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Proposal for instruction set - now on Github
Author: Hubert Lamontagne Date: 2017-09-26 01:37
Agner wrote:
Hubert Lamontagne wrote:
yeengief wrote:

Bonus request: The stack area, or some part of it, is always in a incoherent cache-state: The current core/context is the only one that can write/read there.

I am not sure how this differs from my proposal: Each thread has two stacks. The call stack is private and separate from everything else (and usually so small that it can be cached entirely on-chip). The data stack is private to the thread that owns it. This requires some discipline from the programmer. Only static memory or memory allocated by the main thread is accessible to all threads. Or you may have a special malloc_shared function for allocating memory that is accessible to all threads. Semafors and mutexes are placed in shared memory.

The private data stack is used for function-local variables and register spilling. Since it is private, it does not need coherence across threads. This makes caching of local data simple and efficient. It is also good for safety, of course.

Legacy software needs to turn off the private stack feature if it is sharing local memory between threads. A flag in the executable file header could control this feature.

The difference is that I'm seeing it from the C++ compiler point of view, and how the current ecosystem of malloc+mutexes+threads works in complex applications. As it is, program values that you don't take the address of are indeed truly local, and will show up as renamable values in the SSA pass and not translate into GEP+Load+Store instructions, and can safely be moved into a non-addressable memory like a special stack (or, obviously, a register). This class of values includes register spilling, argument passing, and function return addresses - none of these are "really" visible to the programmer (except for the value), so they can also be moved to a private stack - even in existing C++ code! This doesn't even require special discipline. You can do really crazy optimizations on this "value-only" data, like reordering the reads/writes, assuming perfect alignment and no partial value writes, making page faults and reads/writes from other threads impossible, allowing register renaming and so forth, because you're protected from all the crazy cases since the user doesn't have the address and can't pass it around halfway across the program.

As soon as you take a pointer or reference, it automatically becomes visible to anything that you can pass that pointer to, which turns out to be "the whole rest of the process including other threads". This forces you to give it a real memory address and do real Loads/Stores that run in program order and are visible across all threads. And it's only after that, that you can put in your optimizer pass that tries to remove the Loads/Stores if it can prove that it doesn't change anything in program behavior - which turns out, is really hard. Even just reordering Loads/Stores is hard, because it's hard to prove that function pointers will never overlap and rule out even really dumb cases like input and output buffers to a function overlapping out of alignment.

The difference is that my proposal is based on pointer sharability:
- The main data stack for each thread is NOT thread local and is coherent across the whole process, other threads can read/write in its memory range. It is used for pointer-accessible local values.
- The call stack is local and can also contain anything the compiler KNOWS is non-pointer-accessible such as function call register saves/restores and spillovers and local arrays, and is technically isn't even in paged memory, and isn't accessed with normal load/store instructions.
- Memory allocated using malloc() is accessible from all threads (even if it's allocated by a worker thread). This is necessary for tons of existing C++ code. You can provide a private_malloc() function.

 
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