Introduction

Atomic operations on basic numeric (and pointer) types can be used as a foundation to build higher level construct (e.g. mutexes.) There are also useful on their own (e.g. to implement an atomic counter.) This page outlines a design for adding atomic primops.

The primops

The new primops are modeled after those provided by C11, C++11, GCC, and LLVM.

atomicReadIntArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Int# #)
atomicWriteIntArray# :: MutableByteArray# s -> Int# -> Int# -> State# s -> State# s
atomicReadAddIntArray#
    :: MutableByteArray#     -- Array to modify
    -> Int#                  -- Index, in words
    -> Int#                  -- Amount to add
    -> State# s
    -> (# State# s, Int# #)  -- Value held previously
atomicReadAddIntArray# :: MutableByteArray# -> Int# -> Int# -> State# s -> (# State# s, Int# #)
atomicReadSubIntArray# :: MutableByteArray# -> Int# -> Int# -> State# s -> (# State# s, Int# #)
atomicReadOrIntArray#  :: MutableByteArray# -> Int# -> Int# -> State# s -> (# State# s, Int# #)
atomicReadXorIntArray# :: MutableByteArray# -> Int# -> Int# -> State# s -> (# State# s, Int# #)
atomicReadAndIntArray# :: MutableByteArray# -> Int# -> Int# -> State# s -> (# State# s, Int# #)

Implementation

The primops are implemented as CallishMachOps to allow us to emit LLVM intrinsics when using the LLVM backend. This also allows us to provide a fallback implementation in C on those platforms where we don't want to implement the backend support for these operations. The fallbacks can be implemented using the GCC/LLVM atomic built-ins e.g. __sync_fetch_and_add

Ensuring ordering of non-atomic operations

As the Cmm code generator cannot reorder (TODO is this true?) reads/writes around prim calls (i.e. CallishMachOps) the memory_order_seq_cst semantics should be preserved.

Ordering of non-atomic operations

The memory ordering of non-atomic operations surrounding an atomic operation correspond to the memory_order_seq_cst ordering specified for C11, which provides the strongest ordering guarantee.