[wide-int] Handle more add and sub cases inline

[Richard Sandiford]

Currently add and sub have no fast path for offset_int and widest_int,
they just call the out-of-line version.  This patch handles the
single-HWI cases inline.  At least on x86_64, this only adds one branch
per call; the fast path itself is straight-line code.

On the same fold-const.ii testcase, this reduces the number of
add_large calls from 877507 to 42459.  It reduces the number of
sub_large calls from 25707 to 148.

Tested on x86_64-linux-gnu.  OK to install?

Thanks,
Richard


Index: gcc/wide-int.h
===================================================================
--- gcc/wide-int.h      2013-11-28 13:34:19.596839877 +0000
+++ gcc/wide-int.h      2013-11-28 16:08:11.387731775 +0000
@@ -2234,6 +2234,17 @@ wi::add (const T1 &x, const T2 &y)
       val[0] = xi.ulow () + yi.ulow ();
       result.set_len (1);
     }
+  else if (STATIC_CONSTANT_P (precision > HOST_BITS_PER_WIDE_INT)
+          && xi.len + yi.len == 2)
+    {
+      unsigned HOST_WIDE_INT xl = xi.ulow ();
+      unsigned HOST_WIDE_INT yl = yi.ulow ();
+      unsigned HOST_WIDE_INT resultl = xl + yl;
+      val[0] = resultl;
+      val[1] = (HOST_WIDE_INT) resultl < 0 ? 0 : -1;
+      result.set_len (1 + (((resultl ^ xl) & (resultl ^ yl))
+                          >> (HOST_BITS_PER_WIDE_INT - 1)));
+    }
   else
     result.set_len (add_large (val, xi.val, xi.len,
                               yi.val, yi.len, precision,
@@ -2288,6 +2299,17 @@ wi::sub (const T1 &x, const T2 &y)
       val[0] = xi.ulow () - yi.ulow ();
       result.set_len (1);
     }
+  else if (STATIC_CONSTANT_P (precision > HOST_BITS_PER_WIDE_INT)
+          && xi.len + yi.len == 2)
+    {
+      unsigned HOST_WIDE_INT xl = xi.ulow ();
+      unsigned HOST_WIDE_INT yl = yi.ulow ();
+      unsigned HOST_WIDE_INT resultl = xl - yl;
+      val[0] = resultl;
+      val[1] = (HOST_WIDE_INT) resultl < 0 ? 0 : -1;
+      result.set_len (1 + (((resultl ^ xl) & (xl ^ yl))
+                          >> (HOST_BITS_PER_WIDE_INT - 1)));
+    }
   else
     result.set_len (sub_large (val, xi.val, xi.len,
                               yi.val, yi.len, precision,