Partial fix for #2917

 - add newAlignedPinnedByteArray# for allocating pinned BAs with
   arbitrary alignment

 - the old newPinnedByteArray# now aligns to 16 bytes

Foreign.alloca will use newAlignedPinnedByteArray#, and so might end
up wasting less space than before (we used to align to 8 by default).
Foreign.allocaBytes and Foreign.mallocForeignPtrBytes will get 16-byte
aligned memory, which is enough to avoid problems with SSE
instructions on x86, for example.

There was a bug in the old newPinnedByteArray#: it aligned to 8 bytes,
but would have failed if the header was not a multiple of 8
(fortunately it always was, even with profiling).  Also we
occasionally wasted some space unnecessarily due to alignment in
allocatePinned().

I haven't done anything about Foreign.malloc/mallocBytes, which will
give you the same alignment guarantees as malloc() (8 bytes on
Linux/x86 here).

1 files changed, 4 insertions, 0 deletions

diff --git a/rts/sm/Compact.c b/rts/sm/Compact.c
index 6758cfa560..5f56c1e84b 100644
--- a/rts/sm/Compact.c
+++ b/rts/sm/Compact.c
@@ -490,6 +490,10 @@ update_fwd_large( bdescr *bd )
 
   for (; bd != NULL; bd = bd->link) {
 
+    // nothing to do in a pinned block; it might not even have an object
+    // at the beginning.
+    if (bd->flags & BF_PINNED) continue;
+
     p = bd->start;
     info  = get_itbl((StgClosure *)p);