[perf fix] Add manual GC points to 'if' and 'case'

This resolves issue #2123.

The 'filter' loop would evaluate expressions, and allocate, without
every hitting a GC point.

To break out of a loop or recursion, you will need 'if' or 'case'.  So
I believe this should work everywhere, just as it does the
test/bug-2123.* repro.

I'm not quite sure why the bug depended on get() NOT having a default
arg though.  Could look into that more.

osh/cmd_eval.py

@@ -1726,6 +1726,10 @@ def _Dispatch(self, node, cmd_st):
                 # It will be taken care of by command.Simple, condition, etc.
                 status = self._DoIf(node)
 
+                # Perf bug fix: loops might only execute 'if', but we still
+                # need to GC
+                self._LeafTick()
+
             elif case(command_e.Case):
                 node = cast(command.Case, UP_node)
 
@@ -1734,6 +1738,10 @@ def _Dispatch(self, node, cmd_st):
                 self._MaybeRunDebugTrap()
                 status = self._DoCase(node)
 
+                # Perf bug fix: loops might only execute 'case', but we still
+                # need to GC
+                self._LeafTick()
+
             elif case(command_e.WhileUntil):
                 node = cast(command.WhileUntil, UP_node)
 

test/bug-2123.sh

@@ -4,11 +4,12 @@ demo() {
   local ysh=_bin/cxx-opt/ysh
   ninja $ysh
 
-  #OILS_GC_STATS=1 $ysh test/bug-2123.ysh
+  #OILS_GC_STATS=1 _OILS_GC_VERBOSE=1 $ysh test/bug-2123.ysh
+  time OILS_GC_STATS=1 $ysh test/bug-2123.ysh
 
   # max RSS
   # 246
-  /usr/bin/time --format '%e %M' $ysh test/bug-2123.ysh
+  # /usr/bin/time --format '%e %M' $ysh test/bug-2123.ysh
 }
 
 debug() {

test/bug-2123.ysh

@@ -1,10 +1,15 @@
 # Run with _bin/cxx-opt/ysh
 
 proc filter (; predicate) {
+  #var i = 0
 	for line in (io.stdin) {
 		if (io->evalExpr(predicate, vars={_val: fromJson8(line)})) {
 			write -- $line
 		}
+    #setvar i += 1
+    #if (i % 1000 === 0) {
+    #  echo "i $i"
+    #}
 	}
 }
 
@@ -67,7 +72,10 @@ proc without-default {
   } < $f
 
   #time filter [get(_val, 'missing-key', 0) === 0] < $f >/dev/null
+
+  echo '****** BEGIN filter'
   time filter [get(_val, 'missing-key')] < $f #>/dev/null
+  echo '****** END filter'
 
   write -- 'AFTER -------------------------------------------------------'