[libc] implement the final macros for stdbit.h support (llvm#84798)

Relevant sections of n3096:
- 7.18.1p1
- 7.18.2

Author: nickdesaulniers

libc/docs/c23.rst

@@ -75,7 +75,7 @@ Additions:
   * dfmal
   * fsqrt*
   * dsqrtl
-* stdbit.h (New header)
+* stdbit.h (New header)    |check|
 * stdckdint.h (New header) |check|
 * stddef.h
 

libc/docs/stdbit.rst

@@ -110,10 +110,10 @@ Macros
 =========================  =========
 Macro Name                 Available
 =========================  =========
-__STDC_VERSION_STDBIT_H__
-__STDC_ENDIAN_LITTLE__
-__STDC_ENDIAN_BIG__
-__STDC_ENDIAN_NATIVE__
+__STDC_VERSION_STDBIT_H__  |check|
+__STDC_ENDIAN_LITTLE__     |check|
+__STDC_ENDIAN_BIG__        |check|
+__STDC_ENDIAN_NATIVE__     |check|
 stdc_leading_zeros         |check|
 stdc_leading_ones          |check|
 stdc_trailing_zeros        |check|

libc/include/llvm-libc-macros/stdbit-macros.h

@@ -9,6 +9,11 @@
 #ifndef __LLVM_LIBC_MACROS_STDBIT_MACROS_H
 #define __LLVM_LIBC_MACROS_STDBIT_MACROS_H
 
+#define __STDC_VERSION_STDBIT_H__ 202311L
+#define __STDC_ENDIAN_LITTLE__ __ORDER_LITTLE_ENDIAN__
+#define __STDC_ENDIAN_BIG__ __ORDER_BIG_ENDIAN__
+#define __STDC_ENDIAN_NATIVE__ __BYTE_ORDER__
+
 // TODO(https://github.com/llvm/llvm-project/issues/80509): support _BitInt().
 #ifdef __cplusplus
 inline unsigned stdc_leading_zeros(unsigned char x) {

libc/spec/stdc.td

@@ -805,6 +805,10 @@ def StdC : StandardSpec<"stdc"> {
   HeaderSpec StdBit = HeaderSpec<
       "stdbit.h",
       [
+        Macro<"__STDC_VERSION_STDBIT_H__">,
+        Macro<"__STDC_ENDIAN_LITTLE__">,
+        Macro<"__STDC_ENDIAN_BIG__">,
+        Macro<"__STDC_ENDIAN_NATIVE__">,
         Macro<"stdc_leading_zeros">,
         Macro<"stdc_leading_ones">,
         Macro<"stdc_trailing_zeros">,

libc/test/include/stdbit_test.cpp

@@ -141,3 +141,20 @@ TEST(LlvmLibcStdbitTest, TypeGenericMacroBitCeil) {
   EXPECT_EQ(stdc_bit_ceil(0UL), 0x6DUL);
   EXPECT_EQ(stdc_bit_ceil(0ULL), 0x6EULL);
 }
+
+TEST(LlvmLibcStdbitTest, VersionMacro) {
+  // 7.18.1p2 an integer constant expression with a value equivalent to 202311L.
+  EXPECT_EQ(__STDC_VERSION_STDBIT_H__, 202311L);
+}
+
+TEST(LlvmLibcStdbitTest, EndianMacros) {
+  // 7.18.2p3 The values of the integer constant expressions for
+  // __STDC_ENDIAN_LITTLE__ and __STDC_ENDIAN_BIG__ are not equal.
+  EXPECT_NE(__STDC_ENDIAN_LITTLE__, __STDC_ENDIAN_BIG__);
+  // The standard does allow for __STDC_ENDIAN_NATIVE__ to be an integer
+  // constant expression with an implementation defined value for non-big or
+  // little endianness environments.  I assert such machines are no longer
+  // relevant.
+  EXPECT_TRUE(__STDC_ENDIAN_NATIVE__ == __STDC_ENDIAN_LITTLE__ ||
+              __STDC_ENDIAN_NATIVE__ == __STDC_ENDIAN_BIG__);
+}