[libc-commits] [libc] [libc] Use clang's scoped atomics if available from the compiler (PR #74769)
Joseph Huber via libc-commits
libc-commits at lists.llvm.org
Thu Jan 18 12:37:34 PST 2024
https://github.com/jhuber6 updated https://github.com/llvm/llvm-project/pull/74769
>From 631c24589420865a565f810049b2b21feff6f1b6 Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Thu, 7 Dec 2023 15:44:58 -0600
Subject: [PATCH] [libc] Use clang's scoped atomics if available from the
compiler
Summary:
A recent patch in https://github.com/llvm/llvm-project/pull/72280
provided `clang` the ability to easily use scoped atomics. These are a
special modifier on atomics that some backends support. They are
intended for providing more fine-grained control over the affected
memory of an atomic action. The default is a "system" scope, e.g.
coherence with the GPU and CPU memory on a heterogeneous system. If we
use "device" scope, that implies that the memory is only ordered with
respect to the current GPU.
These builtins are direct replacements for the GCC atomic builitins in
cases where the backend doesn't do anything with the information, so
these should be a drop-in. This introduces some noise, but hopefully it
isn't too contentious.
---
libc/src/__support/CPP/atomic.h | 82 +++++++++++++++++++++++++--------
libc/src/__support/RPC/rpc.h | 17 ++++---
2 files changed, 75 insertions(+), 24 deletions(-)
diff --git a/libc/src/__support/CPP/atomic.h b/libc/src/__support/CPP/atomic.h
index 78dc8d2da3c191d..1c4478dfeab6deb 100644
--- a/libc/src/__support/CPP/atomic.h
+++ b/libc/src/__support/CPP/atomic.h
@@ -26,6 +26,18 @@ enum class MemoryOrder : int {
SEQ_CST = __ATOMIC_SEQ_CST
};
+// These are a clang extension, see the clang documenation for more information:
+// https://clang.llvm.org/docs/LanguageExtensions.html#scoped-atomic-builtins.
+enum class MemoryScope : int {
+#if defined(__MEMORY_SCOPE_SYSTEM) && defined(__MEMORY_SCOPE_DEVICE)
+ SYSTEM = __MEMORY_SCOPE_SYSTEM,
+ DEVICE = __MEMORY_SCOPE_DEVICE,
+#else
+ SYSTEM = 0,
+ DEVICE = 0,
+#endif
+};
+
template <typename T> struct Atomic {
// For now, we will restrict to only arithmetic types.
static_assert(is_arithmetic_v<T>, "Only arithmetic types can be atomic.");
@@ -54,48 +66,82 @@ template <typename T> struct Atomic {
Atomic(const Atomic &) = delete;
Atomic &operator=(const Atomic &) = delete;
- // Atomic load
+ // Atomic load.
operator T() { return __atomic_load_n(&val, int(MemoryOrder::SEQ_CST)); }
- T load(MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_load_n(&val, int(mem_ord));
+ T load(MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_load_n))
+ return __scoped_atomic_load_n(&val, int(mem_ord), (int)(mem_scope));
+ else
+ return __atomic_load_n(&val, int(mem_ord));
}
- // Atomic store
+ // Atomic store.
T operator=(T rhs) {
__atomic_store_n(&val, rhs, int(MemoryOrder::SEQ_CST));
return rhs;
}
- void store(T rhs, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- __atomic_store_n(&val, rhs, int(mem_ord));
+ void store(T rhs, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_store_n))
+ __scoped_atomic_store_n(&val, rhs, int(mem_ord), (int)(mem_scope));
+ else
+ __atomic_store_n(&val, rhs, int(mem_ord));
}
// Atomic compare exchange
- bool compare_exchange_strong(T &expected, T desired,
- MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
+ bool compare_exchange_strong(
+ T &expected, T desired, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
return __atomic_compare_exchange_n(&val, &expected, desired, false,
int(mem_ord), int(mem_ord));
}
- T exchange(T desired, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_exchange_n(&val, desired, int(mem_ord));
+ T exchange(T desired, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_exchange_n))
+ return __scoped_atomic_exchange_n(&val, desired, int(mem_ord),
+ (int)(mem_scope));
+ else
+ return __atomic_exchange_n(&val, desired, int(mem_ord));
}
- T fetch_add(T increment, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_fetch_add(&val, increment, int(mem_ord));
+ T fetch_add(T increment, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_fetch_add))
+ return __scoped_atomic_fetch_add(&val, increment, int(mem_ord),
+ (int)(mem_scope));
+ else
+ return __atomic_fetch_add(&val, increment, int(mem_ord));
}
- T fetch_or(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_fetch_or(&val, mask, int(mem_ord));
+ T fetch_or(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_fetch_or))
+ return __scoped_atomic_fetch_or(&val, mask, int(mem_ord),
+ (int)(mem_scope));
+ else
+ return __atomic_fetch_or(&val, mask, int(mem_ord));
}
- T fetch_and(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_fetch_and(&val, mask, int(mem_ord));
+ T fetch_and(T mask, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_fetch_and))
+ return __scoped_atomic_fetch_and(&val, mask, int(mem_ord),
+ (int)(mem_scope));
+ else
+ return __atomic_fetch_and(&val, mask, int(mem_ord));
}
- T fetch_sub(T decrement, MemoryOrder mem_ord = MemoryOrder::SEQ_CST) {
- return __atomic_fetch_sub(&val, decrement, int(mem_ord));
+ T fetch_sub(T decrement, MemoryOrder mem_ord = MemoryOrder::SEQ_CST,
+ [[maybe_unused]] MemoryScope mem_scope = MemoryScope::DEVICE) {
+ if constexpr (LIBC_HAS_BUILTIN(__scoped_atomic_fetch_sub))
+ return __scoped_atomic_fetch_sub(&val, decrement, int(mem_ord),
+ (int)(mem_scope));
+ else
+ return __atomic_fetch_sub(&val, decrement, int(mem_ord));
}
// Set the value without using an atomic operation. This is useful
diff --git a/libc/src/__support/RPC/rpc.h b/libc/src/__support/RPC/rpc.h
index 08c1dfd10d6d7f3..7b2c89ac4dce48b 100644
--- a/libc/src/__support/RPC/rpc.h
+++ b/libc/src/__support/RPC/rpc.h
@@ -109,14 +109,16 @@ template <bool Invert, typename Packet> struct Process {
/// Retrieve the inbox state from memory shared between processes.
LIBC_INLINE uint32_t load_inbox(uint64_t lane_mask, uint32_t index) const {
- return gpu::broadcast_value(lane_mask,
- inbox[index].load(cpp::MemoryOrder::RELAXED));
+ return gpu::broadcast_value(
+ lane_mask,
+ inbox[index].load(cpp::MemoryOrder::RELAXED, cpp::MemoryScope::SYSTEM));
}
/// Retrieve the outbox state from memory shared between processes.
LIBC_INLINE uint32_t load_outbox(uint64_t lane_mask, uint32_t index) const {
return gpu::broadcast_value(lane_mask,
- outbox[index].load(cpp::MemoryOrder::RELAXED));
+ outbox[index].load(cpp::MemoryOrder::RELAXED,
+ cpp::MemoryScope::SYSTEM));
}
/// Signal to the other process that this one is finished with the buffer.
@@ -126,7 +128,8 @@ template <bool Invert, typename Packet> struct Process {
LIBC_INLINE uint32_t invert_outbox(uint32_t index, uint32_t current_outbox) {
uint32_t inverted_outbox = !current_outbox;
atomic_thread_fence(cpp::MemoryOrder::RELEASE);
- outbox[index].store(inverted_outbox, cpp::MemoryOrder::RELAXED);
+ outbox[index].store(inverted_outbox, cpp::MemoryOrder::RELAXED,
+ cpp::MemoryScope::SYSTEM);
return inverted_outbox;
}
@@ -241,7 +244,8 @@ template <bool Invert, typename Packet> struct Process {
uint32_t slot = index / NUM_BITS_IN_WORD;
uint32_t bit = index % NUM_BITS_IN_WORD;
return bits[slot].fetch_or(static_cast<uint32_t>(cond) << bit,
- cpp::MemoryOrder::RELAXED) &
+ cpp::MemoryOrder::RELAXED,
+ cpp::MemoryScope::DEVICE) &
(1u << bit);
}
@@ -251,7 +255,8 @@ template <bool Invert, typename Packet> struct Process {
uint32_t slot = index / NUM_BITS_IN_WORD;
uint32_t bit = index % NUM_BITS_IN_WORD;
return bits[slot].fetch_and(~0u ^ (static_cast<uint32_t>(cond) << bit),
- cpp::MemoryOrder::RELAXED) &
+ cpp::MemoryOrder::RELAXED,
+ cpp::MemoryScope::DEVICE) &
(1u << bit);
}
};
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