Implement `decodeDouble_Int64#` primop

The existing `decodeDouble_2Int#` primop is rather inconvenient to use
(and in fact is not even used by `integer-gmp`) as the mantissa is split
into 3 components which would actually fit in an `Int64#` value.

However, `decodeDouble_Int64#` is to be used by the new `integer-gmp2`
re-implementation (see #9281).

Moreover, `decodeDouble_2Int#` performs direct bit-wise operations on the
IEEE representation which can be replaced by a combination of the
portable standard C99 `scalbn(3)` and `frexp(3)` functions.

Differential Revision: https://phabricator.haskell.org/D160

1 files changed, 17 insertions, 0 deletions

diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm
index ee50f7fed5..cb4cd5e9fa 100644
--- a/rts/PrimOps.cmm
+++ b/rts/PrimOps.cmm
@@ -22,6 +22,7 @@
  * ---------------------------------------------------------------------------*/
 
 #include "Cmm.h"
+#include "MachDeps.h"
 
 #ifdef __PIC__
 import pthread_mutex_lock;
@@ -807,6 +808,22 @@ stg_decodeDoublezu2Intzh ( D_ arg )
     return (r1, r2, r3, r4);
 }
 
+/* Double# -> (# Int64#, Int# #) */
+stg_decodeDoublezuInt64zh ( D_ arg )
+{
+    CInt exp;
+    I64  mant;
+    W_   mant_ptr;
+
+    STK_CHK_GEN_N (SIZEOF_INT64);
+    reserve BYTES_TO_WDS(SIZEOF_INT64) = mant_ptr {
+        (exp) = ccall __decodeDouble_Int64(mant_ptr "ptr", arg);
+        mant = I64[mant_ptr];
+    }
+
+    return (mant, TO_W_(exp));
+}
+
 /* -----------------------------------------------------------------------------
  * Concurrency primitives
  * -------------------------------------------------------------------------- */