signbit detection (was [perl #39875] -0.0 loses signedness upon numeric comparison)

Message-ID: <Pine.LNX.4.62.0702091121400.10202@fractal.phys.lafayette.edu>

p4raw-id: //depot/perl@30192

1 files changed, 32 insertions, 0 deletions

diff --git a/numeric.c b/numeric.c
index 6720726cfb..20fd7bd2df 100644
--- a/numeric.c
+++ b/numeric.c
@@ -1024,6 +1024,38 @@ Perl_my_frexpl(long double x, int *e) {
 #endif
 
 /*
+=for apidoc Perl_signbit
+
+Return a non-zero integer if the sign bit on an NV is set, and 0 if
+it is not.  
+
+If Configure detects this system has a signbit() that will work with
+our NVs, then we just use it via the #define in perl.h.  Otherwise,
+fall back on this implementation.  As a first pass, this gets everything
+right except -0.0.  Alas, catching -0.0 is the main use for this function,
+so this is not too helpful yet.  Still, at least we have the scaffolding
+in place to support other systems, should that prove useful.
+
+
+Configure notes:  This function is called 'Perl_signbit' instead of a
+plain 'signbit' because it is easy to imagine a system having a signbit()
+function or macro that doesn't happen to work with our particular choice
+of NVs.  We shouldn't just re-#define signbit as Perl_signbit and expect
+the standard system headers to be happy.  Also, this is a no-context
+function (no pTHX_) because Perl_signbit() is usually re-#defined in
+perl.h as a simple macro call to the system's signbit().
+Users should just always call Perl_signbit().
+
+=cut
+*/
+#if !defined(HAS_SIGNBIT)
+int
+Perl_signbit(NV x) {
+    return (x < 0.0) ? 1 : 0;
+}
+#endif
+
+/*
  * Local variables:
  * c-indentation-style: bsd
  * c-basic-offset: 4