[C23] Add INFINITY and NAN macros to <float.h> (llvm#96659)

This is in support of WG14 N2848 which only define the macros if
an infinity or nan are supported. However, because we support builtins
that can produce an infinity or a NAN, and because we have pragmas
that control infinity or NAN behavior, we always define the macros even
if the user passed -ffinite-math-only on the command line.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -338,6 +338,11 @@ C23 Feature Support
 - Properly promote bit-fields of bit-precise integer types to the field's type
   rather than to ``int``. #GH87641
 
+- Added the ``INFINITY`` and ``NAN`` macros to Clang's ``<float.h>``
+  freestanding implementation; these macros were defined in ``<math.h>`` in C99
+  but C23 added them to ``<float.h>`` in
+  `WG14 N2848 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2848.pdf>`_.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 

clang/lib/Headers/float.h

@@ -88,6 +88,12 @@
 #  endif
 #endif
 
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
+    !defined(__STRICT_ANSI__)
+#  undef INFINITY
+#  undef NAN
+#endif
+
 /* Characteristics of floating point types, C99 5.2.4.2.2 */
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) ||              \
@@ -155,6 +161,13 @@
 #  define LDBL_HAS_SUBNORM __LDBL_HAS_DENORM__
 #endif
 
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
+    !defined(__STRICT_ANSI__)
+   /* C23 5.2.5.3.3p29-30 */
+#  define INFINITY (__builtin_inf())
+#  define NAN (__builtin_nan(""))
+#endif
+
 #ifdef __STDC_WANT_IEC_60559_TYPES_EXT__
 #  define FLT16_MANT_DIG    __FLT16_MANT_DIG__
 #  define FLT16_DECIMAL_DIG __FLT16_DECIMAL_DIG__

clang/test/Headers/float.c

@@ -1,9 +1,13 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c89 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c99 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -ffreestanding %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c23 -ffreestanding %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c23 -ffreestanding -ffinite-math-only %s
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++11 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++14 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++17 -ffreestanding %s
+// RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++23 -ffreestanding %s
+// RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++23 -ffreestanding -ffinite-math-only %s
 // expected-no-diagnostics
 
 /* Basic floating point conformance checks against:
@@ -207,6 +211,21 @@
     #error "Mandatory macros {FLT,DBL,LDBL}_MAX_10_EXP are invalid."
 #endif
 
+#if __STDC_VERSION__ >= 202311L || !defined(__STRICT_ANSI__)
+  #ifndef INFINITY
+    #error "Mandatory macro INFINITY is missing."
+  #endif
+  #ifndef NAN
+    #error "Mandatory macro NAN is missing."
+  #endif
+#else
+  #ifdef INFINITY
+    #error "Macro INFINITY should not be defined."
+  #endif
+  #ifdef NAN
+    #error "Macro NAN should not be defined."
+  #endif
+#endif
 
 /* Internal consistency checks */
 _Static_assert(FLT_RADIX == __FLT_RADIX__, "");
@@ -244,3 +263,9 @@ _Static_assert(LDBL_MAX_EXP == __LDBL_MAX_EXP__, "");
 _Static_assert(FLT_MAX_10_EXP == __FLT_MAX_10_EXP__, "");
 _Static_assert(DBL_MAX_10_EXP == __DBL_MAX_10_EXP__, "");
 _Static_assert(LDBL_MAX_10_EXP == __LDBL_MAX_10_EXP__, "");
+
+#if (__STDC_VERSION__ >= 202311L || !defined(__STRICT_ANSI__)) && __FINITE_MATH_ONLY__ == 0
+// Ensure INFINITY and NAN are suitable for use in a constant expression.
+float f1 = INFINITY;
+float f2 = NAN;
+#endif

clang/www/c_status.html

@@ -995,7 +995,7 @@ <h2 id="c2x">C23 implementation status</h2>
     <tr>
       <td>Contradiction about INFINITY macro</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2848.pdf">N2848</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="unreleased" align="center">Clang 19</td>
     </tr>
     <tr>
       <td>Require exact-width integer type interfaces</td>