[libc][semaphore] Add internal unnamed semaphore implementation

libc/config/linux/x86_64/entrypoints.txt

@@ -1233,6 +1233,11 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.pthread.pthread_spin_lock
     libc.src.pthread.pthread_spin_trylock
     libc.src.pthread.pthread_spin_unlock
+
+    # semaphore.h entrypoints
+    libc.src.semaphore.sem_destroy
+    libc.src.semaphore.sem_getvalue
+    libc.src.semaphore.sem_init
     libc.src.pthread.pthread_self
     libc.src.pthread.pthread_setname_np
     libc.src.pthread.pthread_setspecific

libc/hdr/types/CMakeLists.txt

@@ -282,6 +282,14 @@ add_proxy_header_library(
     libc.include.llvm-libc-types.pid_t
 )
 
+add_proxy_header_library(
+  sem_t
+  HDRS
+    sem_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.sem_t
+)
+
 add_proxy_header_library(
   pthread_barrier_t
   HDRS

libc/hdr/types/sem_t.h

@@ -0,0 +1,22 @@
+//===-- Definition of sem_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_HDR_TYPES_SEM_T_H
+#define LLVM_LIBC_HDR_TYPES_SEM_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/sem_t.h"
+
+#else // Overlay mode
+
+#error "Cannot overlay sem_t"
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_SEM_T_H

libc/include/CMakeLists.txt

@@ -471,6 +471,15 @@ add_header_macro(
     .llvm-libc-types.struct_timespec
 )
 
+add_header_macro(
+  semaphore
+  ../libc/include/semaphore.yaml
+  semaphore.h
+  DEPENDS
+    .llvm-libc-types.sem_t
+    .llvm_libc_common_h
+)
+
 add_header_macro(
   spawn
   ../libc/include/spawn.yaml

libc/include/limits.yaml

@@ -52,6 +52,8 @@ macros:
     macro_header: limits-macros.h
   - macro_name: ULLONG_MAX
     macro_header: limits-macros.h
+  - macro_name: SEM_VALUE_MAX
+    macro_header: limits-macros.h
   - macro_name: SCHAR_MIN
     macro_header: limits-macros.h
   - macro_name: UCHAR_MIN

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

@@ -173,6 +173,10 @@
 #define ULLONG_MAX (LLONG_MAX * 2ULL + 1ULL)
 #endif // ULLONG_MAX
 
+#ifndef SEM_VALUE_MAX
+#define SEM_VALUE_MAX INT_MAX
+#endif // SEM_VALUE_MAX
+
 // *_MIN macros
 
 #ifndef SCHAR_MIN

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

@@ -84,6 +84,7 @@ add_header(pthread_rwlockattr_t HDR pthread_rwlockattr_t.h)
 add_header(pthread_spinlock_t HDR pthread_spinlock_t.h DEPENDS .pid_t)
 add_header(pthread_t HDR pthread_t.h DEPENDS .__thread_type)
 add_header(rlim_t HDR rlim_t.h)
+add_header(sem_t HDR sem_t.h DEPENDS .__futex_word)
 if(LIBC_TYPES_TIME_T_IS_32_BIT)
   add_header(time_t HDR time_t_32.h DEST_HDR time_t.h)
 else()

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

@@ -0,0 +1,20 @@
+//===-- Definition of sem_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_SEM_T_H
+#define LLVM_LIBC_TYPES_SEM_T_H
+
+#include "__futex_word.h"
+
+typedef struct {
+  __futex_word __value;        // current semaphore count
+  unsigned int __canary;       // used for sanity check
+  unsigned char __reserved[8]; // for future usage, total fixed size 16 bytes
+} sem_t;
+
+#endif // LLVM_LIBC_TYPES_SEM_T_H

libc/include/semaphore.yaml

@@ -0,0 +1,21 @@
+header: semaphore.h
+standards:
+  - posix
+types:
+  - type_name: sem_t
+functions:
+  - name: sem_destroy
+    return_type: int
+    arguments:
+      - type: sem_t *
+  - name: sem_getvalue
+    return_type: int
+    arguments:
+      - type: sem_t *__restrict
+      - type: int *__restrict
+  - name: sem_init
+    return_type: int
+    arguments:
+      - type: sem_t *
+      - type: int
+      - type: unsigned int

libc/src/CMakeLists.txt

@@ -25,6 +25,7 @@ if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(poll)
   add_subdirectory(pthread)
   add_subdirectory(sched)
+  add_subdirectory(semaphore)
   add_subdirectory(sys)
   add_subdirectory(termios)
 endif()

libc/src/semaphore/CMakeLists.txt

@@ -0,0 +1,49 @@
+add_header_library(
+  posix_semaphore
+  HDRS
+    posix_semaphore.h
+  DEPENDS
+    libc.hdr.types.sem_t
+    libc.src.__support.CPP.atomic
+    libc.src.__support.threads.linux.futex_word_type
+)
+
+add_entrypoint_object(
+  sem_init
+  SRCS
+    sem_init.cpp
+  HDRS
+    sem_init.h
+  DEPENDS
+    .posix_semaphore
+    libc.hdr.errno_macros
+    libc.hdr.limits_macros
+    libc.include.semaphore
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
+  sem_destroy
+  SRCS
+    sem_destroy.cpp
+  HDRS
+    sem_destroy.h
+  DEPENDS
+    .posix_semaphore
+    libc.hdr.errno_macros
+    libc.include.semaphore
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
+  sem_getvalue
+  SRCS
+    sem_getvalue.cpp
+  HDRS
+    sem_getvalue.h
+  DEPENDS
+    .posix_semaphore
+    libc.hdr.errno_macros
+    libc.include.semaphore
+    libc.src.errno.errno
+)

libc/src/semaphore/posix_semaphore.h

@@ -0,0 +1,72 @@
+//===-- Shared helpers for POSIX semaphores -------------------------------===//
+//
+// 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_SRC_SEMAPHORE_POSIX_SEMAPHORE_H
+#define LLVM_LIBC_SRC_SEMAPHORE_POSIX_SEMAPHORE_H
+
+#include "hdr/types/sem_t.h"
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/linux/futex_word.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace sem_utils {
+
+// 0x53 = S, 0x45 = E, 0x4D = M, 0x31 = 1
+// canary value: SEM1 in hex
+LIBC_INLINE_VAR constexpr unsigned int SEM_CANARY = 0x53454D31U;
+
+static_assert(sizeof(__futex_word) == sizeof(FutexWordType));
+static_assert(alignof(__futex_word) >= alignof(FutexWordType));
+
+// TODO:
+// 1. Add named semaphore support: sem_open, sem_close, sem_unlink
+// 2. Add the posting and waiting operations: sem_post, sem_wait,
+//    sem_trywait, sem_timedwait, sem_clockwait.
+
+LIBC_INLINE FutexWordType *value_ptr(sem_t *sem) {
+  return &sem->__value.__word;
+}
+
+// get the atomic reference for sem->__value
+LIBC_INLINE cpp::AtomicRef<FutexWordType> value(sem_t *sem) {
+  return cpp::AtomicRef<FutexWordType>(*value_ptr(sem));
+}
+
+LIBC_INLINE void initialize(sem_t *sem, unsigned int initial_value) {
+  // used in sem_init
+  // init happens before the semaphore is published to any threads
+  // initialize a initialized semaphore is undefined
+  // so RELAXED ordering is enough
+  value(sem).store(initial_value, cpp::MemoryOrder::RELAXED);
+  sem->__canary = SEM_CANARY;
+  for (unsigned char &byte : sem->__reserved)
+    byte = 0;
+}
+
+LIBC_INLINE bool is_valid(const sem_t *sem) {
+  // sanity check for a given semaphore pointer
+  return sem != nullptr && sem->__canary == SEM_CANARY;
+}
+
+LIBC_INLINE void invalidate(sem_t *sem) {
+  // used in sem_destroy
+  // invalidate is safe only when no threads is using
+  // blocked by destroyed semaphore is undefined
+  // use a destroyed semaphore is undefined
+  // RELAXED ordering is enough
+  value(sem).store(0, cpp::MemoryOrder::RELAXED);
+  sem->__canary = 0;
+  for (unsigned char &byte : sem->__reserved)
+    byte = 0;
+}
+
+} // namespace sem_utils
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_SEMAPHORE_POSIX_SEMAPHORE_H

libc/src/semaphore/sem_destroy.cpp

@@ -0,0 +1,29 @@
+//===-- Implementation of sem_destroy ------------------------------------===//
+//
+// 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/semaphore/sem_destroy.h"
+
+#include "src/semaphore/posix_semaphore.h"
+
+#include "hdr/errno_macros.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, sem_destroy, (sem_t * sem)) {
+  if (!sem_utils::is_valid(sem)) {
+    libc_errno = EINVAL;
+    return -1;
+  }
+
+  sem_utils::invalidate(sem);
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/semaphore/sem_destroy.h

@@ -0,0 +1,21 @@
+//===-- Implementation header for sem_destroy ------------------*- 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_SRC_SEMAPHORE_SEM_DESTROY_H
+#define LLVM_LIBC_SRC_SEMAPHORE_SEM_DESTROY_H
+
+#include "hdr/types/sem_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+int sem_destroy(sem_t *sem);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_SEMAPHORE_SEM_DESTROY_H

libc/src/semaphore/sem_getvalue.cpp

@@ -0,0 +1,33 @@
+//===-- Implementation of sem_getvalue -----------------------------------===//
+//
+// 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/semaphore/sem_getvalue.h"
+
+#include "src/semaphore/posix_semaphore.h"
+
+#include "hdr/errno_macros.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, sem_getvalue,
+                   (sem_t *__restrict sem, int *__restrict sval)) {
+  if (!sem_utils::is_valid(sem) || sval == nullptr) {
+    libc_errno = EINVAL;
+    return -1;
+  }
+
+  // get value is informational but not a synchronization op
+  // RELAXED ordering is enough
+  *sval =
+      static_cast<int>(sem_utils::value(sem).load(cpp::MemoryOrder::RELAXED));
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/semaphore/sem_getvalue.h

@@ -0,0 +1,21 @@
+//===-- Implementation header for sem_getvalue -----------------*- 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_SRC_SEMAPHORE_SEM_GETVALUE_H
+#define LLVM_LIBC_SRC_SEMAPHORE_SEM_GETVALUE_H
+
+#include "hdr/types/sem_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+int sem_getvalue(sem_t *__restrict sem, int *__restrict sval);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_SEMAPHORE_SEM_GETVALUE_H