[libc] Add `IN6_IS_ADDR_V4COMPAT`

Full diff: https://github.com/llvm/llvm-project/pull/172646.diff

2 Files Affected:

• (modified) libc/include/llvm-libc-macros/netinet-in-macros.h (+6)
• (modified) libc/test/include/netinet_in_test.cpp (+11)

diff --git a/libc/include/llvm-libc-macros/netinet-in-macros.h b/libc/include/llvm-libc-macros/netinet-in-macros.h
index 1d7bef833f2b8..f22f0bab33772 100644
--- a/libc/include/llvm-libc-macros/netinet-in-macros.h
+++ b/libc/include/llvm-libc-macros/netinet-in-macros.h
@@ -72,6 +72,12 @@
    (__LLVM_LIBC_CAST(reinterpret_cast, uint8_t *, a)[10]) == 0xff &&           \
    (__LLVM_LIBC_CAST(reinterpret_cast, uint8_t *, a)[11]) == 0xff)
 
+#define IN6_IS_ADDR_V4COMPAT(a)                                                \
+  ((__LLVM_LIBC_CAST(reinterpret_cast, uint32_t *, a)[0]) == 0 &&              \
+   (__LLVM_LIBC_CAST(reinterpret_cast, uint32_t *, a)[1]) == 0 &&              \
+   (__LLVM_LIBC_CAST(reinterpret_cast, uint32_t *, a)[2]) == 0 &&              \
+   !IN6_IS_ADDR_UNSPECIFIED(a) && !IN6_IS_ADDR_LOOPBACK(a))
+
 #define IN6_IS_ADDR_MC_NODELOCAL(a)                                            \
   (IN6_IS_ADDR_MULTICAST(a) &&                                                 \
    (__LLVM_LIBC_CAST(reinterpret_cast, uint8_t *, a)[1] & 0xf) == 0x1)
diff --git a/libc/test/include/netinet_in_test.cpp b/libc/test/include/netinet_in_test.cpp
index 2f57080ef8b77..2fb46a9227c07 100644
--- a/libc/test/include/netinet_in_test.cpp
+++ b/libc/test/include/netinet_in_test.cpp
@@ -63,4 +63,15 @@ TEST(LlvmLibcNetinetInTest, IN6Macro) {
   EXPECT_TRUE(IN6_IS_ADDR_V4MAPPED(buff));
   buff[10] = 0;
   buff[11] = 0;
+
+  for (int i = 12; i < 16; ++i) {
+    buff[i] ^= 42;
+    EXPECT_TRUE(IN6_IS_ADDR_V4COMPAT(buff));
+    buff[i] ^= 42;
+  }
+  for (int i = 0; i < 12; ++i) {
+    buff[i] ^= 42;
+    EXPECT_FALSE(IN6_IS_ADDR_V4COMPAT(buff));
+    buff[i] ^= 42;
+  }
 }