From c84b8e597a76b5b81f612182e97f645aee0b2ee2 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 12 Apr 2020 20:23:18 +0200
Subject: [PATCH] LibC: Cache the result of gettid() to avoid syscalls

We now use minherit(MAP_INHERIT_ZERO) to create a gettid() cache that
is automatically invalidated on fork(). This is needed since the TID
will be different in a forked child, and so we can't have a stale
cached TID lying around.

This is a gigantic speedup for LibJS (and everyone else too) :^)
---
 Libraries/LibC/unistd.cpp | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/Libraries/LibC/unistd.cpp b/Libraries/LibC/unistd.cpp
index 10fffdd1a1..679817a937 100644
--- a/Libraries/LibC/unistd.cpp
+++ b/Libraries/LibC/unistd.cpp
@@ -39,6 +39,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/ioctl.h>
+#include <sys/mman.h>
 #include <sys/types.h>
 #include <termios.h>
 #include <unistd.h>
@@ -547,9 +548,27 @@ int ftruncate(int fd, off_t length)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+struct ThreadInfoCache {
+    int tid { 0 };
+};
+
+__thread ThreadInfoCache* thread_info_cache;
+
 int gettid()
 {
-    return syscall(SC_gettid);
+    if (!thread_info_cache) {
+        thread_info_cache = (ThreadInfoCache*)mmap(nullptr, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+        ASSERT(thread_info_cache != MAP_FAILED);
+        if (minherit(thread_info_cache, PAGE_SIZE, MAP_INHERIT_ZERO) < 0) {
+            perror("minherit(MAP_INHERIT_ZERO)");
+            ASSERT_NOT_REACHED();
+        }
+    }
+
+    if (thread_info_cache->tid == 0)
+        thread_info_cache->tid = syscall(SC_gettid);
+
+    return thread_info_cache->tid;
 }
 
 int donate(int tid)