add options and docs

Author: SchrodingerZhu

libc/config/config.json

@@ -59,6 +59,16 @@
     "LIBC_CONF_FREELIST_MALLOC_BUFFER_SIZE": {
       "value": 1073741824,
       "doc": "Default size for the constinit freelist buffer used for the freelist malloc implementation (default 1o 1GB)."
+    },
+  },
+  "unistd": {
+    "LIBC_CONF_ENABLE_TID_CACHE": {
+      "value": true,
+      "doc": "Enable caching mechanism for gettid to avoid syscall (only effective in fullbuild mode, default to true). Please refer to Undefined Behavior documentation for implications."
+    },
+    "LIBC_CONF_ENABLE_PID_CACHE": {
+      "value": true,
+      "doc": "Enable caching mechanism for getpid to avoid syscall (default to true). Please refer to Undefined Behavior documentation for implications."
     }
   }
 }

libc/docs/configure.rst

@@ -43,3 +43,6 @@ to learn about the defaults for your platform and target.
 * **"string" options**
     - ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
     - ``LIBC_CONF_STRING_UNSAFE_WIDE_READ``: Read more than a byte at a time to perform byte-string operations like strlen.
+* **"unistd" options**
+    - ``LIBC_CONF_ENABLE_PID_CACHE``: Enable caching mechanism for getpid to avoid syscall (default to true). Please refer to Undefined Behavior documentation for implications.
+    - ``LIBC_CONF_ENABLE_TID_CACHE``: Enable caching mechanism for gettid to avoid syscall (only effective in fullbuild mode, default to true). Please refer to Undefined Behavior documentation for implications.

libc/docs/dev/undefined_behavior.rst

@@ -95,7 +95,9 @@ Cached ``getpid/gettid``
 Since version ``2.25``, glibc removes its cache mechanism for ``getpid/gettid`` 
 (See the history section in https://man7.org/linux/man-pages/man2/getpid.2.html).
 LLVM's libc still implements the cache as it is useful for fast deadlock detection.
-The cache mechanism is also implemented in MUSL and bionic.
+The cache mechanism is also implemented in MUSL and bionic. The tid/pid cache can 
+be disabled by setting ``LIBC_CONF_ENABLE_TID_CACHE`` and ``LIBC_CONF_ENABLE_PID_CACHE``
+to ``false`` respectively.
 
 Unwrapped ``SYS_clone/SYS_fork/SYS_vfork``
 ------------------------------------------

libc/src/__support/OSUtil/CMakeLists.txt

@@ -16,11 +16,19 @@ add_object_library(
     ${target_os_util}
 )
 
+if (LIBC_CONF_ENABLE_PID_CACHE)
+  set(libc_copt_enable_pid_cache 1)
+else()
+  set(libc_copt_enable_pid_cache 0)
+endif()
+
 if(TARGET libc.src.__support.OSUtil.${LIBC_TARGET_OS}.pid)
   add_object_library(
     pid
     ALIAS
     DEPENDS
       .${LIBC_TARGET_OS}.pid
+    COMPILE_OPTIONS
+      -DLIBC_COPT_ENABLE_PID_CACHE=${libc_copt_enable_pid_cache}
   )
 endif()

libc/src/__support/OSUtil/pid.h

@@ -11,6 +11,11 @@
 #include "hdr/types/pid_t.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/optimization.h"
+
+#ifndef LIBC_COPT_ENABLE_PID_CACHE
+#define LIBC_COPT_ENABLE_PID_CACHE 1
+#endif
+
 namespace LIBC_NAMESPACE {
 
 class ProcessIdentity {
@@ -23,9 +28,13 @@ class ProcessIdentity {
   LIBC_INLINE static void end_fork() { fork_inflight = false; }
   LIBC_INLINE static void refresh_cache() { cache = get_uncached(); }
   LIBC_INLINE static pid_t get() {
+#if LIBC_COPT_ENABLE_PID_CACHE
     if (LIBC_UNLIKELY(fork_inflight))
       return get_uncached();
     return cache;
+#else
+    return get_uncached();
+#endif
   }
 };
 

libc/src/__support/threads/CMakeLists.txt

@@ -35,6 +35,12 @@ if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.mutex)
   )
 endif()
 
+if (LIBC_CONF_ENABLE_TID_CACHE)
+  set(libc_copt_enable_tid_cache 1)
+else()
+  set(libc_copt_enable_tid_cache 0)
+endif()
+
 add_header_library(
   thread_common
   HDRS
@@ -46,6 +52,8 @@ add_header_library(
     libc.src.__support.CPP.string_view
     libc.src.__support.CPP.stringstream
     libc.hdr.types.pid_t
+  COMPILE_OPTIONS
+    -DLIBC_COPT_ENABLE_TID_CACHE=${libc_copt_enable_tid_cache}
 )
 
 if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.thread)

libc/src/__support/threads/thread.h

@@ -9,6 +9,10 @@
 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_THREAD_H
 #define LLVM_LIBC_SRC___SUPPORT_THREADS_THREAD_H
 
+#ifndef LIBC_COPT_ENABLE_TID_CACHE
+#define LIBC_COPT_ENABLE_TID_CACHE 1
+#endif
+
 #include "hdr/types/pid_t.h"
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/CPP/optional.h"
@@ -233,10 +237,15 @@ struct Thread {
 
   LIBC_INLINE void refresh_tid() { this->attrib->tid = get_uncached_tid(); }
   LIBC_INLINE void invalidate_tid() { this->attrib->tid = -1; }
+
   LIBC_INLINE pid_t get_tid() {
+#if LIBC_COPT_ENABLE_TID_CACHE
     if (LIBC_UNLIKELY(this->attrib->tid < 0))
       return get_uncached_tid();
     return this->attrib->tid;
+#else
+    return get_uncached_tid();
+#endif
   }
 };