[C23] Support the SNAN macros in <float.h> (llvm#162858)

These macros were brought in as part of the TS 18661-1 integration in
C23, and then renamed in WG14 N2710.

Test coverage is being added to WG14 N3364 from C2y because that paper
was fixing a bug with the way these macros are handled with unary + and
- operators, so all the existing test coverage was there, just needed to
use a header file instead of defining the macros manually.

Fixes llvm#162830

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -187,6 +187,9 @@ C2y Feature Support
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
+- Added ``FLT_SNAN``, ``DBL_SNAN``, and ``LDBL_SNAN`` to Clang's ``<float.h>``
+  header in C23 and later modes. This implements
+  `WG14 N2710 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2710.htm>`_.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/lib/Headers/float.h

@@ -89,6 +89,9 @@
     !defined(__STRICT_ANSI__)
 #  undef INFINITY
 #  undef NAN
+#  undef FLT_SNAN
+#  undef DBL_SNAN
+#  undef LDBL_SNAN
 #endif
 
 /* Characteristics of floating point types, C99 5.2.4.2.2 */
@@ -160,9 +163,15 @@
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
     !defined(__STRICT_ANSI__)
+   /* C23 5.2.5.3.2p28 */
+#  define FLT_SNAN (__builtin_nansf(""))
+#  define DBL_SNAN (__builtin_nans(""))
+#  define LDBL_SNAN (__builtin_nansl(""))
+
    /* C23 5.2.5.3.3p29-30 */
 #  define INFINITY (__builtin_inff())
 #  define NAN (__builtin_nanf(""))
+
    /* C23 5.2.5.3.3p32 */
 #  define FLT_NORM_MAX __FLT_NORM_MAX__
 #  define DBL_NORM_MAX __DBL_NORM_MAX__

clang/test/C/C2y/n3364.c

@@ -1,17 +1,24 @@
-// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic -emit-llvm -o - %s
-// RUN: %clang_cc1 -verify -Wall -pedantic -emit-llvm -o - %s
+// RUN: %clang_cc1 -verify -std=c2y -ffreestanding -Wall -pedantic -emit-llvm -o - %s
+// RUN: %clang_cc1 -verify -ffreestanding -Wall -pedantic -emit-llvm -o - %s
 // expected-no-diagnostics
 
 /* WG14 N3364: Yes
  * Give consistent wording for SNAN initialization v3
  *
  * Ensure that initializing from a signaling NAN (optionally with a unary + or
  * -) at translation time behaves correctly at runtime.
+ *
+ * This also serves as a test for C23's WG14 N2710 which introduces these
+ * macros into float.h in Clang 22.
  */
 
-#define FLT_SNAN __builtin_nansf("1")
-#define DBL_SNAN __builtin_nans("1")
-#define LD_SNAN __builtin_nansl("1")
+#if __STDC_VERSION__ >= 202311L
+#include <float.h>
+#else
+#define FLT_SNAN __builtin_nansf("")
+#define DBL_SNAN __builtin_nans("")
+#define LDBL_SNAN __builtin_nansl("")
+#endif
 
 float f1 = FLT_SNAN;
 float f2 = +FLT_SNAN;
@@ -27,9 +34,9 @@ double d3 = -DBL_SNAN;
 // CHECK: @d2 = {{.*}}global double 0x7FF0000000000001
 // CHECK: @d3 = {{.*}}global double 0xFFF0000000000001
 
-long double ld1 = LD_SNAN;
-long double ld2 = +LD_SNAN;
-long double ld3 = -LD_SNAN;
+long double ld1 = LDBL_SNAN;
+long double ld2 = +LDBL_SNAN;
+long double ld3 = -LDBL_SNAN;
 // CHECK: @ld1 = {{.*}}global {{double 0x7FF0000000000001|x86_fp80 0xK7FFF8000000000000001|fp128 0xL00000000000000017FFF000000000000}}
 // CHECK: @ld2 = {{.*}}global {{double 0x7FF0000000000001|x86_fp80 0xK7FFF8000000000000001|fp128 0xL00000000000000017FFF000000000000}}
 // CHECK: @ld3 = {{.*}}global {{double 0xFFF0000000000001|x86_fp80 0xKFFFF8000000000000001|fp128 0xL0000000000000001FFFF000000000000}}