Should unw_word_t be 32-bits on 32-bit platforms?

[Albert Wong (王重傑)]

Sounds like a 32-bit unw_word_t is the general consensus (I'll double-check
for ABI requirements on monday) . Couple of comments inline.


On Sun, Jun 1, 2014 at 10:07 AM, David Majnemer <david.majnemer at gmail.com>
wrote:

> It should be a uint32_t for ARM If ABI, rather than API, compatibility
> with HP libunwind is a goal.
>

I wan't quite sure how to interpret this statement. Do you mean preserving
API compatibility is the goal?  If so, we're going to have some problems
with floating point support.  Antoine (CC'd) is currently working on the
patch and IIRC he's been finding a need to poke a few functions through
into the libunwind API to facilitate the personality routines demand saving
of VFP registers in phase 1 and then restoring them before phase 2.  This
is mostly a heads up...we'll see what the code looks like when it's done.


> On Sat, May 31, 2014 at 10:33 PM, Jean-Daniel Dupas <
> devlists at shadowlab.org> wrote:
>
>>
>> Le 1 juin 2014 à 06:42, Saleem Abdulrasool <compnerd at compnerd.org> a
>> écrit :
>>
>> On Sat, May 31, 2014 at 6:21 PM, Albert Wong (王重傑) <ajwong at google.com>
>> wrote:
>>
>>> Hello cfe-commits,
>>>
>>> As part of our work to implement EHABI, we experimentally tried to
>>> typedef unw_word_t so it was 32-bits on arm platforms.  It felt wasteful of
>>> memory and CPU cycles to work in 64-bits when everything on the
>>> architecture is 32-bits.
>>>
>>> Unfortunately, this caused a problem with all the
>>> _LIBUNWIND_TRACE_UNWINDING format specifiers that specified %llX for
>>> unw_word_t values.  I can see 3 solutions to this:
>>>
>>>   (1) Continue making unw_word_t 64-bit regardless of the architecture's
>>> native word size.
>>>   (2) Change unw_word_t to 32-bit and cast every log line argument to
>>> long-long so the format
>>>        strings stay constant.
>>>   (3) Change unw_word_t to 32-bit and use PRIXPTR in the format strings
>>> (we'd have to add
>>>        compatibility defines for platforms that don't have these macros)
>>> to avoid needing the cast.
>>>
>>> Both options 2 and 3 are severely annoying code-wise. On the other hand,
>>> introducing inefficiency -- even if it's only for the exception handling
>>> path -- just to support tracing statements that are compiled out in release
>>> mode seems unfortunately.
>>>
>>> My idealist side wants to suggest going with (3).  My more pragmatic
>>> "this likely is just noise compared to table lookups" side wants to go with
>>> (1).
>>>
>>
>> I think I would agree with your idealist side.  What happens if someone
>> tries to do bitwise operations on the incorrect type?  Hopefully, most of
>> them would be caught by the compiler.  It feels that it might be nicer to
>> go with option 3 even though it will cause a slight bit more churn.
>>
>>
>>
>> Instead of using PRIXPTR, you may just redefine your own macro next to
>> the declaration of unw_word_t, so if someone want to use 64bit size
>> unw_word_t on 32 bit platform, it will just have to change the typedef and
>> macro definition at a single place.
>>
>
This is possible, but I'm unsure what the benefit of providing an extra
configuration option here is. Supporting multiple word sizes on one
architecture sound like pretty scary bit of complexity to add...

-Albert


>
>> Suggestions?
>>>
>>> -Albert
>>>
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>>>
>>
>>
>> --
>> Saleem Abdulrasool
>> compnerd (at) compnerd (dot) org
>> _______________________________________________
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>>
>>
>> -- Jean-Daniel
>>
>>
>>
>>
>>
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