LibC: Remove duplicate gs touch during gettid()/getpid() fast path

While profiling I noticed that gettid() was hitting gs register
twice, once for the initial fetch of s_cache_tid out of TLS for
the initialization check, and then again when we return the actual
value.

Optimize the implementation to cache the value so we avoid the
double fetch during the 99% case where it's already set. With
this change gettid() goes from being the 3rd most sampled function
in test-js, to pretty much disappearing into  ~20th place.

Additionally add the same optimization to getpid().

1 files changed, 12 insertions, 6 deletions

diff --git a/Libraries/LibC/unistd.cpp b/Libraries/LibC/unistd.cpp
index 9ff0605957..feee5faaea 100644
--- a/Libraries/LibC/unistd.cpp
+++ b/Libraries/LibC/unistd.cpp
@@ -202,9 +202,12 @@ gid_t getgid()
 
 pid_t getpid()
 {
-    if (!s_cached_pid)
-        s_cached_pid = syscall(SC_getpid);
-    return s_cached_pid;
+    int cached_pid = s_cached_pid;
+    if (!cached_pid) {
+        cached_pid = syscall(SC_getpid);
+        s_cached_pid = cached_pid;
+    }
+    return cached_pid;
 }
 
 pid_t getppid()
@@ -608,9 +611,12 @@ int truncate(const char* path, off_t length)
 
 int gettid()
 {
-    if (!s_cached_tid)
-        s_cached_tid = syscall(SC_gettid);
-    return s_cached_tid;
+    int cached_tid = s_cached_tid;
+    if (!cached_tid) {
+        cached_tid = syscall(SC_gettid);
+        s_cached_tid = cached_tid;
+    }
+    return cached_tid;
 }
 
 int donate(int tid)