[M3devel] aligned_procedures: suggest "closure_marker_size"

[Rodney M. Bates]

Before anybody fiddles with closure markers, *please* do *both* the following:

1)  Let me know.  M3gdb needs to both recognize and construct closures
     (and it currently does.)  Changing them will break it, unless it is
     modified accordingly.

2)  Make sure there is some reasonable way m3gdb can tell by looking at
     the object code being debugged, whether it was generated by a
     compiler version that uses the old or the new closure marker
     representation.

I have worked hard to make m3gdb able to adapt to the various compilers
and versions, but periodically I get undermined on 1) and/or 2) above
by some quiet change somebody makes without thinking about m3gdb.

Also, think about the implications on linking together code produced
by different compiler versions.  I am quite sure changing the closure
representation would mean *all* linked-in libraries would have to be
recompiled, along with the main program, by the same compiler version.

Right now, closure markers are always the same size as pointers, and I think
there are multiple places in the compiler and runtime, in addition to m3gdb,
that rely on this.  They would all have to be located and fixed.  And I
don't think they all key off any single declaration, e.g. closure_marker_size.

Jay K wrote:
> yes, but I think it is target-specific. IA64 would use 16 bytes.
> It isn't even in library code, but generated code.
>  
>  - Jay
> 
>  
> ------------------------------------------------------------------------
> From: hosking at cs.purdue.edu
> Date: Wed, 27 Jan 2010 10:57:09 -0500
> To: jay.krell at cornell.edu
> CC: m3devel at elegosoft.com
> Subject: Re: [M3devel] aligned_procedures: suggest "closure_marker_size"
> 
> If we declare it as a 32-bit subrange it should just work, right?
> 
> Antony Hosking | Associate Professor | Computer Science | Purdue University
> 305 N. University Street | West Lafayette | IN 47907 | USA
> Office +1 765 494 6001 | Mobile +1 765 427 5484
> 
> 
> 
> 
> On 27 Jan 2010, at 09:01, Jay K wrote:
> 
>     MIPS64, SPARC64 and maybe others could probably all benefit slightly
>     from
>       the closure marker being a 4 byte -1 instead of an INTEGER.
>      
>      
>      That is: 64bit architectures with a fixed size 4 byte instruction
>     where alignment is checked
> 
>      
>      That is, we should probably make their be a per-target variable
>     "closure marker size"
>       that is 4 for all current targets (IA64 should probably be 16 though),
>       though one would have to look into the various instruction encodings.
> 
> 
>      - Jay
> 
>