Eliminate zero_static_objects_list()

Summary:
[Revised version of D1076 that was committed and then backed out]

In a workload with a large amount of code, zero_static_objects_list()
takes a significant amount of time, and furthermore it is in the
single-threaded part of the GC.

This patch uses a slightly fiddly scheme for marking objects on the
static object lists, using a flag in the low 2 bits that flips between
two states to indicate whether an object has been visited during this
GC or not.  We also have to take into account objects that have not
been visited yet, which might appear at any time due to runtime linking.

Test Plan: validate

Reviewers: austin, ezyang, rwbarton, bgamari, thomie

Reviewed By: bgamari, thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1106

1 files changed, 5 insertions, 2 deletions

diff --git a/rts/sm/GCThread.h b/rts/sm/GCThread.h
index cbe4346afe..d42b89f973 100644
--- a/rts/sm/GCThread.h
+++ b/rts/sm/GCThread.h
@@ -131,8 +131,11 @@ typedef struct gc_thread_ {
                                    //  during GC without accessing the block
                                    //   allocators spin lock. 
 
-    StgClosure* static_objects;      // live static objects
-    StgClosure* scavenged_static_objects;   // static objects scavenged so far
+    // These two lists are chained through the STATIC_LINK() fields of static
+    // objects.  Pointers are tagged with the current static_flag, so before
+    // following a pointer, untag it with UNTAG_STATIC_LIST_PTR().
+    StgClosure* static_objects;            // live static objects
+    StgClosure* scavenged_static_objects;  // static objects scavenged so far
 
     W_ gc_count;                 // number of GCs this thread has done