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[Caml-list] OCaml SPARC asm compiler implementation question
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Date: 2002-06-03 (16:45)
From: John Carr <jfc@M...>
Subject: [Caml-list] OCaml SPARC asm compiler implementation question

ocaml 3.04 uses registers %l6 and %l7 for allocation in compiled SPARC
code.  %l6 is the address of the last object allocated.  %l7 is a
pointer to a word holding the lower limit of the youngest generation,
young_limit in minor_gc.c.

The alloc instruction sequence looks like:
	ld	[%l7],%g1	! load young_limit
	sub	%l6,N,%l6	! decrement alloc pointer
	cmp	%l6,%g1		! bounds check
	bgeu	label		! branch if out of memory

I have three questions:

Why does %l7 hold the address of young_limit instead of its value?
The alloc instruction sequence would be reduced by one instruction and
1-3 cycles (depending on chip) if the load could be omitted.  The MIPS
implementation does not have this indirection.

Looking at this code made me wonder, what fraction of operations in a
typical ocaml program are allocations?  _Garbage Collection_ quotes
statistics for several functional languages, including a version of ML,
but I don't think it mentions ocaml.

Are many people using ocaml on UltraSPARC processors?  The code is
optimized for older SPARCs and substantial improvement is possible
for UltraSPARC.  (For example, the integer load-double and store-double
instructions are faster than a pair of load or store instructions on
older SPARCs, are substantially slower on UltraSPARC, and trap to
emulation on the Hal SPARC-64.)  More on this in a future message.

    --John Carr (jfc@mit.edu)
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