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, 5 insertions, 0 deletions

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 3779a79b70..942adb0394 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -814,6 +814,11 @@ primop  NewPinnedByteArrayOp_Char "newPinnedByteArray#" GenPrimOp
    {Create a mutable byte array that the GC guarantees not to move.}
    with out_of_line = True
 
+primop  NewAlignedPinnedByteArrayOp_Char "newAlignedPinnedByteArray#" GenPrimOp
+   Int# -> Int# -> State# s -> (# State# s, MutableByteArray# s #)
+   {Create a mutable byte array, aligned by the specified amount, that the GC guarantees not to move.}
+   with out_of_line = True
+
 primop  ByteArrayContents_Char "byteArrayContents#" GenPrimOp
    ByteArray# -> Addr#
    {Intended for use with pinned arrays; otherwise very unsafe!}