[gcc r13-2449] Fold __builtin_signbit to nonzero instead of 1.

[gcc r13-2449] Fold __builtin_signbit to nonzero instead of 1.

[Aldy Hernandez]

https://gcc.gnu.org/g:1de9aa45529a98a5760a21570b181b78dbd11b3c

commit r13-2449-g1de9aa45529a98a5760a21570b181b78dbd11b3c
Author: Aldy Hernandez <aldyh@redhat.com>
Date:   Mon Sep 5 07:36:25 2022 +0200

    Fold __builtin_signbit to nonzero instead of 1.
    
    After Joseph's comment wrt __builtin_signbit, I have been unable to
    convince myself that arbitrarily folding __builtin_signbit () of a
    negative number to 1 is correct.
    
    For example, on the true side of x < -5.0 we know the sign bit is set,
    but on the false side, we know nothing because X may be a NAN.  I
    don't want to put ourselves in a position where the same call to
    __builtin_signbit can return two different things (1 or negative
    whatever), so I'm going to return nonzero which is correct across the
    board until someone can convince me otherwise.
    
    Setting the range to nonzero still allows us to fold conditionals
    checking __fold_builtin, while not actually propagating a 1.  Zero
    propagation still works.
    
    That being said, I still think we should be folding
    __builtin_signbit's of negative numbers to whatever the hardware
    returns, instead of 1/0 like what the front ends do.
    
    gcc/ChangeLog:
    
            * gimple-range-fold.cc
            (fold_using_range::range_of_builtin_int_call): Fold a set signbit
            in __builtin_signbit to nonzero.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.dg/tree-ssa/vrp-float-signbit-2.c: New test.

Diff:
---
 gcc/gimple-range-fold.cc                           |  5 +----
 .../gcc.dg/tree-ssa/vrp-float-signbit-2.c          | 24 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/gcc/gimple-range-fold.cc b/gcc/gimple-range-fold.cc
index d8497fc9be7..3543f0980b8 100644
--- a/gcc/gimple-range-fold.cc
+++ b/gcc/gimple-range-fold.cc
@@ -1032,10 +1032,7 @@ fold_using_range::range_of_builtin_int_call (irange &r, gcall *call,
 	    if (tmp.get_signbit ().varying_p ())
 	      return false;
 	    if (tmp.get_signbit ().yes_p ())
-	      {
-		tree one = build_one_cst (type);
-		r.set (one, one);
-	      }
+	      r.set_nonzero (type);
 	    else
 	      r.set_zero (type);
 	    return true;
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-2.c b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-2.c
new file mode 100644
index 00000000000..954c7ebd4f8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/vrp-float-signbit-2.c
@@ -0,0 +1,24 @@
+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-evrp" }
+
+// Test that the only thing we know about the signbit about negative number is
+// that it's not 0.
+
+void link_error ();
+
+int num;
+
+void func(float x)
+{
+  if (x < -5.0)
+    {
+      num = __builtin_signbit (x);
+
+      // We may not know the exact signbit, but we know it's not 0.
+      if (!__builtin_signbit (x))
+	link_error ();
+    }
+}
+
+// { dg-final { scan-tree-dump-not "num = \[-0-9\];" "evrp" } }
+// { dg-final { scan-tree-dump-not "link_error" "evrp" } }