[bitc-dev] Implementing closure conversion

[Jonathan S. Shapiro]

I've been looking (with dismay) at closure conversion. More precisely:
at the challenges of correctly implementing it in the bootstrap
compiler.

Many procedures do not require a closure environment pointer. If a
procedure only references globals, there is no need to build an
explicitly separate environment for it.

Adding a closure environment pointer to a procedure's arguments if you
don't need one is very damaging on those architectures that pass
parameters in registers. It is almost certain to occupy one of those
registers, and if you are not going to use it that seems like a waste.

Some procedures *do* require a closure environment pointer. This means,
in particular, that when a function is passed as an argument we need to
allow for the possibility.

There are three possible implementations for all this:

1. Emit all procedures to take a closure environment pointer, even when
they do not need one. This implies that calls out to C will require a
wrapper procedure to strip the closure environment argument.

2. Emit closure environment pointer arguments only when the procedure
needs one. For such procedures, allocate a runtime closure structure
that inserts the additional argument at the front of the argument list
at runtime.

3. Do a bunch of more complicated static analysis to determine if a
procedure is actually captured and only add the closure argument for
procedures that are "at risk".


I had hoped to do option (2), and this is currently implemented for
IA-32. Then I started looking at various calling convention
specifications, and I realized that for SPARC in particular this is
going to be truly horrible. If we were going to ASM I could work around
it, but since we are emitting C it's a real pain in the butt.

Because this is a bootstrap compiler, I am currently leaning toward
adopting option (1) instead. Does anybody know of a better option that
we can do *quickly*? I'm really trying to avoid emitting
procedure-specific closure insertion code.


shap