X86 Serializing Instructions

The following instructions include a store fence as part of their semantics: SFENCE (part of SSE). MFENCE (part of SSE2). XCHG (part of the base x86). LOCK-prefixed instruction (the LOCK prefix is part of the base x86). A fully serializing instruction. A memory access of the UC memory type.

Sfence makes sure that all prior stores in program order become globally visible before any later stores in program order become globally visible. There are two differences compared to what you've written. First, sfence does not serialize issued prior stores; it serializes all prior stores irrespective of whether they have been issued or not. Second, it serializes with respect to only all later stores; not all later instructions.

That's what is meant by 'serializing operation' within the context of sfence.You've quoted only the first sentence from the documentation, but every sentence matters. @PeterCordes - probably Intel is just playing fast and loose with terminology here.

I don't think they really meant to say that the ordering has anything to do with dispatch or issue (regardless of the naming convention) since that's an internal detail not useful in any ISA doc (and if they were talking about issue to an EU it really makes no sense since that happens out of order). They really just mean 'that came first in program order' like Hadi says. It's also confusing they mention stores that happen before, but not stores that happen after, as if there is some asymmetry - but there isn't.–May 24 '18 at 3:05. That's again probably just some leaking of internal details: the way it works is mostly by ensuring that any outstanding stores (weakly ordered ones) go into L1 or are otherwise ordered - but the thing is really a two-way fence for stores. The has totally changed the language and is much clearer, talking not about 'serialization' but about the ordering guarantees, and removing the hint of asymmetry. Serialization is only mentioned in the short description: 'Serializes store operations.'

–May 24 '18 at 3:07. The use of the word serialization is also a bit overloaded in the Intel manual since a 'serializing instruction' is a special thing (as you are aware), and sfence is not one of those. IMO the current text is much better. Also I lied when I said the hint of asymmetry is gone - the first sentence is: Orders processor execution relative to all memory stores prior to the SFENCE instruction. The next setence, however, clears it up IMO: The processor ensures that every store prior to SFENCE is globally visible before any store after SFENCE becomes globally visible.–May 24 '18 at 3:08. The English word:. occurring in a series rather than simultaneously.Computers.a) of or relating to the apparent or actual performance of data-processing operations one at a time (distinguished fromparallel).b) of or relating to the transmission or processing of eachpart of a whole in sequence, as each bit of a byte or each byte of acomputer word (distinguished from parallel).( can also mean converting an object representation to a bit-stream or byte-stream which can be stored to disk or sent over a network outside of the program.

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But that's not the meaning that applies in the context of sfence).Database is a more closely related concept.SFENCE orders the global visibility of earlier stores with respect to SFENCE itself, and later stores. Serializing = imposing an order on things, stopping them from overlapping or happening in parallel.Note that in Intel terminology, 'serializing instruction' has a special meaning: an instruction that flushes the store buffer and the out-of-order instruction pipeline before any later instructions can execute. (They can decode and maybe even issue into the out-of-order core, but not execute).sfence is not a 'serializing instruction' in that sense; it only orders NT stores with respect to each other and regular stores.

(Regular stores are already ordered with respect to each other, so sfence has no effect if there are no NT stores in flight. All you need for correct release semantics is to put regular stores in the right order, e.g. With a compiler barrier to stop compile-time reordering.)'serializing' in Intel's definition of sfence is just the plain English meaning of the term, not the 'serializing instruction' x86 special meaning.Current wording of:Intel rewrote the opening paragraph to say 'orders' instead of 'serializes', except in the short description: Serializes store operations.The main Description is:Orders processor execution relative to all memory stores prior to the SFENCE instruction. The processor ensures that every store prior to SFENCE is globally visible before any store after SFENCE becomes globally visible. The SFENCE instruction is ordered with respect to memory stores, other SFENCE instructions, MFENCE instructions, and any serializing instructions (such as the CPUID instruction). It is not ordered with respect to memory loads or the LFENCE instruction.The first sentence is still kind of bogus, though. Execution isn't ordered, only commit to L1d cache.

Yeah the first sentence still has a hint of confusion. But it says 'relative to all memory stores', so you could read it as 'orders processor execution only as is necessary to order stores' which turns out to be 'not at all'.

If sort of makes sense if you think of the second sentence, which is really the key, as clarifying and being specific about the result of the first. It's unfortunate that it says processor execution since that sounds tantalizing like some kind of instruction serialization, but we know it's not.–May 24 '18 at 6:19. An sfence prevents stores before the fence from being re-ordered with respect to stores after the fence. Don't focus on the 'serializing' part: Intel has removed the text you quoted from the current version of the manual (you linked an obsolete source).The 1 (emphasis mine):Orders processor execution relative to all memory stores prior to theSFENCE instruction. The processor ensures that every store prior toSFENCE is globally visible before any store after SFENCE becomesglobally visible. The SFENCE instruction is ordered with respect tomemory stores, other SFENCE instructions, MFENCE instructions, and anyserializing instructions (such as the CPUID instruction). It is notordered with respect to memory loads or the LFENCE instruction.Weakly ordered memory types can be used to achieve higher processorperformance through such techniques as out-of-order issue,write-combining, and write-collapsing.

The degree to which a consumerof data recognizes or knows that the data is weakly ordered variesamong applications and may be unknown to the producer of this data.The SFENCE instruction provides a performance-efficient way ofensuring store ordering between routines that produce weakly-orderedresults and routines that consume this data.The second (emphasized) line is the key: this guys is there to orders stores.It doesn't (necessarily) make stores become visible sooner - that happens naturally on a coherent architecture like x86. It doesn't necessarily serialize instructions surrounding the fence, including stores: it just makes sure stores aren't apparently reordered across the barrier.Here's a secret though: this instruction is mostly useless in x86 code. The x86 memory model already guarantees that normal stores are already exactly ordered with respect to each other: stores from a given CPU become visible in program order to all other CPUs, so sfence doesn't add anything.

The only exceptions, where sfence can be useful is with relatively obscure stuff like or really obscure stuff like WC memory types. If you aren't using that, you don't need this instruction.1 I've also linked an unofficial source as there is no official HTML source that I'm aware of - but I checked that it is up-to-date on sfence as of May 2018.