From cb93f4c02a9d6fc368b32ad1c7838b4347ae4703 Mon Sep 17 00:00:00 2001
From: Mike Gerwitz <mike.gerwitz@ryansg.com>
Date: Wed, 2 Dec 2020 14:56:38 -0500
Subject: [PATCH] [DEV-8947] Guided TCO: Reassign argument values after
 processing all expressions

I did say it was _experimental_ guided TRO.

This waits to perform the actual argument reassignment until after
processing the expressions associated with the new arguments, since they
will otherwise be replaced when their original values are still needed.
---
 RELEASES.md                      | 11 ++++++++
 src/current/compiler/js-calc.xsl | 45 +++++++++++++++++++++-----------
 2 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/RELEASES.md b/RELEASES.md
index 317a62de..7571ffa3 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -13,6 +13,17 @@ TAME developers: Add new changes under a "NEXT" heading as part of the
 commits that introduce the changes.  To make a new release, run
 `tools/mkrelease`, which will handle updating the heading for you.
 
+NEXT
+====
+This release provides tail-call optimizations aimed at the query system in
+core.
+
+Compiler
+--------
+- [bugfix] Recursive calls using TCO will wait to overwrite their function
+  arguments until all expressions calculating the new argument values have
+  completed.
+
 v17.6.5 (2020-12-03)
 ====================
 This release improves Summary Page performance when populating the page with
diff --git a/src/current/compiler/js-calc.xsl b/src/current/compiler/js-calc.xsl
index dc375f58..a21b0b4a 100644
--- a/src/current/compiler/js-calc.xsl
+++ b/src/current/compiler/js-calc.xsl
@@ -960,27 +960,42 @@
 
   <variable name="arg-prefix" select="concat( ':', $name, ':' )" />
 
+  <text>(/*TCO*/function(){</text>
+
   <!-- reassign function arguments -->
-  <for-each select="
-      root(.)/preproc:symtable/preproc:sym[
-        @type='func'
-        and @name=$name
-      ]/preproc:sym-ref
-    ">
+  <variable name="args" as="element(c:arg)*">
+    <for-each select="
+        root(.)/preproc:symtable/preproc:sym[
+          @type='func'
+          and @name=$name
+        ]/preproc:sym-ref
+      ">
 
-    <variable name="pname" select="substring-after( @name, $arg-prefix )" />
-    <variable name="arg" select="$self/c:arg[@name=$pname]" />
+      <variable name="pname" select="substring-after( @name, $arg-prefix )" />
+      <variable name="arg" select="$self/c:arg[@name=$pname]" />
 
-    <!-- if the call specified this argument, then use it -->
-    <if test="$arg">
-      <sequence select="concat( '/*TCO*/', $pname, '=' )" />
-      <apply-templates select="$arg/c:*[1]" mode="compile" />
-      <text>,</text>
-    </if>
+      <!-- if the call specified this argument, then use it -->
+      <sequence select="$arg" />
+    </for-each>
+  </variable>
+
+  <!-- store reassignments first in a temporary variable, since the
+       expressions may reference the original arguments and we do not want
+       to overwrite yet -->
+  <for-each select="$args">
+    <sequence select="concat( 'const __tco_', @name, '=' )" />
+    <apply-templates select="c:*[1]" mode="compile" />
+    <text>;</text>
+  </for-each>
+
+  <!-- perform final reassignments, now that expressions no longer need the
+       original values  -->
+  <for-each select="$args">
+    <sequence select="concat( @name, '=__tco_', @name, ';' )" />
   </for-each>
 
   <!-- return value, which doesn't matter since it won't be used -->
-  <text>0</text>
+  <text>return 0;})()</text>
 
   <!-- don't support c:when here; not worth the effort -->
   <if test="./c:when">