Merge pull request #4142 from endragor/gc-allow-skip-stack

Added stackSize parameter to GC_step

doc/gc.txt

@@ -56,7 +56,7 @@ file as well). With this switch the GC supports the following operations:
 
 .. code-block:: nim
   proc GC_setMaxPause*(MaxPauseInUs: int)
-  proc GC_step*(us: int, strongAdvice = false)
+  proc GC_step*(us: int, strongAdvice = false, stackSize = -1)
 
 The unit of the parameters ``MaxPauseInUs`` and ``us`` is microseconds.
 
@@ -75,7 +75,13 @@ These two procs are the two modus operandi of the realtime GC:
     This allows the GC to perform some work for up to ``us`` time. This is
     useful to call in a main loop to ensure the GC can do its work. To
     bind all GC activity to a ``GC_step`` call, deactivate the GC with
-    ``GC_disable`` at program startup.
+    ``GC_disable`` at program startup. If ``strongAdvice`` is set to ``true``,
+    GC will be forced to perform collection cycle. Otherwise, GC may decide not
+    to do anything, if there is not much garbage to collect.
+    You may also specify the current stack size via ``stackSize`` parameter.
+    It can improve performance, when you know that there are no unique Nim
+    references below certain point on the stack. Make sure the size you specify
+    is greater than the potential worst case size.
 
 These procs provide a "best effort" realtime guarantee; in particular the
 cycle collector is not aware of deadlines yet. Deactivate it to get more

lib/system/gc.nim

@@ -980,7 +980,19 @@ when withRealTime:
       collectCTBody(gch)
     release(gch)
 
-  proc GC_step*(us: int, strongAdvice = false) = GC_step(gch, us, strongAdvice)
+  proc GC_step*(us: int, strongAdvice = false, stackSize = -1) {.noinline.} =
+    var stackTop {.volatile.}: pointer
+    let prevStackBottom = gch.stackBottom
+    if stackSize >= 0:
+      stackTop = addr(stackTop)
+      when stackIncreases:
+        gch.stackBottom = cast[pointer](
+          cast[ByteAddress](stackTop) - sizeof(pointer) * 6 - stackSize)
+      else:
+        gch.stackBottom = cast[pointer](
+          cast[ByteAddress](stackTop) + sizeof(pointer) * 6 + stackSize)
+    GC_step(gch, us, strongAdvice)
+    gch.stackBottom = prevStackBottom
 
 when not defined(useNimRtl):
   proc GC_disable() =

lib/system/gc2.nim

@@ -956,7 +956,19 @@ when withRealTime:
         strongAdvice:
       collectCTBody(gch)
 
-  proc GC_step*(us: int, strongAdvice = false) = GC_step(gch, us, strongAdvice)
+  proc GC_step*(us: int, strongAdvice = false, stackSize = -1) {.noinline.} =
+    var stackTop {.volatile.}: pointer
+    let prevStackBottom = gch.stackBottom
+    if stackSize >= 0:
+      stackTop = addr(stackTop)
+      when stackIncreases:
+        gch.stackBottom = cast[pointer](
+          cast[ByteAddress](stackTop) - sizeof(pointer) * 6 - stackSize)
+      else:
+        gch.stackBottom = cast[pointer](
+          cast[ByteAddress](stackTop) + sizeof(pointer) * 6 + stackSize)
+    GC_step(gch, us, strongAdvice)
+    gch.stackBottom = prevStackBottom
 
 when not defined(useNimRtl):
   proc GC_disable() =