[RFC] Add op1_range for __builtin_signbit.

[Aldy Hernandez <aldyh@redhat.com>]

This is the op1_range range-op entry for __builtin_signbit.  It allows
us to wind back through a call to signbit.

For example, on the true side of if (__builtin_signbit(x_5) != 0) we
can crop down the range of x_5 to:

	[frange] float [-Inf, -0.0 (-0x0.0p+0)] -NAN

Similarly on the false side, we can crop to:

	[frange] float [0.0 (0x0.0p+0), +Inf] +NAN

This brings about an interesting question, can we fold the second
conditional here as always false?

	void foo(float x)
	{
	  if (__builtin_signbit (x))
	    {
	      if (x > 0.0)
	        link_error();
	    }
	}

The only values for x at the second conditional are negative values,
or -NAN, so it will always evaluate to false.  ISTM that we
*shouldn't* fold this conditional as there is user observable behavior
if there is a signaling NAN.  For that matter, that is exactly what we
do in ranger-ops.  We leave the conditional in place, but ranger
is able to determine that "x" is UNDEFINED on the path leading up to
link_error:

=========== BB 3 ============
Imports: x_3(D)
Exports: x_3(D)
x_3(D)	[frange] float [-Inf, -0.0 (-0x0.0p+0)] -NAN
    <bb 3> :
    if (x_3(D) > 0.0)
      goto <bb 4>; [INV]
    else
      goto <bb 5>; [INV]

3->4  (T) x_3(D) : 	[frange] UNDEFINED
3->5  (F) x_3(D) : 	[frange] float [-Inf, -0.0 (-0x0.0p+0)] -NAN

This would allow users of ranger to query x_3 on the 3->4 and notice
it's unreachable, without VRP removing the conditional.

I believe this is correct, but would like confirmation from the experts.

gcc/ChangeLog:

	* gimple-range-op.cc: Add op1_range entry for __builtin_signbit.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-signbit-3.c: New test.
---
 gcc/gimple-range-op.cc                        | 20 +++++++++++++++++++
 .../gcc.dg/tree-ssa/vrp-float-signbit-3.c     | 14 +++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-3.c

diff --git a/gcc/gimple-range-op.cc b/gcc/gimple-range-op.cc
index 42ebc7d6ce5..abc33e7af0c 100644
--- a/gcc/gimple-range-op.cc
+++ b/gcc/gimple-range-op.cc
@@ -306,6 +306,7 @@ class cfn_signbit : public range_operator_float
 {
 public:
   using range_operator_float::fold_range;
+  using range_operator_float::op1_range;
   virtual bool fold_range (irange &r, tree type, const frange &lh,
 			   const irange &, relation_kind) const
   {
@@ -320,6 +321,25 @@ public:
       }
    return false;
   }
+  virtual bool op1_range (frange &r, tree type, const irange &lhs,
+			  const frange &, relation_kind) const override
+  {
+    if (lhs.zero_p ())
+      {
+	r.set (type, dconst0, frange_val_max (type));
+	r.update_nan (false);
+	return true;
+      }
+    if (!lhs.contains_p (build_zero_cst (lhs.type ())))
+      {
+	REAL_VALUE_TYPE dconstm0 = dconst0;
+	dconstm0.sign = 1;
+	r.set (type, frange_val_min (type), dconstm0);
+	r.update_nan (true);
+	return true;
+      }
+    return false;
+  }
 } op_cfn_signbit;
 
 // Implement range operator for CFN_BUILT_IN_TOUPPER and CFN_BUILT_IN_TOLOWER.
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-3.c b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-3.c
new file mode 100644
index 00000000000..dd3090aeb10
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-3.c
@@ -0,0 +1,14 @@
+// { dg-do compile }
+// { dg-options "-O2 -ffast-math -fdump-tree-evrp" }
+
+void link_error();
+
+void foo(float x)
+{
+  if (__builtin_signbit (x))
+    {
+      if (x > 0.0)
+	link_error();
+    }
+}
+// { dg-final { scan-tree-dump-not "link_error" "evrp" } }
-- 
2.37.1