compiler: eliminate wasteful treg nulling.

* stdlib/optimize.tl (live-info): New slot, clobbered.
(basic-block): New slot, cycle. Struct also inherits
clobbered slot from live-info.
(basic-block print): Print clobbered and cycle.
(basic-blocks local-liveness): Calculate clobbered
for each instruction and from that for the basic block.
(basic-blocks identify-cycle-members): New method.
Discovers which basic blocks are part of any cycle,
and identifies them by setting the new cycle slot
to t.
(basic-blocks do-peephole-block): New local functions
here for determining whether a register has been
clobbered before the first instruction, either
in the same basic block or any ancestors. Only
works when the block is not part of a cycle.
We add a peephole pattern matching move instructions
that set tregs to (t 0)/nil. When we are not in a
cycle block, and the treg has not previously been
clobbered, we know it is clean: it still has the initial
nil value set by the VM and we can remove the instruction.

* stdlib/compiler.tl (compiler optimize): Call the
identify-cycle-members method before peephole.

1 files changed, 1 insertions, 0 deletions

diff --git a/stdlib/compiler.tl b/stdlib/compiler.tl
index fe4db13e..c3043d38 100644
--- a/stdlib/compiler.tl
+++ b/stdlib/compiler.tl
@@ -1840,6 +1840,7 @@
             (while* (and (>= olev 6)
                          (neql nblocks (set nblocks bb.(num-blocks))))
               bb.(calc-liveness)
+              bb.(identify-cycle-members)
               bb.(peephole)
               bb.(link-graph)
               bb.(thread-jumps)