Thread overview
While loop on global variable optimised away?
May 11
ichneumwn
May 11
Johan
May 11
ichneumwn
May 11

Hi Forum,

I have a snippet of code as follows:

extern(C) extern __gshared uint g_count;

// inside a class member function:
  while(g_count) <= count) {}

This is from a first draft of the code without proper thread synchronisation. The global variable g_count is updated from a bit of C++ code. As soon as I turn the optimiser on, the code never gets passed this point, leading me to suspect it gets turned into

  while(true) {}

If modify the code in the following way:

  import core.volatile : volatileLoad;

  while(volatileLoad(&g_count) <= count) {}

it works again.

My question is, have I hit a compiler bug (ldc 1.28.1, aarch64 [Raspberry Pi]) or is this part of the language design. I would have thought since D use thread-local storage by default, that for a __gshared variable it would be understood that it can get modified by another thread. Access through atomic function would prevent the compiler from optimising this away as well, but if I were to use a Mutex inside the loop, there is no way for the compiler to tell what that Mutex is protecting and it might still decide to optimise the test away (assuming that is what is happening, did not attempt to look at the assembler code).

Cheers

May 11
Compiler optimizations should not be defined by a programming language specification.

This will be on LLVM.
May 11

On Wednesday, 11 May 2022 at 09:34:20 UTC, ichneumwn wrote:

>

Hi Forum,

I have a snippet of code as follows:

extern(C) extern __gshared uint g_count;

// inside a class member function:
  while(g_count) <= count) {}

This is from a first draft of the code without proper thread synchronisation. The global variable g_count is updated from a bit of C++ code. As soon as I turn the optimiser on, the code never gets passed this point, leading me to suspect it gets turned into

  while(true) {}

If modify the code in the following way:

  import core.volatile : volatileLoad;

  while(volatileLoad(&g_count) <= count) {}

it works again.

My question is, have I hit a compiler bug (ldc 1.28.1, aarch64 [Raspberry Pi]) or is this part of the language design. I would have thought since D use thread-local storage by default, that for a __gshared variable it would be understood that it can get modified by another thread.

This is part of the language spec. The language assumes that there is a single thread running, and any thread synchronization must be done by the user. This is well known from C and C++, from which D (implicitly afaik) borrows the memory model.

Example: imagine loading a struct with 2 ulongs from shared memory: auto s = global_struct_variable;. Loading the data into local storage s - e.g. CPU registers - would happen in two steps, first member1, then member2 (simplified, let's assume it spans across a cache boundary, etc..). During that load sequence, another thread might write to the struct. If the language must have defined behavior in that situation (other thread write), then a global mutex lock/unlock must be done before/after every read and write of shared data. That'd be a big performance impact on multithreading. Instead, single-thread execution is assumed, and thus the optimization is valid.

Your solution with volatileLoad is correct.

>

Access through atomic function would prevent the compiler from optimising this away as well, but if I were to use a Mutex inside the loop, there is no way for the compiler to tell what that Mutex is protecting and it might still decide to optimise the test away (assuming that is what is happening, did not attempt to look at the assembler code).

Any function call (inside the loop) for which it cannot be proven that it never modifies your memory variable will work. That's why I'm pretty sure that mutex lock/unlock will work.

On Wednesday, 11 May 2022 at 09:37:26 UTC, rikki cattermole wrote:

>

Compiler optimizations should not be defined by a programming language specification.

This is not true. Compiler optimizations are valid if and only if they can be proven by the programming language specification. A compiler optimization can never change valid program behavior. If an optimization does change behavior, then either the program is invalid per the language spec, or the optimization is bugged (or the observed behavior change is outside the language spec, such as how long a program takes to execute).

-Johan

May 11

On Wednesday, 11 May 2022 at 10:01:18 UTC, Johan wrote:

>

Any function call (inside the loop) for which it cannot be proven that it never modifies your memory variable will work. That's why I'm pretty sure that mutex lock/unlock will work.

Thank you, in C I would not have been surprised. It was the default thread local storage that made me question it. Your remark about the mutex lock/unlock is very helpful, I was starting to get worried I would need to keep a very careful watch on those __gshareds [well, more than usual :) ]

6 days ago

On Wednesday, 11 May 2022 at 10:01:18 UTC, Johan wrote:

>

Any function call (inside the loop) for which it cannot be proven that it never modifies your memory variable will work. That's why I'm pretty sure that mutex lock/unlock will work.

I think the common semantics ought to be that everything written by thread A before it releases the mutex will be visible to thread B when it aquires the same mutex, and any assumptions beyond this are nonportable?