[libc] implement the final macros for stdbit.h support

[nickdesaulniers]

Relevant sections of n3096:

• 7.18.1p1
• 7.18.2

[llvmbot]

@llvm/pr-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

Changes

Relevant sections of n3096:

• 7.18.1p1
• 7.18.2

---

Full diff: https://github.com/llvm/llvm-project/pull/84798.diff

5 Files Affected:

• (modified) libc/docs/c23.rst (+1-1)
• (modified) libc/docs/stdbit.rst (+4-4)
• (modified) libc/include/llvm-libc-macros/stdbit-macros.h (+5)
• (modified) libc/spec/stdc.td (+4)
• (modified) libc/test/include/stdbit_test.cpp (+17)

diff --git a/libc/docs/c23.rst b/libc/docs/c23.rst
index 24cef8539393df..3f64722bc8e644 100644
--- a/libc/docs/c23.rst
+++ b/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
 
diff --git a/libc/docs/stdbit.rst b/libc/docs/stdbit.rst
index 9b4974cf1479b1..d42f7938246293 100644
--- a/libc/docs/stdbit.rst
+++ b/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|
diff --git a/libc/include/llvm-libc-macros/stdbit-macros.h b/libc/include/llvm-libc-macros/stdbit-macros.h
index 10c0fac3c8dd86..c5b2f097783440 100644
--- a/libc/include/llvm-libc-macros/stdbit-macros.h
+++ b/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) {
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
index d91f5c1f723345..8d03f2b30368b2 100644
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -804,6 +804,10 @@ 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">,
diff --git a/libc/test/include/stdbit_test.cpp b/libc/test/include/stdbit_test.cpp
index 6c12665c4454d0..dedab3d82bb2db 100644
--- a/libc/test/include/stdbit_test.cpp
+++ b/libc/test/include/stdbit_test.cpp
@@ -224,3 +224,20 @@ 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__);
+}