[AArch64] Use scvtf fbits option where appropriate

[Wilco Dijkstra]

Hi Joel,

A few comments below:

+/* If X is a positive CONST_DOUBLE with a value that is the reciprocal of a
+   power of 2 (i.e 1/2^n) return the number of float bits. e.g. for x==(1/2^n)
+   return log2 (n). Otherwise return 0.  */
+int
+aarch64_fpconst_pow2_recip (rtx x)
+{
+  REAL_VALUE_TYPE r0;
+
+  if (!CONST_DOUBLE_P (x))
+    return 0;
+
+  r0 = *CONST_DOUBLE_REAL_VALUE (x);
+  if (exact_real_inverse (DFmode, &r0)
+      && !REAL_VALUE_NEGATIVE (r0))
+    {
+      if (exact_real_truncate (DFmode, &r0))

Truncate to double? That doesn't do anything...

+	{
+	  HOST_WIDE_INT value = real_to_integer (&r0);
+	  value = value & 0xffffffff;
+	  if ((value != 0) && ( (value & (value - 1)) == 0))
+	    {
+	      int ret = exact_log2 (value);
+	      gcc_assert (IN_RANGE (ret, 0, 31));
+	      return ret;
+	    }

Wouldn't it be easier to just do exact_log2 (real_to_integer (&r0))
and then check the range is in 1..31?

--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -6016,6 +6016,40 @@
   [(set_attr "type" "f_cvtf2i")]
 )
 
+(define_insn "*aarch64_<su_optab>cvtf_<fcvt_target>_<GPF:mode>2_mult"
+  [(set (match_operand:GPF 0 "register_operand" "=w,w")
+	(mult:GPF (FLOATUORS:GPF
+		   (match_operand:<FCVT_TARGET> 1 "register_operand" "w,?r"))
+		   (match_operand 2 "aarch64_fp_pow2_recip""Dt,Dt")))]

We should add a comment before both define_insn similar to the other
conversions, explaining what they do and why there are 2 separate patterns
(the default versions of the conversions appear to be missing a comment too).

Wilco