Re: [Gcl-devel] ia64 and garbage collection

[Camm Maguire <camm at enhanced dot com>]

Greetings, and thanks for the speedy replies!  This really shows off
the power of the net!  I've just put together a quick working gc for
gcl on the ia64, and it appears to be building maxima now without
problems!  A world record turn around, if you asked me :-).

My current patch is below.  Comments most welcome.  And of course, a
few questions remaining!


Jim Wilson <wilson@redhat.com> writes:

> IA-64 has a register stack.  This is somewhat like the register windows
> on sparcs.  If you want to be able to view all of the registers, you need
> to flush the register stack to memory somehow.  This can be done by calling
> getcontext as David Mosberger mentioned, but can also be done by using the
> flushrs instruction, either directly via a builtin, or by a small assembly
> language stub (which is what boehm-gc does).

1) Does the __builtin function identically to the asm in the Boehm-gc?
   Returning the same address too?

> 
> IA-64 has two stacks.  One is the regular stack that holds procedure activation
> frames.  The other stack is called the backing store, and holds the part of
> the register stack that has been flushed to memory.  (This is unlike the sparc
> which uses a single stack for both.)  You will have to search both stacks.
> 

2) Looking over the boehm code, am I correct in guessing that this is
   also the case on hppa (HP PA-RISC)?

> In order to get your GC working, you will have to spend a little time reading
> IA-64 architecture manuals to understand how all of this stuff works.  Or
> alternatively study a working example.  boehm-gc for instance already supports
> IA-64.

3)  I'd appreciate your comments on the patch below if possible.

> 
> Jim


Andrew Haley <aph@cambridge.redhat.com> writes:

> You don't seem to have mentioned that the IA-64 has *two* stacks: one
> contains registers and one structures and arrays.  If you want to

perhaps this is because I was unaware of it :-).

> flush the register stack to memory for the sake of the garbage
> collector, that's easy: just call __builtin_ia64_flushrs().
> 

Thanks for the tip!

> Andrew.
> 


David Mosberger <davidm@napali.hpl.hp.com> writes:

> >>>>> On Tue, 30 Jul 2002 13:33:12 -0400, Camm Maguire <camm@enhanced.com> said:
> 
>   Camm> 1) Is there a work around, i.e. a way in which I can ensure
>   Camm> clearing all registers back to the stack at a certain point in
>   Camm> the program?
> 
> A call to getcontext() does have the effect of forcing all
> callee-saved (preserved) registers into memory (and the "scratch"
> registers won't be live, by virtue of getcontext() being a regular
> function call).

And thank you for this suggestion!  How does this differ from the
flushrs solution?

> 
> 	--david
> 


=============================================================================
#if defined(__ia64__) /* from boehm-gc code */
	asm("        .text");
	asm("        .psr abi64");
	asm("        .psr lsb");
	asm("        .lsb");
	asm("");
	asm("        .text");
	asm("        .align 16");
	asm("        .global GC_save_regs_in_stack");
	asm("        .proc GC_save_regs_in_stack");
	asm("GC_save_regs_in_stack:");
	asm("        .body");
	asm("        flushrs");
	asm("        ;;");
	asm("        mov r8=ar.bsp");
	asm("        br.ret.sptk.few rp");
	asm("        .endp GC_save_regs_in_stack");

void * GC_save_regs_in_stack();
#endif


....

(fn is set to a stack marking function, 'mark_stack_carefully')
(C_GC_OFFSET is defined to 0, but allows for marking addresses at
fractional word boundaries if set to 2)

#if defined(__ia64__)
    {
       extern void * __libc_ia64_register_backing_store_base;
       void * bst=GC_save_regs_in_stack();
       void * bsb=__libc_ia64_register_backing_store_base;

       if (bsb>bst)
          (*fn)(bsb,bst,C_GC_OFFSET);
       else
          (*fn)(bst,bsb,C_GC_OFFSET);
       
    }
#endif
=============================================================================

-- 
Camm Maguire			     			camm@enhanced.com
==========================================================================
"The earth is but one country, and mankind its citizens."  --  Baha'u'llah