Implement shrinkSmallMutableArray# and resizeSmallMutableArray#.

This is a part of GHC Proposal #25: "Offer more array resizing primitives".
Resources related to the proposal:

  - Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/121
  - Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0025-resize-boxed.rst

Only shrinkSmallMutableArray# is implemented as a primop since a
library-space implementation of resizeSmallMutableArray# (in GHC.Exts)
is no less efficient than a primop would be. This may be replaced by
a primop in the future if someone devises a strategy for growing
arrays in-place. The library-space implementation always copies the
array when growing it.

This commit also tweaks the documentation of the deprecated
sizeofMutableByteArray#, removing the mention of concurrency. That
primop is unsound even in single-threaded applications. Additionally,
the non-negativity assertion on the existing shrinkMutableByteArray#
primop has been removed since this predicate is trivially always true.

1 files changed, 18 insertions, 1 deletions

diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm
index b66c561dcb..b5930363a1 100644
--- a/rts/PrimOps.cmm
+++ b/rts/PrimOps.cmm
@@ -174,12 +174,13 @@ stg_isMutableByteArrayPinnedzh ( gcptr mba )
 stg_shrinkMutableByteArrayzh ( gcptr mba, W_ new_size )
 // MutableByteArray# s -> Int# -> State# s -> State# s
 {
-   ASSERT(new_size >= 0);
    ASSERT(new_size <= StgArrBytes_bytes(mba));
 
    OVERWRITING_CLOSURE_OFS(mba, (BYTES_TO_WDS(SIZEOF_StgArrBytes) +
                                  ROUNDUP_BYTES_TO_WDS(new_size)));
    StgArrBytes_bytes(mba) = new_size;
+   // See the comments in overwritingClosureOfs for an explanation
+   // of the interaction with LDV profiling.
    LDV_RECORD_CREATE(mba);
 
    return ();
@@ -224,6 +225,22 @@ stg_resizzeMutableByteArrayzh ( gcptr mba, W_ new_size )
    }
 }
 
+// shrink size of SmallMutableArray in-place
+stg_shrinkSmallMutableArrayzh ( gcptr mba, W_ new_size )
+// SmallMutableArray# s -> Int# -> State# s -> State# s
+{
+   ASSERT(new_size <= StgSmallMutArrPtrs_ptrs(mba));
+
+   OVERWRITING_CLOSURE_OFS(mba, (BYTES_TO_WDS(SIZEOF_StgSmallMutArrPtrs) +
+                                 new_size));
+   StgSmallMutArrPtrs_ptrs(mba) = new_size;
+   // See the comments in overwritingClosureOfs for an explanation
+   // of the interaction with LDV profiling.
+   LDV_RECORD_CREATE(mba);
+
+   return ();
+}
+
 // RRN: This one does not use the "ticketing" approach because it
 // deals in unboxed scalars, not heap pointers.
 stg_casIntArrayzh( gcptr arr, W_ ind, W_ old, W_ new )