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tlively
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It's unfortunate that the test gets so huge, but LGTM.
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To allow #2331 to roll, I forgot that upstream and fastcomp handle sbrk differently. This fixes that, and handles the upstream case where we import sbrk itself from JS. We can simplify this after emscripten-core/emscripten#9397 lands, however, it may also be nice to keep the backwards compatibility for running on existing wasm files in the wild.
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Currently emscripten links the wasm, then links the JS, then computes the final static allocations and in particular the location of the sbrk ptr (i.e. the location in memory of the brk location). Emscripten then imports that into the asm.js or wasm as
env.DYNAMICTOP_PTR. However, this didn't work in the wasm backend where we didn't have support for importing globals from JS, so we implement sbrk in JS.I am proposing that we change this model to allow us to write sbrk in C and compile it to wasm. To do so, that C code can import an extern C function,
emscripten_get_sbrk_ptr(), which basically just returns that location. The PostEmscripten pass can even apply that value at compile time, so we avoid the function call, and end up in the optimal place, see #2325 and emscripten PRs will be opened once other stuff lands.However, the SafeHeap pass must be updated to handle this, or our CI will break in the middle. This PR fixes that, basically making it check if
env.DYNAMICTOP_PTRexists, or if not then looking forenv.emscripten_get_sbrk_ptr, so that it can handle both.