[Bug libstdc++/84949] -ffast-math bugged with respect to NaNs

[kretz at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949

Matthias Kretz <kretz at kde dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kretz at kde dot org

--- Comment #6 from Matthias Kretz <kretz at kde dot org> ---
I'd like to make a case for numeric_limits<floating-point>::is_iec559 to follow
__STDC_IEC_559__. I.e. the following patch:

--- a/libstdc++-v3/include/std/limits
+++ b/libstdc++-v3/include/std/limits
@@ -1649,7 +1649,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       denorm_min() _GLIBCXX_USE_NOEXCEPT { return __FLT_DENORM_MIN__; }

       static _GLIBCXX_USE_CONSTEXPR bool is_iec559
+#ifdef __STDC_IEC_559__
   = has_infinity && has_quiet_NaN && has_denorm == denorm_present;
+#else
+  = false;
+#endif
       static _GLIBCXX_USE_CONSTEXPR bool is_bounded = true;
       static _GLIBCXX_USE_CONSTEXPR bool is_modulo = false;

@@ -1724,7 +1728,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       denorm_min() _GLIBCXX_USE_NOEXCEPT { return __DBL_DENORM_MIN__; }

       static _GLIBCXX_USE_CONSTEXPR bool is_iec559
+#ifdef __STDC_IEC_559__
   = has_infinity && has_quiet_NaN && has_denorm == denorm_present;
+#else
+  = false;
+#endif
       static _GLIBCXX_USE_CONSTEXPR bool is_bounded = true;
       static _GLIBCXX_USE_CONSTEXPR bool is_modulo = false;

@@ -1799,7 +1807,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       denorm_min() _GLIBCXX_USE_NOEXCEPT { return __LDBL_DENORM_MIN__; }

       static _GLIBCXX_USE_CONSTEXPR bool is_iec559
+#ifdef __STDC_IEC_559__
   = has_infinity && has_quiet_NaN && has_denorm == denorm_present;
+#else
+  = false;
+#endif
       static _GLIBCXX_USE_CONSTEXPR bool is_bounded = true;
       static _GLIBCXX_USE_CONSTEXPR bool is_modulo = false;


The __STDC_IEC_559__ macro is not defined when -ffast-math is active. is_iec559
requires "true if and only if the type adheres to ISO/IEC/IEEE 60559". I don't
have IEC559 at hand, but I believe assuming no NaN, inf, or -0 can occur makes
the floating point types not adhere to IEC559.
And IIUC, if __STDC_IEC_559__ is defined, then has_infinity, has_quiet_NaN and
has_denorm must all be true. So
+#ifdef __STDC_IEC_559__
+  = true;
+#else
+  = false;
+#endif
should be correct.