From fdd7a41eec615cf3d77a95896a6183326e60c2ca Mon Sep 17 00:00:00 2001
From: Simon Marlow <marlowsd@gmail.com>
Date: Fri, 12 Sep 2008 10:01:40 +0000
Subject: [PATCH] Fix some bugs in the stack-reducing code (#2571)

---
 rts/Schedule.c      |   16 ++++++++--------
 rts/sm/BlockAlloc.c |   16 ++++++++++++----
 rts/sm/Storage.c    |    7 ++++---
 3 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/rts/Schedule.c b/rts/Schedule.c
index a41dd67..3f42814 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -2192,7 +2192,7 @@ static StgTSO *
 threadStackUnderflow (Task *task STG_UNUSED, StgTSO *tso)
 {
     bdescr *bd, *new_bd;
-    lnat new_tso_size_w, tso_size_w;
+    lnat free_w, tso_size_w;
     StgTSO *new_tso;
 
     tso_size_w = tso_sizeW(tso);
@@ -2207,19 +2207,19 @@ threadStackUnderflow (Task *task STG_UNUSED, StgTSO *tso)
     // while we are moving the TSO:
     lockClosure((StgClosure *)tso);
 
-    new_tso_size_w = round_to_mblocks(tso_size_w/2);
-
-    debugTrace(DEBUG_sched, "thread %ld: reducing TSO size from %lu words to %lu",
-               (long)tso->id, tso_size_w, new_tso_size_w);
+    // this is the number of words we'll free
+    free_w = round_to_mblocks(tso_size_w/2);
 
     bd = Bdescr((StgPtr)tso);
-    new_bd = splitLargeBlock(bd, new_tso_size_w / BLOCK_SIZE_W);
-    new_bd->free = bd->free;
+    new_bd = splitLargeBlock(bd, free_w / BLOCK_SIZE_W);
     bd->free = bd->start + TSO_STRUCT_SIZEW;
 
     new_tso = (StgTSO *)new_bd->start;
     memcpy(new_tso,tso,TSO_STRUCT_SIZE);
-    new_tso->stack_size = new_tso_size_w - TSO_STRUCT_SIZEW;
+    new_tso->stack_size = new_bd->free - new_tso->stack;
+
+    debugTrace(DEBUG_sched, "thread %ld: reducing TSO size from %lu words to %lu",
+               (long)tso->id, tso_size_w, tso_sizeW(new_tso));
 
     tso->what_next = ThreadRelocated;
     tso->_link = new_tso; // no write barrier reqd: same generation
diff --git a/rts/sm/BlockAlloc.c b/rts/sm/BlockAlloc.c
index daf9fb0..280ebfc 100644
--- a/rts/sm/BlockAlloc.c
+++ b/rts/sm/BlockAlloc.c
@@ -565,6 +565,9 @@ freeChain_lock(bdescr *bd)
     RELEASE_SM_LOCK;
 }
 
+// splitBlockGroup(bd,B) splits bd in two.  Afterward, bd will have B
+// blocks, and a new block descriptor pointing to the remainder is
+// returned.
 bdescr *
 splitBlockGroup (bdescr *bd, nat blocks)
 {
@@ -575,16 +578,21 @@ splitBlockGroup (bdescr *bd, nat blocks)
     }
 
     if (bd->blocks > BLOCKS_PER_MBLOCK) {
-        nat mblocks;
+        nat low_mblocks, high_mblocks;
         void *new_mblock;
         if ((blocks - BLOCKS_PER_MBLOCK) % (MBLOCK_SIZE / BLOCK_SIZE) != 0) {
             barf("splitLargeBlock: not a multiple of a megablock");
         }
-        mblocks = 1 + (blocks - BLOCKS_PER_MBLOCK) / (MBLOCK_SIZE / BLOCK_SIZE);
-        new_mblock = (void *) ((P_)MBLOCK_ROUND_DOWN(bd) + mblocks * MBLOCK_SIZE_W);
+        low_mblocks = 1 + (blocks - BLOCKS_PER_MBLOCK) / (MBLOCK_SIZE / BLOCK_SIZE);
+        high_mblocks = (bd->blocks - blocks) / (MBLOCK_SIZE / BLOCK_SIZE);
+
+        new_mblock = (void *) ((P_)MBLOCK_ROUND_DOWN(bd) + low_mblocks * MBLOCK_SIZE_W);
         initMBlock(new_mblock);
         new_bd = FIRST_BDESCR(new_mblock);
-        new_bd->blocks = MBLOCK_GROUP_BLOCKS(mblocks);
+        new_bd->blocks = MBLOCK_GROUP_BLOCKS(high_mblocks);
+
+        ASSERT(blocks + new_bd->blocks == 
+               bd->blocks + BLOCKS_PER_MBLOCK - MBLOCK_SIZE/BLOCK_SIZE);
     }
     else
     {
diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index 8d237c1..e10304c 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -661,8 +661,8 @@ allocatedBytes( void )
     return allocated;
 }
 
-// split N blocks off the start of the given bdescr, returning the 
-// remainder as a new block group.  We treat the remainder as if it
+// split N blocks off the front of the given bdescr, returning the
+// new block group.  We treat the remainder as if it
 // had been freshly allocated in generation 0.
 bdescr *
 splitLargeBlock (bdescr *bd, nat blocks)
@@ -680,6 +680,7 @@ splitLargeBlock (bdescr *bd, nat blocks)
     new_bd->step    = g0s0;
     new_bd->flags   = BF_LARGE;
     new_bd->free    = bd->free;
+    ASSERT(new_bd->free <= new_bd->start + new_bd->blocks * BLOCK_SIZE_W);
 
     // add the new number of blocks to the counter.  Due to the gaps
     // for block descriptor, new_bd->blocks + bd->blocks might not be
@@ -687,7 +688,7 @@ splitLargeBlock (bdescr *bd, nat blocks)
     bd->step->n_large_blocks += bd->blocks;
 
     return new_bd;
-}    
+}
 
 /* -----------------------------------------------------------------------------
    allocateLocal()
-- 
1.7.10.4