改进 InterlockedCompareExchange() 的原子读取

Question

Assuming architecture is ARM64 or x86-64.

I want to make sure if these two are equivalent:

1. a = _InterlockedCompareExchange64((__int64*)p, 0, 0);
2. MyBarrier(); a = *(volatile __int64*)p; MyBarrier();

Where MyBarrier() is a memory barrier (hint) of compiler level, like __asm__ __volatile__ ("" ::: "memory") . So method 2 is supposed to be faster than method 1.

I heard that _Interlocked() functions would also imply memory barrier of both compiler and hardware level.

I heard that read (proper-aligned) intrinsic data is atomic on these architectures, but I am not sure if method 2 could be widely used?

(ps. because I think CPU will handle data dependency automatically so hardware barrier is not much considered here.)

Thank you for any advise / correction on this.

---

Here is some benchmarks on Ivy Bridge (i5 laptop).

(1E+006 loops: 27ms ):

; __int64 a = _InterlockedCompareExchange64((__int64*)p, 0, 0);
xor eax, eax
lock cmpxchg QWORD PTR val$[rsp], rbx

(1E+006 loops: 27ms ):

; __faststorefence(); __int64 a = *(volatile __int64*)p;
lock or DWORD PTR [rsp], 0
mov rcx, QWORD PTR val$[rsp]

(1E+006 loops: 7ms ):

; _mm_sfence(); __int64 a = *(volatile __int64*)p;
sfence
mov rcx, QWORD PTR val$[rsp]

(1E+006 loops: 1.26ms , not synchronized?):

; __int64 a = *(volatile __int64*)p;
mov rcx, QWORD PTR val$[rsp]

Answer 1

For the second version to be functionally equivalent, you obviously need atomic 64-bit reads, which is true on your platform.

However, _MemoryBarrier() is not a "hint to the compiler". _MemoryBarrier() on x86 prevents compiler and CPU reordering, and also ensures global visibility after the write. You also probably only need the first _MemoryBarrier() , the second one could be replaced with a _ReadWriteBarrier() unless a is also a shared variable - but you don't even need that since you are reading through a volatile pointer, which will prevent any compiler reordering in MSVC.

When you create this replacement, you basically end up with pretty much the same result :

// a = _InterlockedCompareExchange64((__int64*)&val, 0, 0);
xor eax, eax
lock cmpxchg QWORD PTR __int64 val, r8 ; val

// _MemoryBarrier(); a = *(volatile __int64*)&val;
lock or DWORD PTR [rsp], r8d
mov rax, QWORD PTR __int64 val ; val

Running these two in a loop, on my i7 Ivy Bridge laptop, gives equal results, within 2-3%.

However, with two memory barriers, the "optimized version" is actually around 2x slower.

So the better question is: Why are you using _InterlockedCompareExchange64 at all? If you need atomic access to a variable, use std::atomic , and an optimizing compiler should compile it to the most optimized version for your architecture, and add all the necessary barriers to prevent reordering and ensure cache coherency.