When should I use _mm_sfence _mm_lfence and _mm_mfence

Question

I read the "Intel Optimization guide Guide For Intel Architecture".

However, I still have no idea about when should I use

_mm_sfence()
_mm_lfence()
_mm_mfence()

Could anyone explain when these should be used when writing multi-threaded code?

Answer 1

If you're using NT stores, you might want _mm_sfence or maybe even _mm_mfence. The use-cases for _mm_lfence are much more obscure.

If not, just use C++11 std::atomic and let the compiler worry about the asm details of controlling memory ordering.

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x86 has a strongly-ordered memory model, but C++ has a very weak memory model (same for C). For acquire/release semantics, you only need to prevent compile-time reordering. See Jeff Preshing's Memory Ordering At Compile Time article.

_mm_lfence and _mm_sfence do have the necessary compiler-barrier effect, but they will also cause the compiler to emit a useless lfence or sfence asm instruction that makes your code run slower.

There are better options for controlling compile-time reordering when you aren't doing any of the obscure stuff that would make you want sfence.

For example, GNU C/C++ asm("" ::: "memory") is a compiler barrier (all values have to be in memory matching the abstract machine because of the "memory" clobber), but no asm instructions are emitted.

If you're using C++11 std::atomic, you can simply do shared_var.store(tmp, std::memory_order_release). That's guaranteed to become globally visible after any earlier C assignments, even to non-atomic variables.

_mm_mfence is potentially useful if you're rolling your own version of C11 / C++11 std::atomic, because an actual mfence instruction is one way to get sequential consistency, i.e. to stop later loads from reading a value until after preceding stores become globally visible. See Jeff Preshing's Memory Reordering Caught in the Act.

But note that mfence seems to be slower on current hardware than using a locked atomic-RMW operation. e.g. xchg [mem], eax is also a full barrier, but runs faster, and does a store. On Skylake, the way mfence is implemented prevents out-of-order execution of even non-memory instruction following it. See the bottom of this answer.

In C++ without inline asm, though, your options for memory barriers are more limited (How many memory barriers instructions does an x86 CPU have?). mfence isn't terrible, and it is what gcc and clang currently use to do sequential-consistency stores.

Seriously just use C++11 std::atomic or C11 stdatomic if possible, though; It's easier to use and you get quite good code-gen for a lot of things. Or in the Linux kernel, there are already wrapper functions for inline asm for the necessary barriers. Sometimes that's just a compiler barrier, sometimes it's also an asm instruction to get stronger run-time ordering than the default. (e.g. for a full barrier).

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No barriers will make your stores appear to other threads any faster. All they can do is delay later operations in the current thread until earlier things happen. The CPU already tries to commit pending non-speculative stores to L1d cache as quickly as possible.

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_mm_sfence is by far the most likely barrier to actually use manually in C++

The main use-case for _mm_sfence() is after some _mm_stream stores, before setting a flag that other threads will check.

See Enhanced REP MOVSB for memcpy for more about NT stores vs. regular stores, and x86 memory bandwidth. For writing very large buffers (larger than L3 cache size) that definitely won't be re-read any time soon, it can be a good idea to use NT stores.

NT stores are weakly-ordered, unlike normal stores, so you need sfence if you care about publishing the data to another thread. If not (you'll eventually read them from this thread), then you don't. Or if you make a system call before telling another thread the data is ready, that's also serializing.

sfence (or some other barrier) is necessary to give you release/acquire synchronization when using NT stores. C++11 std::atomic implementations leave it up to you to fence your NT stores, so that atomic release-stores can be efficient.

#include <atomic>
#include <immintrin.h>

struct bigbuf {
    int buf[100000];
    std::atomic<unsigned> buf_ready;
};

void producer(bigbuf *p) {
  __m128i *buf = (__m128i*) (p->buf);

  for(...) {
     ...
     _mm_stream_si128(buf,   vec1);
     _mm_stream_si128(buf+1, vec2);
     _mm_stream_si128(buf+2, vec3);
     ...
  }

  _mm_sfence();    // All weakly-ordered memory shenanigans stay above this line
  // So we can safely use normal std::atomic release/acquire sync for buf
  p->buf_ready.store(1, std::memory_order_release);
}

Then a consumer can safely do if(p->buf_ready.load(std::memory_order_acquire)) { foo = p->buf[0]; ... } without any data-race Undefined Behaviour. The reader side does not need _mm_lfence; the weakly-ordered nature of NT stores is confined entirely to the core doing the writing. Once it becomes globally visible, it's fully coherent and ordered according to the normal rules.

Other use-cases include ordering clflushopt to control the order of data being stored to memory-mapped non-volatile storage. (e.g. an NVDIMM using Optane memory, or DIMMs with battery-backed DRAM exist now.)

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_mm_lfence is almost never useful as an actual load fence. Loads can only be weakly ordered when loading from WC (Write-Combining) memory regions, like video ram. Even movntdqa (_mm_stream_load_si128) is still strongly ordered on normal (WB = write-back) memory, and doesn't do anything to reduce cache pollution. (prefetchnta might, but it's hard to tune and can make things worse.)

TL:DR: if you aren't writing graphics drivers or something else that maps video RAM directly, you don't need _mm_lfence to order your loads.

lfence does have the interesting microarchitectural effect of preventing execution of later instructions until it retires. e.g. to stop _rdtsc() from reading the cycle-counter while earlier work is still pending in a microbenchmark. (Applies always on Intel CPUs, but on AMD only with an MSR setting: Is LFENCE serializing on AMD processors?. Otherwise lfence runs 4 per clock on Bulldozer family, so clearly not serializing.)

Since you're using intrinsics from C/C++, the compiler is generating code for you. You don't have direct control over the asm, but you might possibly use _mm_lfence for things like Spectre mitigation if you can get the compiler to put it in the right place in the asm output: right after a conditional branch, before a double array access. (like foo[bar[i]]). If you're using kernel patches for Spectre, I think the kernel will defend your process from other processes, so you'd only have to worry about this in a program that uses a JIT sandbox and is worried about being attacked from within its own sandbox.

Answer 2

Here is my understanding, hopefully accurate and simple enough to make sense:

(Itanium) IA64 architecture allows memory reads and writes to be executed in any order, so the order of memory changes from the point of view of another processor is not predictable unless you use fences to enforce that writes complete in a reasonable order.

From here on, I am talking about x86, x86 is strongly ordered.

On x86, Intel does not guarantee that a store done on another processor will always be immediately visible on this processor. It is possible that this processor speculatively executed the load (read) just early enough to miss the other processor's store (write). It only guarantees the order that writes become visible to other processors is in program order. It does not guarantee that other processors will immediately see any update, no matter what you do.

Locked read/modify/write instructions are fully sequentially consistent. Because of this, in general you already handle missing the other processor's memory operations because a locked xchg or cmpxchg will sync it all up, you will acquire the relevant cache line for ownership immediately and will update it atomically. If another CPU is racing with your locked operation, either you will win the race and the other CPU will miss the cache and get it back after your locked operation, or they will win the race, and you will miss the cache and get the updated value from them.

lfence stalls instruction issue until all instructions before the lfence are completed. mfence specifically waits for all preceding memory reads to be brought fully into the destination register, and waits for all preceding writes to become globally visible, but does not stall all further instructions as lfence would. sfence does the same for only stores, flushes write combiner, and ensures that all stores preceding the sfence are globally visible before allowing any stores following the sfence to begin execution.

Fences of any kind are rarely needed on x86, they are not necessary unless you are using write-combining memory or non-temporal instructions, something you rarely do if you are not a kernel mode (driver) developer. Normally, x86 guarantees that all stores are visible in program order, but it does not make that guarantee for WC (write combining) memory or for "non-temporal" instructions that do explicit weakly ordered stores, such as movnti.

So, to summarize, stores are always visible in program order unless you have used special weakly ordered stores or are accessing WC memory type. Algorithms using locked instructions like xchg, or xadd, or cmpxchg, etc, will work without fences because locked instructions are sequentially consistent.

Answer 3

The intrinsic calls you mention all simply insert an sfence, lfence or mfence instruction when they are called. So the question then becomes "What are the purposes of those fence instructions"?

The short answer is that lfence is completely useless^* and sfence almost completely useless for memory ordering purposes for user-mode programs in x86. On the other hand, mfence serves as a full memory barrier, so you might use it in places where you need a barrier if there isn't already some nearby lock-prefixed instruction providing what you need.

The longer-but-still short answer is...

lfence

lfence is documented to order loads prior to the lfence with respect to loads after, but this guarantee is already provided for normal loads without any fence at all: that is, Intel already guarantees that "loads aren't reordered with other loads". As a practical matter, this leaves the purpose of lfence in user-mode code as an out-of-order execution barrier, useful perhaps for carefully timing certain operations.

sfence

sfence is documented to order stores before and after in the same way that lfence does for loads, but just like loads the store order is already guaranteed in most cases by Intel. The primary interesting case where it doesn't is the so-called non-temporal stores such as movntdq, movnti, maskmovq and a few other instructions. These instructions don't play by the normal memory ordering rules, so you can put an sfence between these stores and any other stores where you want to enforce the relative order. mfence works for this purpose too, but sfence is faster.

mfence

Unlike the other two, mfence actually does something: it serves as a full memory barrier, ensuring that all of the previous loads and stores will have completed¹ before any of the subsequent loads or stores begin execution. This answer is too short to explain the concept of a memory barrier fully, but an example would be Dekker's algorithm, where each thread wanting to enter a critical section stores to a location and then checks to see if the other thread has stored something to its location. For example, on thread 1:

mov   DWORD [thread_1_wants_to_enter], 1  # store our flag
mov   eax,  [thread_2_wants_to_enter]     # check the other thread's flag
test  eax, eax
jnz   retry
; critical section

Here, on x86, you need a memory barrier in between the store (the first mov), and the load (the second mov), otherwise each thread could see zero when they read the other's flag because the x86 memory model allows loads to be re-ordered with earlier stores. So you could insert an mfence barrier as follows to restore sequential consistency and the correct behavior of the algorithm:

mov   DWORD [thread_1_wants_to_enter], 1  # store our flag
mfence
mov   eax,  [thread_2_wants_to_enter]     # check the other thread's flag
test  eax, eax
jnz   retry
; critical section

In practice, you don't see mfence as much as you might expect, because x86 lock-prefixed instructions have the same full-barrier effect, and these are often/always (?) cheaper than an mfence.

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¹ E.g., loads will have been satisfied and stores will have become globally visible (although it would be implemented differently as long as the visible effect wrt ordering is "as if" that occurred).