0, 0, 0
>>>>
>>>>  Likewise.  There seems to be an established practice for this header
>>>> across architectures to have no space between macro arguments or before
>>>> the opening parenthesis.  This might help with the alignment.
>>>
>>> I still think it makes sense to follow the glibc style though, even if
>>> other archs don't.
>>>
>>> Let me know if it should be a different way and I'll update it.
>>
>>  There is the issue of the discrepancy compared to the libgcc version, and
>> while `diff -l' and `patch -l' solve that for manual processing, more
>> sophisticated tools may not cope and require manual intervention.
>>
>>  Again, I would suggest getting a second opinion.
> 
> The current option matches the rest of the file, so I think it makes
> sense to leave as is.
> 
> If this is a problem we can just change the spacing for the entire
> file in a future patch.
> 
> Alistair
> 
>>
>>   Maciej