[libc] Implement barriers for pthreads

libc/config/linux/x86_64/entrypoints.txt

@@ -1039,6 +1039,9 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.pthread.pthread_join
     libc.src.pthread.pthread_key_create
     libc.src.pthread.pthread_key_delete
+    libc.src.pthread.pthread_barrier_init
+    libc.src.pthread.pthread_barrier_wait
+    libc.src.pthread.pthread_barrier_destroy
     libc.src.pthread.pthread_mutex_destroy
     libc.src.pthread.pthread_mutex_init
     libc.src.pthread.pthread_mutex_lock

libc/hdr/CMakeLists.txt

@@ -72,6 +72,15 @@ add_proxy_header_library(
     libc.include.fenv
 )
 
+add_proxy_header_library(
+  pthread_macros
+  HDRS
+    pthread_macros.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-macros.pthread_macros
+    libc.include.pthread
+)
+
 add_proxy_header_library(
   sched_macros
   HDRS

libc/hdr/pthread_macros.h

@@ -0,0 +1,22 @@
+//===-- Definition of macros from pthread.h -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_PTHREAD_MACROS_H
+#define LLVM_LIBC_HDR_PTHREAD_MACROS_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-macros/pthread-macros.h"
+
+#else // Overlay mode
+
+#include <pthread.h>
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_PTHREAD_MACROS_H

libc/hdr/types/CMakeLists.txt

@@ -237,6 +237,22 @@ add_proxy_header_library(
     libc.include.llvm-libc-types.pid_t
 )
 
+add_proxy_header_library(
+  pthread_barrier_t
+  HDRS
+    pthread_barrier_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.pthread_barrier_t
+)
+
+add_proxy_header_library(
+  pthread_barrierattr_t
+  HDRS
+    pthread_barrierattr_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.pthread_barrierattr_t
+)
+
 add_proxy_header_library(
   atexithandler_t
   HDRS

libc/hdr/types/pthread_barrier_t.h

@@ -0,0 +1,22 @@
+//===-- Definition of macros from pthread_barrier_t.h ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIER_T_H
+#define LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIER_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/pthread_barrier_t.h"
+
+#else // Overlay mode
+
+#error "Cannot overlay pthread_barrier_t"
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIER_T_H

libc/hdr/types/pthread_barrierattr_t.h

@@ -0,0 +1,22 @@
+//===-- Definition of macros from pthread_barrierattr_t.h -----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIERATTR_T_H
+#define LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIERATTR_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/pthread_barrierattr_t.h"
+
+#else // Overlay mode
+
+#error "Cannot overlay pthread_barrierattr_t"
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_PTHREAD_BARRIERATTR_T_H

libc/include/CMakeLists.txt

@@ -390,6 +390,8 @@ add_header_macro(
     .llvm-libc-types.pthread_attr_t
     .llvm-libc-types.pthread_condattr_t
     .llvm-libc-types.pthread_key_t
+    .llvm-libc-types.pthread_barrier_t
+    .llvm-libc-types.pthread_barrierattr_t
     .llvm-libc-types.pthread_mutex_t
     .llvm-libc-types.pthread_mutexattr_t
     .llvm-libc-types.pthread_once_t

libc/include/llvm-libc-macros/pthread-macros.h

@@ -22,6 +22,8 @@
 #define PTHREAD_MUTEX_STALLED 0
 #define PTHREAD_MUTEX_ROBUST 1
 
+#define PTHREAD_BARRIER_SERIAL_THREAD -1
+
 #define PTHREAD_ONCE_INIT {0}
 
 #define PTHREAD_PROCESS_PRIVATE 0

libc/include/llvm-libc-types/CMakeLists.txt

@@ -10,6 +10,7 @@ add_header(__exec_envp_t HDR __exec_envp_t.h)
 add_header(__futex_word HDR __futex_word.h)
 add_header(pid_t HDR pid_t.h)
 add_header(__mutex_type HDR __mutex_type.h DEPENDS .__futex_word .pid_t)
+add_header(__barrier_type HDR __barrier_type.h)
 add_header(__pthread_once_func_t HDR __pthread_once_func_t.h)
 add_header(__pthread_start_t HDR __pthread_start_t.h)
 add_header(__pthread_tss_dtor_t HDR __pthread_tss_dtor_t.h)
@@ -53,6 +54,8 @@ add_header(pthread_condattr_t HDR pthread_condattr_t.h DEPENDS .clockid_t)
 add_header(pthread_key_t HDR pthread_key_t.h)
 add_header(pthread_mutex_t HDR pthread_mutex_t.h DEPENDS .__futex_word .__mutex_type)
 add_header(pthread_mutexattr_t HDR pthread_mutexattr_t.h)
+add_header(pthread_barrier_t HDR pthread_barrier_t.h)
+add_header(pthread_barrierattr_t HDR pthread_barrierattr_t.h)
 add_header(pthread_once_t HDR pthread_once_t.h DEPENDS .__futex_word)
 add_header(pthread_rwlock_t HDR pthread_rwlock_t.h DEPENDS .__futex_word .pid_t)
 add_header(pthread_rwlockattr_t HDR pthread_rwlockattr_t.h)

libc/include/llvm-libc-types/__barrier_type.h

@@ -0,0 +1,21 @@
+//===-- Definition of __barrier_type type ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES__BARRIER_TYPE_H
+#define LLVM_LIBC_TYPES__BARRIER_TYPE_H
+
+typedef struct __attribute__((aligned(8 /* alignof (Barrier) */))) {
+  unsigned expected;
+  unsigned waiting;
+  bool blocking;
+  char entering[24 /* sizeof (CndVar) */];
+  char exiting[24 /* sizeof (CndVar) */];
+  char mutex[24 /* sizeof (Mutex) */];
+} __barrier_type;
+
+#endif // LLVM_LIBC_TYPES__BARRIER_TYPE_H

libc/include/llvm-libc-types/pthread_barrier_t.h

@@ -0,0 +1,15 @@
+//===-- Definition of pthread_barrier_t type --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_PTHREAD_BARRIER_T_H
+#define LLVM_LIBC_TYPES_PTHREAD_BARRIER_T_H
+
+#include "include/llvm-libc-types/__barrier_type.h"
+typedef __barrier_type pthread_barrier_t;
+
+#endif // LLVM_LIBC_TYPES_PTHREAD_BARRIER_T_H

libc/include/llvm-libc-types/pthread_barrierattr_t.h

@@ -0,0 +1,16 @@
+//===-- Definition of pthread_barrierattr_t type --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_PTHREAD_BARRIERATTR_T_H
+#define LLVM_LIBC_TYPES_PTHREAD_BARRIERATTR_T_H
+
+typedef struct {
+  bool pshared;
+} pthread_barrierattr_t;
+
+#endif // LLVM_LIBC_TYPES_PTHREAD_BARRIERATTR_T_H

libc/include/pthread.yaml

@@ -6,6 +6,8 @@ types:
   - type_name: pthread_once_t
   - type_name: pthread_mutex_t
   - type_name: pthread_mutexattr_t
+  - type_name: pthread_barrier_t
+  - type_name: pthread_barrierattr_t
   - type_name: pthread_key_t
   - type_name: pthread_condattr_t
   - type_name: __pthread_tss_dtor_t
@@ -277,6 +279,26 @@ functions:
     arguments:
       - type: pthread_mutexattr_t *__restrict
       - type: int
+  - name: pthread_barrier_init
+    standards:
+      - POSIX
+    return_type: int
+    arguments:
+      - type: pthread_barrier_t *__restrict
+      - type: const pthread_barrierattr_t *__restrict
+      - type: int
+  - name: pthread_barrier_wait
+    standards:
+      - POSIX
+    return_type: int
+    arguments:
+      - type: pthread_barrier_t *
+  - name: pthread_barrier_destroy
+    standards:
+      - POSIX
+    return_type: int
+    arguments:
+      - type: pthread_barrier_t *
   - name: pthread_once
     standards:
       - POSIX

libc/src/__support/threads/linux/CMakeLists.txt

@@ -121,3 +121,14 @@ add_object_library(
     libc.src.__support.threads.linux.raw_mutex
     libc.src.__support.CPP.mutex
 )
+
+add_object_library(
+  barrier
+  HDRS
+    barrier.h
+  SRCS
+    barrier.cpp
+  DEPENDS
+    libc.src.__support.threads.CndVar
+    libc.src.__support.threads.mutex
+)

libc/src/__support/threads/linux/barrier.cpp

@@ -0,0 +1,85 @@
+//===-- Implementation of Barrier class ------------- ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/threads/linux/barrier.h"
+#include "hdr/errno_macros.h"
+#include "src/__support/threads/CndVar.h"
+#include "src/__support/threads/mutex.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+int Barrier::init(Barrier *b,
+                  [[maybe_unused]] const pthread_barrierattr_t *attr,
+                  unsigned count) {
+  LIBC_ASSERT(attr == nullptr); // TODO implement barrierattr
+  if (count == 0)
+    return EINVAL;
+
+  b->expected = count;
+  b->waiting = 0;
+  b->blocking = true;
+
+  int err;
+  err = CndVar::init(&b->entering);
+  if (err != 0)
+    return err;
+
+  err = CndVar::init(&b->exiting);
+  if (err != 0)
+    return err;
+
+  auto mutex_err = Mutex::init(&b->m, false, false, false, false);
+  if (mutex_err != MutexError::NONE)
+    return EAGAIN;
+
+  return 0;
+}
+
+int Barrier::wait() {
+  m.lock();
+
+  // if the barrier is emptying out threads, wait until it finishes
+  while (!blocking)
+    entering.wait(&m);
+  waiting++;
+
+  if (waiting < expected) {
+    // block threads until waiting = expected
+    while (blocking)
+      exiting.wait(&m);
+  } else {
+    // this is the last thread to call wait(), so lets wake everyone up
+    blocking = false;
+    exiting.broadcast();
+  }
+  waiting--;
+
+  if (waiting == 0) {
+    // all threads have exited the barrier, let's let the ones waiting to enter
+    // continue
+    blocking = true;
+    entering.broadcast();
+    m.unlock();
+
+    // POSIX dictates that the barrier should return a special value to just one
+    // thread, so we can arbitrarily choose this thread
+    return PTHREAD_BARRIER_SERIAL_THREAD;
+  }
+  m.unlock();
+
+  return 0;
+}
+
+int Barrier::destroy(Barrier *b) {
+  CndVar::destroy(&b->entering);
+  CndVar::destroy(&b->exiting);
+  Mutex::destroy(&b->m);
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/__support/threads/linux/barrier.h

@@ -0,0 +1,50 @@
+//===-- A platform independent abstraction layer for barriers --*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_BARRIER_H
+#define LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_BARRIER_H
+
+#include "hdr/pthread_macros.h"
+#include "include/llvm-libc-types/pthread_barrier_t.h"
+#include "include/llvm-libc-types/pthread_barrierattr_t.h"
+#include "src/__support/threads/CndVar.h"
+#include "src/__support/threads/mutex.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+// NOTE: if the size of this class changes, you must ensure that the size of
+// pthread_barrier_t (found in include/llvm-libc/types/pthread_barrier_t.h) is
+// the same size
+class Barrier {
+private:
+  unsigned expected;
+  unsigned waiting;
+  bool blocking;
+  CndVar entering;
+  CndVar exiting;
+  Mutex m;
+
+public:
+  static int init(Barrier *b, const pthread_barrierattr_t *attr,
+                  unsigned count);
+  static int destroy(Barrier *b);
+  int wait();
+};
+
+static_assert(
+    sizeof(Barrier) == sizeof(pthread_barrier_t),
+    "The public pthread_barrier_t type cannot accommodate the internal "
+    "barrier type.");
+
+static_assert(alignof(Barrier) == alignof(pthread_barrier_t),
+              "The public pthread_barrier_t type has a different alignment "
+              "than the internal barrier type.");
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_BARRIER_H