New wi::bitreverse function.

This patch provides a wide-int implementation of bitreverse, that
implements both of Richard Sandiford's suggestions from the review at
https://gcc.gnu.org/pipermail/gcc-patches/2023-May/618215.html of an
improved API (as a stand-alone function matching the bswap refactoring),
and an implementation that works with any bit-width precision.

2023-06-05  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* wide-int.cc (wi::bitreverse_large): New function implementing
	bit reversal of an integer.
	* wide-int.h (wi::bitreverse): New (template) function prototype.
	(bitreverse_large): Prototype helper function/implementation.
	(wi::bitreverse): New template wrapper around bitreverse_large.

2 files changed, 42 insertions, 0 deletions

diff --git a/gcc/wide-int.cc b/gcc/wide-int.cc
index 1e4c046..24bdce2 100644
--- a/gcc/wide-int.cc
+++ b/gcc/wide-int.cc
@@ -766,6 +766,33 @@ wi::bswap_large (HOST_WIDE_INT *val, const HOST_WIDE_INT *xval,
   return canonize (val, len, precision);
 }
 
+/* Bitreverse the integer represented by XVAL and LEN into VAL.  Return
+   the number of blocks in VAL.  Both XVAL and VAL have PRECISION bits.  */
+unsigned int
+wi::bitreverse_large (HOST_WIDE_INT *val, const HOST_WIDE_INT *xval,
+		      unsigned int len, unsigned int precision)
+{
+  unsigned int i, s;
+
+  for (i = 0; i < len; i++)
+    val[i] = 0;
+
+  for (s = 0; s < precision; s++)
+    {
+      unsigned int block = s / HOST_BITS_PER_WIDE_INT;
+      unsigned int offset = s & (HOST_BITS_PER_WIDE_INT - 1);
+      if (((safe_uhwi (xval, len, block) >> offset) & 1) != 0)
+	{
+	  unsigned int d = (precision - 1) - s;
+	  block = d / HOST_BITS_PER_WIDE_INT;
+	  offset = d & (HOST_BITS_PER_WIDE_INT - 1);
+          val[block] |= 1 << offset;
+	}
+    }
+
+  return canonize (val, len, precision);
+}
+
 /* Fill VAL with a mask where the lower WIDTH bits are ones and the bits
    above that up to PREC are zeros.  The result is inverted if NEGATE
    is true.  Return the number of blocks in VAL.  */
diff --git a/gcc/wide-int.h b/gcc/wide-int.h
index e4723ad..498d14d 100644
--- a/gcc/wide-int.h
+++ b/gcc/wide-int.h
@@ -553,6 +553,7 @@ namespace wi
   UNARY_FUNCTION zext (const T &, unsigned int);
   UNARY_FUNCTION set_bit (const T &, unsigned int);
   UNARY_FUNCTION bswap (const T &);
+  UNARY_FUNCTION bitreverse (const T &);
 
   BINARY_FUNCTION min (const T1 &, const T2 &, signop);
   BINARY_FUNCTION smin (const T1 &, const T2 &);
@@ -1748,6 +1749,8 @@ namespace wi
 			      unsigned int, unsigned int, unsigned int);
   unsigned int bswap_large (HOST_WIDE_INT *, const HOST_WIDE_INT *,
 			    unsigned int, unsigned int);
+  unsigned int bitreverse_large (HOST_WIDE_INT *, const HOST_WIDE_INT *,
+				 unsigned int, unsigned int);
   
   unsigned int lshift_large (HOST_WIDE_INT *, const HOST_WIDE_INT *,
 			     unsigned int, unsigned int, unsigned int);
@@ -2281,6 +2284,18 @@ wi::bswap (const T &x)
   return result;
 }
 
+/* Bitreverse the integer X.  */
+template <typename T>
+inline WI_UNARY_RESULT (T)
+wi::bitreverse (const T &x)
+{
+  WI_UNARY_RESULT_VAR (result, val, T, x);
+  unsigned int precision = get_precision (result);
+  WIDE_INT_REF_FOR (T) xi (x, precision);
+  result.set_len (bitreverse_large (val, xi.val, xi.len, precision));
+  return result;
+}
+
 /* Return the mininum of X and Y, treating them both as having
    signedness SGN.  */
 template <typename T1, typename T2>