From 96948355a291c4bba4a148a8bfe3cde9389e75d9 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Sat, 2 May 2020 12:46:06 -0700
Subject: compiler: treat nested load-time forms.

load-time forms nested in load-time forms have no special
semantics; it's wasteful to separately hoist them into load
time and store their value in their own D register.
We must be careful: this is not true if a nested form occurs
in a lambda.

* share/txr/stdlib/compiler.tl (*load-time*): New special
variable.
(compiler comp-lambda): Bind *load-time* to nil around
the compilation of the lambda, so load-time forms in the
lambda are hoisted to load time, even if the lambda itself
is wrapped in a load-time form.
(compiler comp-load-time-lit): Bind *load-time* true around
the compilation of the form. If *load-time* is already true,
then skip the special load-time logic and just compile the
enclosed form; the surrounding load-time compilation is taking
care of the load-time hoisting.

* txr.1: Document that load-time forms nested in load-time
forms don't do anything, except in the lambda case.
---
 share/txr/stdlib/compiler.tl | 33 +++++++++++++++++--------------
 txr.1                        | 46 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+), 14 deletions(-)

diff --git a/share/txr/stdlib/compiler.tl b/share/txr/stdlib/compiler.tl
index ca86db11..f1c4ca09 100644
--- a/share/txr/stdlib/compiler.tl
+++ b/share/txr/stdlib/compiler.tl
@@ -199,6 +199,8 @@
 
 (defvar *dedup*)
 
+(defvar *load-time*)
+
 (defun dedup (obj)
   (cond
     ((null obj) nil)
@@ -815,7 +817,8 @@
 
 (defmeth compiler comp-lambda (me oreg env form)
   (mac-param-bind form (op par-syntax . body) form
-    (let* ((pars (new (fun-param-parser par-syntax form)))
+    (let* ((*load-time* nil)
+           (pars (new (fun-param-parser par-syntax form)))
            (need-frame (or (plusp pars.nfix) pars.rest))
            (nenv (if need-frame (new env up env co me) env))
            lexsyms fvars specials need-dframe)
@@ -1239,19 +1242,21 @@
 
 (defmeth compiler comp-load-time-lit (me oreg env form)
   (mac-param-bind form (op loaded-p exp) form
-    (if loaded-p
-      me.(compile oreg env ^(quote ,exp))
-      (compile-in-toplevel me
-        (let* ((dreg me.(alloc-dreg))
-               (exp me.(compile dreg (new env co me) exp))
-               (lt-frag (new (frag dreg
-                                   ^(,*exp.code
-                                     ,*(maybe-mov dreg exp.oreg))
-                                   exp.fvars
-                                   exp.ffuns))))
-          (misleading-ref-check exp env form)
-          (push lt-frag me.lt-frags)
-          (new (frag dreg nil)))))))
+    (cond
+      (loaded-p me.(compile oreg env ^(quote ,exp)))
+      (*load-time* me.(compile oreg env exp))
+      (t (compile-in-toplevel me
+           (let* ((*load-time* t)
+                  (dreg me.(alloc-dreg))
+                  (exp me.(compile dreg (new env co me) exp))
+                  (lt-frag (new (frag dreg
+                                      ^(,*exp.code
+                                         ,*(maybe-mov dreg exp.oreg))
+                                      exp.fvars
+                                      exp.ffuns))))
+             (misleading-ref-check exp env form)
+             (push lt-frag me.lt-frags)
+             (new (frag dreg nil))))))))
 
 (defun maybe-mov (to-reg from-reg)
   (if (nequal to-reg from-reg)
diff --git a/txr.1 b/txr.1
index e6f39589..c8da18f7 100644
--- a/txr.1
+++ b/txr.1
@@ -71443,6 +71443,52 @@ the immediately enclosing form which surrounds the
 .code load-time
 form.
 
+A
+.code load-time
+form may be nested inside another
+.code load-time
+form. In this situation, two cases occur.
+
+If the two forms are not embedded in a
+.codn lambda ,
+or else are embedded in the same
+.codn lambda ,
+then the inner
+.code load-time
+form is superfluous due to the presence of the outer
+.codn load-time .
+That is to say, the inner
+.mono
+.meti (load-time << form )
+.onom
+expression is equivalent to
+.metn form ,
+because the outer form already establishes its evaluation to be in a load-time
+context.
+
+If the inner
+.code load-time
+form occurs in a
+.codn lambda ,
+but the outer form occurs outside of that
+.codn lambda ,
+then the semantics of the inner
+.code load-time
+form is relevant and necessary. This is because expressions occurring in a
+.code lambda
+are evaluated when the
+.code lambda
+is called, which may take place from a non-load-time context, even if the
+.code lambda
+itself was produced in a load-time context.
+
+An expression being embedded in a
+.code lambda
+means that it appears either in the
+.code lambda
+body, or else in the parameter list as the initializing
+expression for an optional parameter.
+
 .TP* Notes:
 
 When interpreted code containing
-- 
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