compiler: fix overestimated register count.

The compiler internally allocates registers that are never
inserted into code (but correctly recycled). Thus the treg
bump counter overestimates the actual number of registers that
a VM requires; but that's how we use it! That causes wasted
stack space.

What we can do to get a tightly clamped register count
is simply to let the assembler report the highest-numbered
treg operand that it finds in the code.

(In the future we will need something better to optimize the
register allocation. Why: because closures have their own VM
instance with its own registers. If an overall VM uses
registers t00 through t09, but a certain closure uses
only t02 through t06, that closure's frame only needs to
allocate seven registers.)

* share/txr/stdlib/asm.tl (assembler): New slot, max-treg.
(assembler parse-args): Whenever a register operand is parsed,
if it is a t register, update max-treg.

* share/txr/stdlib/compiler.tl (usr:compile-toplevel): When
creating the vm-desc, determine the nreg parameter from the
assembler's max-treg value, rather than from the compiler's
treg allocation bump counter.

2 files changed, 4 insertions, 1 deletions

diff --git a/share/txr/stdlib/asm.tl b/share/txr/stdlib/asm.tl
index 33eed4bf..1c4e76c2 100644
--- a/share/txr/stdlib/asm.tl
+++ b/share/txr/stdlib/asm.tl
@@ -45,6 +45,7 @@
   (defstruct assembler nil
     buf
     bstr
+    (max-treg 0)
     (labdef (hash))
     (labref (hash))
     (:static imm-width (relate '(si mi bi) '(10 16 32)))
@@ -169,6 +170,8 @@
                          (not (< parg 1024)))
                 oc.(synerr "argument ~a of ~s isn't a small register"
                            n syntax))
+              (when (and (member type '(r rs d ds)) (< parg 256))
+                (set me.max-treg (max parg me.max-treg)))
               parg))
           pattern (rest syntax) (range 1)))
 
diff --git a/share/txr/stdlib/compiler.tl b/share/txr/stdlib/compiler.tl
index 0a46e0ae..bab0d341 100644
--- a/share/txr/stdlib/compiler.tl
+++ b/share/txr/stdlib/compiler.tl
@@ -1302,7 +1302,7 @@
       co.(free-treg oreg)
       co.(check-treg-leak)
       as.(asm ^(,*frag.code (end ,frag.oreg)))
-      (vm-make-desc co.nlev co.treg-cntr as.buf co.(get-datavec) co.(get-funvec)))))
+      (vm-make-desc co.nlev (succ as.max-treg) as.buf co.(get-datavec) co.(get-funvec)))))
 
 (defvarl %file-suff-rx% #/[.][^\\\/.]+/)