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

[Jean-Daniel Dupas]

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.

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




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