Re: Something needs to happen with shared, and soon.

[David Nadlinger]

On Wednesday, 14 November 2012 at 15:08:35 UTC, Andrei 
Alexandrescu wrote:
Sorry, I was imprecise. We need to (a) define intrinsics for 
loading and storing data with high-level semantics (a short 
list: acquire, release, acquire+release, and 
sequentially-consistent) and THEN (b) implement the needed code 
generation appropriately for each architecture. Indeed on x86 
there is little need to insert fence instructions, BUT there is 
a definite need for the compiler to prevent certain 
reorderings. That's why implementing shared data operations 
(whether implicit or explicit) as sheer library code is NOT 
possible.

Sorry, I didn't see this message of yours before replying (the 
perils of threaded news readers…).

You are right about the fact that we need some degree of compiler 
support for atomic instructions. My point was that is it already 
available, otherwise it would have been impossible to implement 
core.atomic.{atomicLoad, atomicStore} (for DMD inline asm is 
used, which prohibits compiler code motion).

Thus, »we«, meaning on a language level, don't need to change 
anything about the current situations, with the possible 
exception of adding finer-grained control to 
core.atomic.MemoryOrder/mysnc [1]. It is the duty of the compiler 
writers to provide the appropriate means to implement druntime on 
their code generation infrastructure – and indeed, the 
situation in DMD could be improved, using inline asm is hitting a 
fly with a sledgehammer.

David


[1] I am not sure where the point of diminishing returns is here, 
although it might make sense to provide the same options as 
C++11. If I remember correctly, D1/Tango supported a lot more 
levels of synchronization.