[M3devel] closure marker

[Jay K]

A little bit of research done (just searching the web):
 Mips, Alpha, PowerPC, HPPA, ARM, SPARC


all appear to use a 32bit instruction
presumably aligned but I couldn't confirm for all of them.
  It seems likely that if the processor cares about data alignment, then code will be aligned the same.


Looks like SH has 16bit instructions.


I think we should go ahead soon and change the marker to be elementsize, count, initialize size=32bits, count = 1 for all targets.
With IA64 the expected exception with size=64bits, count=2.
  It packs 3 41bit instructions + 5bit template code into 128bit quantities, presumably 64bit aligned.
  We can revisit at that time.
And really size=64, count=1 might suffice.

I'm a little torn.
A 64bit marker is more certain.
Code has been this way forever.
etc.

 - Jay


----------------------------------------
> From: jay.krell at cornell.edu
> To: hosking at cs.purdue.edu; m3devel at elegosoft.com
> Subject: closure marker
> Date: Wed, 26 May 2010 15:13:38 +0000
>
>
> As I understand, currently we define the "closure marker" to be INTEGER sized and -1, and guaranteed aligned or not.
> 64bit systems that care about alignment pay quite a penality imho.
>
>
> I need to research, but I strongly suspect we should have Target.ClosureSize (bits) instead of Target.AlignedProcedures.
>
>
> If a system with a 64bit INTEGER has 4 byte instructions, that are always 4 byte aligned, and has instructions that can load a 4 byte integer, then we'd just set Target.ClosureSize = 32. The alignment stuff would fall away.
>
>
> I suspect this covers PPC64, SPARC64, ALPHA64, HPPA64, but I'd have to research their intruction sets.
>   Do they have 4 byte instructions, that are always 4 byte aligned?
>
>
> IA64 probably would have Target.ClosureSize=128 and the frontend would generate two guaranteed-aligned 64bit loads.
>
>
> More general might be
>   Target.ClosureElementSize  (bits)
>   Target.ClosureElementCount
>
>
> IA64 could then use Target.ClosureElementSize = 64, Target.ClosureElementCount = 2.
> Whereas most others would have Target.ClosureElementSize  = 32, Target.ClosureElementSize  = 1.
> ClosureElementSize would be chosen to be match guaranteed code alignment.
>
>
>
>   - Jay
>
>