Store a destination step in the block descriptor

At the moment, this just saves a memory reference in the GC inner loop
(worth a percent or two of GC time).  Later, it will hopefully let me
experiment with partial steps, and simplifying the generation/step
infrastructure.

2 files changed, 25 insertions, 14 deletions

diff --git a/includes/rts/storage/Block.h b/includes/rts/storage/Block.h
index 849f99f430..e99a03e76c 100644
--- a/includes/rts/storage/Block.h
+++ b/includes/rts/storage/Block.h
@@ -48,22 +48,26 @@
 
 #ifndef CMINUSMINUS
 typedef struct bdescr_ {
-  StgPtr start;			/* start addr of memory */
-  StgPtr free;			/* first free byte of memory */
-  struct bdescr_ *link;		/* used for chaining blocks together */
-  union { 
-      struct bdescr_ *back;	/* used (occasionally) for doubly-linked lists*/
-      StgWord *bitmap;
-      StgPtr  scan;             /* scan pointer for copying GC */
-  } u;
-  unsigned int gen_no;		/* generation */
-  struct step_ *step;		/* step */
-  StgWord32 blocks;		/* no. of blocks (if grp head, 0 otherwise) */
-  StgWord32 flags;              /* block is in to-space */
+    StgPtr start;              /* start addr of memory */
+    StgPtr free;               /* first free byte of memory */
+    struct bdescr_ *link;      /* used for chaining blocks together */
+    union {
+        struct bdescr_ *back;  /* used (occasionally) for doubly-linked lists*/
+        StgWord *bitmap;
+        StgPtr  scan;           /* scan pointer for copying GC */
+    } u;
+
+    struct step_ *step;		/* step */
+    struct step_ *dest;		/* destination step */
+
+    StgWord32 blocks;		/* no. of blocks (if grp head, 0 otherwise) */
+
+    StgWord16 gen_no;
+    StgWord16 flags;            /* block flags, see below */
 #if SIZEOF_VOID_P == 8
-  StgWord32 _padding[2];
+    StgWord32 _padding[2];
 #else
-  StgWord32 _padding[0];
+    StgWord32 _padding[0];
 #endif
 } bdescr;
 #endif
diff --git a/includes/rts/storage/GC.h b/includes/rts/storage/GC.h
index b30582dfd7..aa0531382f 100644
--- a/includes/rts/storage/GC.h
+++ b/includes/rts/storage/GC.h
@@ -214,4 +214,11 @@ void dirty_MUT_VAR(StgRegTable *reg, StgClosure *p);
 /* (needed when dynamic libraries are used). */
 extern rtsBool keepCAFs;
 
+INLINE_HEADER void initBdescr(bdescr *bd, step *step)
+{
+    bd->step   = step;
+    bd->gen_no = step->gen_no;
+    bd->dest   = step->to;
+}
+
 #endif /* RTS_STORAGE_GC_H */