3 ms·

I don't think that's correct? Can you give an example of an algorithm on x86 or ARM that requires a consume barrier to operate correctly? It's a noop. Depend

by ajross 3y ago

I don't think that's correct? Can you give an example of an algorithm on x86 or ARM that requires a consume barrier to operate correctly? It's a noop. Dependent loads on those hardware architectures do not reorder, and by definition dependent loads can't be reordered by the compiler (because you can't write time travel code that executes a load before the instruction that loads its pointer!). Consume does not AFAICT act as a memory clobber or optimization barrier to the compiler.

gpderetta 3y ago
consume obviously doesn't need a barrier on arm or x86, but it still needs a source level annotation. relaxed is not enough as the compiler could otherwise break the required dependency (for example by introducing a speculation). In fact consume is in practice not implemented optimally by compilers because it is very very hard to guarantee that the data dependency is not broken in all cases. I.e. the quirks of Alpha have no bearing on the existence of consume. A different annotation between relaxed and acquire would still needed to specifically capture the semantics of data dependencies (that's the theory at least, in practice even consume and the [[carries_dependency]] annotations are not enough).
jcranmer 3y ago
> Consume does not AFAICT act as a memory clobber or optimization barrier to the compiler. This is your confusion. It is precisely an optimization barrier. Specifically, it's an acquire barrier, but only for certain data-dependent subsequent loads. Compilers universally treat it as a global acquire barrier because it turns out that it's too difficult to ensure that language-level data dependencies translate to hardware-level data dependencies. On x86, the hardware never needs acquire barriers, so the relaxation to an acquire barrier has no effect. However, although the hardware doesn't need acquire barriers, the compiler still needs to know where those exist in the program so that it doesn't reorder the loads anyways. Meanwhile, on Alpha, the hardware still needs an acquire barrier in the case where there's a data dependency. So in both the ultra-weak and the rather strong memory models, translating memory_order_consume as memory_order_acquire has no actual difference to the hardware outcomes. This is why memory_order_consume is really for ARM et al--the compiler still needs to know about the existence of the barrier, but the hardware doesn't need the barrier. And this is why there's still efforts going on to fix memory_order_consume; the people who work on ARM and the like want to get to the point where they can write these codes without having the compiler emit acquire barriers.
- ajross 3y ago
  > It is precisely an optimization barrier. Specifically, it's an acquire barrier, but only for certain data-dependent subsequent loads. I don't understand what that means. Compiler barriers are about changing the ordering of instructions. "DEpendent" loads have defined ordering already, you can't issue a load before loading the pointer register you're dereferencing! I have to ask again: can you give me an example from any toolchain and any architecture other than Alpha where a consume generates a detectable difference in the generated code?
  - gpderetta 3y ago
    The article has an explicit example of how a compiler can break the dependency by adding speculation (for example after PGO). In practice it is unlikely that a plain load through a pointer will break, but consume allows for more complex dependencies beyond pointer dereferences.
  - jcranmer 3y ago
    Compiler optimizations need not preserve data dependencies. For example, consider this fragment of code: if (i == 3) { y = x[i]; } The compiler will happily replace this code with: if (i == 3) { y = x[3]; } Now, the load of `x[i]` is no longer data-dependent on `i`. This is a real optimization that happens in compilers, and the Linux kernel (which relies on something akin to release/consume, although not directly dependent on the C11 memory model) has documentation specifically warning against using the results of the consume-like load in comparisons precisely because this optimization exists and has caused problems in the past. This is also why compilers have generally ignored memory_order_consume, by-the-by. Supporting consume 'properly' (i.e., other than strengthening it into a memory_order_acquire) requires suppressing optimizations that do not preserve data dependencies, but consume leaks out too much to the point that it's basically not possible to prove that the dependency isn't part of a consume chain, and the optimization is too beneficial to be worth turning off entirely.
    - ajross 3y ago
      Once more, and then I'll leave this thread. This is circular. I asked for an example of where you'd need to use a consume load for correctness[1] on a modern architecture, and you gave me an example of where a consume load would generate different code. That's not responsive. What you posted was a situation where a compiler would take something that is a dependent load syntactically and turn it into an independent load. And that's interesting, but on an architecture where dependent loads are always correctly ordered it's not a justification for having consume in the standard. (Edit: it's also true that this is a regime where we've seen security bugs due to optimization exactly like this. And that's true. But that's also not a memory ordering issue and it's, again, not resolvable by use of the C++ consume load feature). So we're back to "consume exists so you can write C++ code in 2023 to run on an architecture Compaq cancelled in 2003". Right? [1] Though it's true that my second phrasing was more ambiguous than my first.

by ajross 3y ago

gpderetta 3y ago

consume obviously doesn't need a barrier on arm or x86, but it still needs a source level annotation. relaxed is not enough as the compiler could otherwise break the required dependency (for example by introducing a speculation). In fact consume is in practice not implemented optimally by compilers because it is very very hard to guarantee that the data dependency is not broken in all cases. I.e. the quirks of Alpha have no bearing on the existence of consume. A different annotation between relaxed and acquire would still needed to specifically capture the semantics of data dependencies (that's the theory at least, in practice even consume and the [[carries_dependency]] annotations are not enough).

jcranmer 3y ago

> Consume does not AFAICT act as a memory clobber or optimization barrier to the compiler. This is your confusion. It is precisely an optimization barrier. Specifically, it's an acquire barrier, but only for certain data-dependent subsequent loads. Compilers universally treat it as a global acquire barrier because it turns out that it's too difficult to ensure that language-level data dependencies translate to hardware-level data dependencies. On x86, the hardware never needs acquire barriers, so the relaxation to an acquire barrier has no effect. However, although the hardware doesn't need acquire barriers, the compiler still needs to know where those exist in the program so that it doesn't reorder the loads anyways. Meanwhile, on Alpha, the hardware still needs an acquire barrier in the case where there's a data dependency. So in both the ultra-weak and the rather strong memory models, translating memory_order_consume as memory_order_acquire has no actual difference to the hardware outcomes. This is why memory_order_consume is really for ARM et al--the compiler still needs to know about the existence of the barrier, but the hardware doesn't need the barrier. And this is why there's still efforts going on to fix memory_order_consume; the people who work on ARM and the like want to get to the point where they can write these codes without having the compiler emit acquire barriers.

ajross 3y ago
> It is precisely an optimization barrier. Specifically, it's an acquire barrier, but only for certain data-dependent subsequent loads. I don't understand what that means. Compiler barriers are about changing the ordering of instructions. "DEpendent" loads have defined ordering already, you can't issue a load before loading the pointer register you're dereferencing! I have to ask again: can you give me an example from any toolchain and any architecture other than Alpha where a consume generates a detectable difference in the generated code?
- gpderetta 3y ago
  The article has an explicit example of how a compiler can break the dependency by adding speculation (for example after PGO). In practice it is unlikely that a plain load through a pointer will break, but consume allows for more complex dependencies beyond pointer dereferences.
- jcranmer 3y ago
  Compiler optimizations need not preserve data dependencies. For example, consider this fragment of code: if (i == 3) { y = x[i]; } The compiler will happily replace this code with: if (i == 3) { y = x[3]; } Now, the load of `x[i]` is no longer data-dependent on `i`. This is a real optimization that happens in compilers, and the Linux kernel (which relies on something akin to release/consume, although not directly dependent on the C11 memory model) has documentation specifically warning against using the results of the consume-like load in comparisons precisely because this optimization exists and has caused problems in the past. This is also why compilers have generally ignored memory_order_consume, by-the-by. Supporting consume 'properly' (i.e., other than strengthening it into a memory_order_acquire) requires suppressing optimizations that do not preserve data dependencies, but consume leaks out too much to the point that it's basically not possible to prove that the dependency isn't part of a consume chain, and the optimization is too beneficial to be worth turning off entirely.
  - ajross 3y ago
    Once more, and then I'll leave this thread. This is circular. I asked for an example of where you'd need to use a consume load for correctness[1] on a modern architecture, and you gave me an example of where a consume load would generate different code. That's not responsive. What you posted was a situation where a compiler would take something that is a dependent load syntactically and turn it into an independent load. And that's interesting, but on an architecture where dependent loads are always correctly ordered it's not a justification for having consume in the standard. (Edit: it's also true that this is a regime where we've seen security bugs due to optimization exactly like this. And that's true. But that's also not a memory ordering issue and it's, again, not resolvable by use of the C++ consume load feature). So we're back to "consume exists so you can write C++ code in 2023 to run on an architecture Compaq cancelled in 2003". Right? [1] Though it's true that my second phrasing was more ambiguous than my first.