Optimize clearNursery by short-circuiting when we get to currentNursery

This is a note to myself so I don't forget about this. Essentially, we can do something like this (this particular patch variant untested):

diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index 36776b9..0311042 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -598,6 +598,11 @@ clearNursery (Capability *cap)
         ASSERT(bd->gen_no == 0);
         ASSERT(bd->gen == g0);
         IF_DEBUG(sanity,memset(bd->start, 0xaa, BLOCK_SIZE));
+        if (bd == cap->r.rCurrentNursery) {
+            IF_DEBUG(sanity, for (bd = bd->link; bd; bd = bd->link)
+                                ASSERT(bd->free == bd->start));
+            break;
+        }
     }
     }
 }

This is due to invariants about how we manage the currentNursery pointer. But we need a note about it, and I need to test it more carefully. This optimization probably doesn't help too much on normal GHC, but when I have lots of nurseries it helps quite a bit.

Trac metadata

Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Runtime System
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CC	simonmar
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