Implement ranger folder for __builtin_signbit.

Now that we keep track of the signbit, we can use it to fold __builtin_signbit.

I am assuming I don't have try too hard to get the actual signbit
number and 1 will do.  Especially, since we're inconsistent in trunk whether
we fold the builtin or whether we calculate it at runtime.

abulafia:~$ cat a.c
float nzero = -0.0;

main(){
    printf("0x%x\n", __builtin_signbit(-0.0));
    printf("0x%x\n", __builtin_signbit(nzero));
}
abulafia:~$ gcc a.c -w && ./a.out
0x1
0x80000000

It is amazing that we've been failing to fold something as simple as
this:

	if (x > 5.0)
	  num = __builtin_signbit (x);

It does the right thing now :-P.

gcc/ChangeLog:

	* gimple-range-fold.cc
	(fold_using_range::range_of_builtin_int_call): Add case for
	CFN_BUILT_IN_SIGNBIT.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-signbit-1.c: New test.

2 files changed, 32 insertions, 0 deletions

diff --git a/gcc/gimple-range-fold.cc b/gcc/gimple-range-fold.cc
index b0b2210..d8497fc 100644
--- a/gcc/gimple-range-fold.cc
+++ b/gcc/gimple-range-fold.cc
@@ -1023,6 +1023,26 @@ fold_using_range::range_of_builtin_int_call (irange &r, gcall *call,
 	break;
       }
 
+    case CFN_BUILT_IN_SIGNBIT:
+      {
+	arg = gimple_call_arg (call, 0);
+	frange tmp;
+	if (src.get_operand (tmp, arg))
+	  {
+	    if (tmp.get_signbit ().varying_p ())
+	      return false;
+	    if (tmp.get_signbit ().yes_p ())
+	      {
+		tree one = build_one_cst (type);
+		r.set (one, one);
+	      }
+	    else
+	      r.set_zero (type);
+	    return true;
+	  }
+	break;
+      }
+
     case CFN_BUILT_IN_TOUPPER:
       {
 	arg = gimple_call_arg (call, 0);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-1.c b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-1.c
new file mode 100644
index 0000000..3fa783e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-1.c
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-evrp" }
+
+int num;
+
+void func(float x)
+{
+  if (x > 5.0)
+    num = __builtin_signbit (x);
+}
+
+// { dg-final { scan-tree-dump-times "num = 0;" 1 "evrp" } }