[libc][arc4random 3/4] implement global and local random state

libc/src/__support/CMakeLists.txt

@@ -398,6 +398,30 @@ add_header_library(
     libc.src.__support.macros.attributes
 )
 
+add_header_library(
+  aba_ptr
+  HDRS
+    aba_ptr.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.src.__support.common
+    libc.src.__support.threads.sleep
+)
+
+add_header_library(
+  mpmc_stack
+  HDRS
+    mpmc_stack.h
+  DEPENDS
+    libc.src.__support.aba_ptr
+    libc.src.__support.common
+    libc.src.__support.CPP.atomic
+    libc.src.__support.CPP.new
+    libc.src.__support.CPP.optional
+    libc.src.__support.CPP.type_traits
+)
+
+
 add_subdirectory(FPUtil)
 add_subdirectory(OSUtil)
 add_subdirectory(StringUtil)

libc/src/__support/aba_ptr.h

@@ -0,0 +1,83 @@
+//===-- Transactional Ptr for ABA prevention --------------------*- 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___SUPPORT_TAGGED_POINTER_H
+#define LLVM_LIBC_SRC___SUPPORT_TAGGED_POINTER_H
+
+#include "hdr/types/size_t.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/sleep.h"
+
+#ifdef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
+#define LIBC_ABA_PTR_IS_ATOMIC true
+#else
+#define LIBC_ABA_PTR_IS_ATOMIC false
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+template <class T, bool IsAtomic> struct AbaPtrImpl {
+  union Impl {
+    struct alignas(2 * alignof(void *)) Atomic {
+      T *ptr;
+      size_t tag;
+    } atomic;
+    struct Mutex {
+      T *ptr;
+      bool locked;
+    } mutex;
+  } impl;
+
+  LIBC_INLINE constexpr AbaPtrImpl(T *ptr)
+      : impl(IsAtomic ? Impl{.atomic{ptr, 0}} : Impl{.mutex{ptr, false}}) {}
+
+  /// User must guarantee that operation is redoable.
+  template <class Op> LIBC_INLINE void transaction(Op &&op) {
+    if constexpr (IsAtomic) {
+      for (;;) {
+        typename Impl::Atomic snapshot, next;
+        __atomic_load(&impl.atomic, &snapshot, __ATOMIC_RELAXED);
+        next.ptr = op(snapshot.ptr);
+        // Wrapping add for unsigned integers.
+        next.tag = snapshot.tag + 1;
+        if (__atomic_compare_exchange(&impl.atomic, &snapshot, &next, true,
+                                      __ATOMIC_ACQ_REL, __ATOMIC_RELAXED))
+          return;
+      }
+    } else {
+      // Acquire the lock.
+      while (__atomic_exchange_n(&impl.mutex.locked, true, __ATOMIC_ACQUIRE))
+        while (__atomic_load_n(&impl.mutex.locked, __ATOMIC_RELAXED))
+          LIBC_NAMESPACE::sleep_briefly();
+
+      impl.mutex.ptr = op(impl.mutex.ptr);
+      // Release the lock.
+      __atomic_store_n(&impl.mutex.locked, false, __ATOMIC_RELEASE);
+    }
+  }
+
+  LIBC_INLINE T *get() const {
+    if constexpr (IsAtomic) {
+      // Weak micro-architectures typically regards simultaneous partial word
+      // loading and full word loading as a race condition. While there are
+      // implementations that uses racy read anyway, we still load the whole
+      // word to avoid any complications.
+      typename Impl::Atomic snapshot;
+      __atomic_load(&impl.atomic, &snapshot, __ATOMIC_RELAXED);
+      return snapshot.ptr;
+    } else {
+      return impl.mutex.ptr;
+    }
+  }
+};
+
+template <class T> using AbaPtr = AbaPtrImpl<T, LIBC_ABA_PTR_IS_ATOMIC>;
+} // namespace LIBC_NAMESPACE_DECL
+
+#undef LIBC_ABA_PTR_IS_ATOMIC
+#endif

libc/src/__support/mpmc_stack.h

@@ -0,0 +1,107 @@
+//===-- Simple Lock-free MPMC Stack -----------------------------*- 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___SUPPORT_MPMC_STACK_H
+#define LLVM_LIBC_SRC___SUPPORT_MPMC_STACK_H
+
+#include "src/__support/CPP/atomic.h"
+#include "src/__support/CPP/new.h"
+#include "src/__support/CPP/optional.h"
+#include "src/__support/aba_ptr.h"
+
+namespace LIBC_NAMESPACE_DECL {
+template <class T> class MPMCStack {
+  struct Node {
+    cpp::Atomic<size_t> visitor;
+    Node *next;
+    T value;
+
+    LIBC_INLINE Node(T val) : visitor(0), next(nullptr), value(val) {}
+  };
+  AbaPtr<Node> head;
+
+public:
+  static_assert(cpp::is_copy_constructible<T>::value,
+                "T must be copy constructible");
+  LIBC_INLINE constexpr MPMCStack() : head(nullptr) {}
+  LIBC_INLINE bool push(T value) {
+    AllocChecker ac;
+    Node *new_node = new (ac) Node(value);
+    if (!ac)
+      return false;
+    head.transaction([new_node](Node *old_head) {
+      new_node->next = old_head;
+      return new_node;
+    });
+    return true;
+  }
+  LIBC_INLINE bool push_all(T values[], size_t count) {
+    struct Guard {
+      size_t count;
+      Node **allocated;
+      LIBC_INLINE Guard(Node *allocated[]) : count(0), allocated(allocated) {}
+      LIBC_INLINE ~Guard() {
+        for (size_t i = 0; i < count; ++i)
+          delete allocated[i];
+      }
+      LIBC_INLINE void add(Node *node) { allocated[count++] = node; }
+      LIBC_INLINE void clear() { count = 0; }
+    };
+    // Variable sized array is a GNU extension.
+    __extension__ Node *allocated[count];
+    {
+      Guard guard(allocated);
+      for (size_t i = 0; i < count; ++i) {
+        AllocChecker ac;
+        Node *new_node = new (ac) Node(values[i]);
+        if (!ac)
+          return false;
+        guard.add(new_node);
+        if (i != 0)
+          new_node->next = allocated[i - 1];
+      }
+      guard.clear();
+    }
+    head.transaction([&allocated, count](Node *old_head) {
+      allocated[0]->next = old_head;
+      return allocated[count - 1];
+    });
+    return true;
+  }
+  LIBC_INLINE cpp::optional<T> pop() {
+    cpp::optional<T> res = cpp::nullopt;
+    Node *node = nullptr;
+    head.transaction([&](Node *current_head) -> Node * {
+      if (!current_head) {
+        res = cpp::nullopt;
+        return nullptr;
+      }
+      node = current_head;
+      node->visitor.fetch_add(1);
+      res = cpp::optional<T>{node->value};
+      Node *next = node->next;
+      node->visitor.fetch_sub(1);
+      return next;
+    });
+    // On a successful transaction, a node is popped by us. So we must delete
+    // it. When we are at here, no one else can acquire
+    // new reference to the node, but we still need to wait until other threads
+    // inside the transaction who may potentially be holding a reference to the
+    // node.
+    if (res) {
+      // Spin until the node is no longer in use.
+      while (node->visitor.load() != 0)
+        LIBC_NAMESPACE::sleep_briefly();
+      delete node;
+    }
+    return res;
+  }
+};
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif

libc/src/stdlib/linux/CMakeLists.txt

@@ -9,3 +9,23 @@ add_entrypoint_object(
     libc.src.signal.raise
     libc.src.stdlib._Exit
 )
+
+add_header_library(
+  vdso_rng
+  HDRS
+    vdso_rng.h
+  DEPENDS
+    libc.src.__support.threads.thread # For __cxa_thread_atexit_impl
+    libc.src.__support.CPP.algorithm
+    libc.src.__support.CPP.bit
+    libc.src.__support.CPP.mutex
+    libc.src.__support.CPP.optional
+    libc.src.__support.OSUtil.linux.vdso
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.macros.config
+    libc.src.__support.mpmc_stack
+    libc.src.__support.threads.callonce
+    libc.src.__support.threads.linux.raw_mutex
+    libc.src.sys.auxv.getauxval # TODO: remove public entrypoint dependency
+    libc.include.sys_syscall
+)