diff -ruN llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/SmallVector.cpp llvm-project-llvmorg-12.0.0/llvm/lib/Support/SmallVector.cpp
--- llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/SmallVector.cpp	2021-04-06 19:38:18.000000000 +0300
+++ llvm-project-llvmorg-12.0.0/llvm/lib/Support/SmallVector.cpp	2021-06-09 16:18:35.039546181 +0300
@@ -132,7 +132,7 @@
 // Both uint32_t and uint64_t instantiations are needed for 64-bit builds.
 // This instantiation will never be used in 32-bit builds, and will cause
 // warnings when sizeof(Size_T) > sizeof(size_t).
-#if SIZE_MAX > UINT32_MAX
+#if SIZE_MAX > UINT32_MAX && !defined(__serenity__)
 template class llvm::SmallVectorBase<uint64_t>;
 
 // Assertions to ensure this #if stays in sync with SmallVectorSizeType.
diff -ruN llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Path.inc llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Path.inc
--- llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Path.inc	2021-04-06 19:38:18.000000000 +0300
+++ llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Path.inc	2021-06-09 16:24:37.446095863 +0300
@@ -108,7 +108,7 @@
 #endif
 
 #if defined(__NetBSD__) || defined(__DragonFly__) || defined(__GNU__) || \
-    defined(__MVS__)
+    defined(__MVS__) || defined(__serenity__)
 #define STATVFS_F_FLAG(vfs) (vfs).f_flag
 #else
 #define STATVFS_F_FLAG(vfs) (vfs).f_flags
@@ -524,7 +524,7 @@
 
   // vmount entry not found; "remote" is the conservative answer.
   return false;
-#elif defined(__MVS__)
+#elif defined(__MVS__) || defined(__serenity__)
   // The file system can have an arbitrary structure on z/OS; must go with the
   // conservative answer.
   return false;
diff -ruN llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Program.inc llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Program.inc
--- llvm-orig/llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Program.inc	2021-04-06 19:38:18.000000000 +0300
+++ llvm-project-llvmorg-12.0.0/llvm/lib/Support/Unix/Program.inc	2021-06-10 11:04:28.765989133 +0300
@@ -335,7 +335,7 @@
 namespace llvm {
 namespace sys {
 
-#ifndef _AIX
+#if !defined(_AIX) && !defined(__serenity__)
 using ::wait4;
 #else
 static pid_t (wait4)(pid_t pid, int *status, int options, struct rusage *usage);
@@ -344,7 +344,7 @@
 } // namespace sys
 } // namespace llvm
 
-#ifdef _AIX
+#if defined(_AIX) || defined(__serenity__)
 #ifndef _ALL_SOURCE
 extern "C" pid_t (wait4)(pid_t pid, int *status, int options,
                          struct rusage *usage);
@@ -357,7 +357,7 @@
 
   // AIX wait4 does not work well with WNOHANG.
   if (!(options & WNOHANG))
-    return ::wait4(pid, status, options, usage);
+    return ::waitpid(pid, status, options);
 
   // For WNOHANG, we use waitid (which supports WNOWAIT) until the child process
   // has terminated.
@@ -374,7 +374,7 @@
   // The child has already terminated, so a blocking wait on it is okay in the
   // absence of indiscriminate `wait` calls from the current process (which
   // would cause the call here to fail with ECHILD).
-  return ::wait4(pid, status, options & ~WNOHANG, usage);
+  return ::waitpid(pid, status, options & ~WNOHANG);
 }
 #endif
 
@@ -519,10 +519,10 @@
 
 bool llvm::sys::commandLineFitsWithinSystemLimits(StringRef Program,
                                                   ArrayRef<StringRef> Args) {
-  static long ArgMax = sysconf(_SC_ARG_MAX);
+  static long ArgMax = 4096;
   // POSIX requires that _POSIX_ARG_MAX is 4096, which is the lowest possible
   // value for ARG_MAX on a POSIX compliant system.
-  static long ArgMin = _POSIX_ARG_MAX;
+  static long ArgMin = 4096;
 
   // This the same baseline used by xargs.
   long EffectiveArgMax = 128 * 1024;
diff -ruN llvm-orig/llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink.cpp llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink.cpp
--- llvm-orig/llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink.cpp	2021-04-06 19:38:18.000000000 +0300
+++ llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink.cpp	2021-06-09 19:52:15.384543089 +0300
@@ -660,7 +660,7 @@
 
 Expected<std::unique_ptr<TargetProcessControl>>
 LLVMJITLinkRemoteTargetProcessControl::ConnectToExecutor() {
-#ifndef LLVM_ON_UNIX
+#if !defined(LLVM_ON_UNIX) || defined(__serenity__)
   // FIXME: Add TCP support for Windows.
   return make_error<StringError>("-" + OutOfProcessExecutorConnect.ArgStr +
                                      " not supported on non-unix platforms",
diff -ruN llvm-orig/llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink-executor/llvm-jitlink-executor.cpp llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink-executor/llvm-jitlink-executor.cpp
--- llvm-orig/llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink-executor/llvm-jitlink-executor.cpp	2021-04-06 19:38:18.000000000 +0300
+++ llvm-project-llvmorg-12.0.0/llvm/tools/llvm-jitlink/llvm-jitlink-executor/llvm-jitlink-executor.cpp	2021-06-09 16:43:06.154952293 +0300
@@ -45,7 +45,7 @@
 }
 
 int openListener(std::string Host, int Port) {
-#ifndef LLVM_ON_UNIX
+#if !defined(LLVM_ON_UNIX) || defined(__serenity__)
   // FIXME: Add TCP support for Windows.
   printErrorAndExit("listen option not supported");
   return 0;