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Re: Continuations, basic blocks, loops and register allocation

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From:
Leopold Toetsch
Date:
November 17, 2004 00:38
Subject:
Re: Continuations, basic blocks, loops and register allocation
Message ID:
200411170748.iAH7mwq32639@thu8.leo.home
Matt Fowles <ubermatt@gmail.com> wrote:

> ...  Thus you can consider all of the
> following questions (even though they will be phrased as statements).

> 1)  After a full continuation is taken all of the registers must be
> considered invalid.

Calling a subroutine allocates a new register frame, that subs register
frame pointer in the context points to these fresh registers.

A continuation restores the context it captured, i.e. at the place,
where it was created. This is true for all continuations. Inside the
context there is a *pointer* to a register frame, which is therefore
restored too.

The effect of taking a continuation is therefore to restore registers to
that state where the continuation was created. Due to calling conventions
a part of the registers is volatile (used during a call or as return
results), while the other part is non-volatile.

Until here there is no difference between return or full continuation.

The effect of a full continuation can be to create a loop, where the
known control flow doesn't show a loop. Without further syntax to denote
such loops 1) is true. This register invalidation happens, if a
preserved register was e.g. only used once after the call and then that
register got reassigned, which is allowed for a linear control flow but
not inside a loop.

This has per se nothing to do with a continuation. If you got an opcode
that does *silently* a "goto again_label" the CFG doesn't cope with the
loop, because it isn't there and things start breaking. The effect of a
full continuation *is* to create such loops.

> 2)  After a return continuation is taken, the registers can be trusted.

Yes, according to usage in pdd03.

> 3)  If someone takes a full continuation, all return continuations
> down the callstack must be promoted.

If one *creates* a full continuation ...

> 4)  After a function call, some magic needs to happen so that the code
> knows whether it came back to itself via a return continuation and can
> trust its registers, or it came back via a full continuation and
> cannot trust them.

No. It's too late for magic. Either the CFG is known at compile time or
refetching in the presence of full continuations is mandatory. For both
the code must reflect the facts.

> Corrections welcome,
> Matt

leo

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