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WASI: Implement experimental threading support #16207

Merged
merged 9 commits into from
Jun 27, 2023
Merged

WASI: Implement experimental threading support #16207

merged 9 commits into from
Jun 27, 2023

Commits on Jun 26, 2023

  1. wasm-ld: implement --export-memory flag 

    This flag allows the user to force export the memory to the host
environment. This is useful when the memory is imported from the
host but must also be exported. This is (currently) required
to pass the memory validation for runtimes when using threads.
In this future this may become an error instead.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  2. Compilation: allow threads for Wasm when shared-memory is enabled 

    When the user enabled the linker-feature 'shared-memory' we do not force
a singlethreaded build. The linker already verifies all other CPU features
required for threads are enabled. This is true for both WASI and
freestanding.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  3. std: implement Futex for WebAssembly 

    Implements std's `Futex` for the WebAssembly target using Wasm's
`atomics` instruction set. When the `atomics` cpu feature is disabled
we emit a compile-error.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  4. std: implement Thread spawn for WASI 

    This implements a first version to spawn a WASI-thread. For a new thread
to be created, we calculate the size required to store TLS, the new stack,
and metadata. This size is then allocated using a user-provided allocator.

After a new thread is spawn, the HOST will call into our bootstrap procedure.
This bootstrap procedure will then initialize the TLS segment and set the
newly spawned thread's TID. It will also set the stack pointer to the newly
created stack to ensure we do not clobber the main thread's stack.

When bootstrapping the thread is completed, we will call the user's
function on this new thread.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  5. store allocator & remove global assembly 

    We now store the original allocator that was used to allocate the
memory required for the thread. This allocator can then be used
in any cleanup functionality to ensure the memory is freed correctly.

Secondly, we now use a function to set the stack pointer instead of
generating a function using global assembly. This is a lot cleaner
and more readable.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  6. std: implement join for WASI-threads 

    We now reset the Thread ID to 0 and wake up the main thread listening
for the thread to finish. We use inline assembly as we cannot use
the stack to set the thread ID as it could possibly clobber any
of the memory.

Currently, we leak the memory that was allocated for the thread.
We need to implement a way where we can clean up the memory without
using the stack (as the stack is stored inside this same memory).
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  7. free allocated memory upon call join 

    When `join` detects a thread has completed, it will free the allocated
memory of the thread. For this we must first copy the allocator. This is
required as the allocated memory holds a reference to the original
allocator. If we free the memory, we would end up with UB as the
allocator would free itself.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  8. std: implement detach for WASI-threads 

    When a thread is detached from the main thread, we automatically
cleanup any allocated memory. For this we first reset the stack-pointer
to the original stack-pointer of the main-thread so we can safely clear
the memory which also contains the thread's stack.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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  9. default to single-threaded for WebAssembly 

    When targeting WebAssembly, we default to building a single-threaded build
as threads are still experimental. The user however can enable a multi-
threaded build by specifying '-fno-single-threaded'. It's a compile-error
to enable this flag, but not also enable shared-memory.
    @Luukdegram
    Luukdegram committed Jun 26, 2023
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