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, 18 insertions, 0 deletions

diff --git a/rts/StgPrimFloat.c b/rts/StgPrimFloat.c
index 123e77b806..6e78546b3e 100644
--- a/rts/StgPrimFloat.c
+++ b/rts/StgPrimFloat.c
@@ -17,6 +17,10 @@
 
 #define IEEE_FLOATING_POINT 1
 
+#if FLT_RADIX != 2
+# error FLT_RADIX != 2 not supported
+#endif
+
 /*
  * Encoding and decoding Doubles.  Code based on the HBC code
  * (lib/fltcode.c).
@@ -158,6 +162,20 @@ __decodeDouble_2Int (I_ *man_sign, W_ *man_high, W_ *man_low, I_ *exp, StgDouble
     }
 }
 
+/* This is expected to replace uses of __decodeDouble_2Int() in the long run */
+StgInt
+__decodeDouble_Int64 (StgInt64 *const mantissa, const StgDouble dbl)
+{
+    if (dbl) {
+        int exp = 0;
+        *mantissa = (StgInt64)scalbn(frexp(dbl, &exp), DBL_MANT_DIG);
+        return exp-DBL_MANT_DIG;
+    } else {
+        *mantissa = 0;
+        return 0;
+    }
+}
+
 /* Convenient union types for checking the layout of IEEE 754 types -
    based on defs in GNU libc <ieee754.h>
 */