compiler: optimize away discard register.

When compiling the procedural special forms prog and prog1,
there are subforms whose values are discarded.  The
compilation of those forms still requires an output register,
which is passed down.  In certain cases, wasteful moves of data
into that register are generated, which we can eliminate.

* share/txr/stdlib/compiler.tl (struct compiler): New slot,
discards. Holds t-registers that are marked as discard.
(compiler alloc-discard-treg): New method.
(compiler free-treg): Remove freed treg from discard list.
(compiler maybe-alloc-treg): If the given register is a
discard, we must allocate.
(compiler maybe-mov): New method, replacing maybe-mov function.
(compiler comp-if): Replace maybe-mov function calls with method.
In the (if test then) case, avoid referencing oreg register
after a maybe-mov since it may be a discard such that the
maybe-mov produced no code; reference the original register.
(comp-progn): Allocate oreg-discard with alloc-discard-treg
method instead of alloc-treg.
(comp-prog1): Same thing with igreg.
(comp-for): For compiling the test expression, use the same
output register as what was used for the init block. Do not
borrow oreg for this, which may be a discard that will be
removed by the maybe-mov.
(compiler (comp-setq, comp-ift, comp-switch, comp-block,
comp-catch, comp-let, comp-fbind, comp-lambda-impl, comp-or,
comp-tree-case, comp-load-time-lit): Replace maybe-mov
function calls with method.
(maybe-mov): Function removed, replaced by method.

0 files changed, 0 insertions, 0 deletions