Dependency on Abseil making WASM/YoWASP builds impossible

[whitequark]

Abseil (with CMake, at least) has a hard dependency on threading, which breaks builds on platforms without threads. The upstream worked around this for Bazel builds but not CMake. Unfortunately, the workaround (or its CMake equivalent) fixes the immediate issue (no -pthread) but it doesn't actually let the build complete on platforms without atomics, at least not for the library that nextpnr wants to use.

[litghost]

@whitequark What's the CMake invocation to build for WASM? Given the Bazel changes, it looks the right answer is to change the find_package(Threads REQUIRED) to:

if(NOT ${CMAKE_SYSTEM_NAME} STREQUAL Emscripten)
  find_package(Threads REQUIRED)
endif()

etc. But I cannot test this without a way to run the emscripten build.

[whitequark]

Given the Bazel changes, it looks the right answer is to change the find_package(Threads REQUIRED) to:

That doesn't help; there's a ton of references to atomics, which are not currently a part of WASI.

That said, the WASI build is done by this script.

[litghost]

Given the Bazel changes, it looks the right answer is to change the find_package(Threads REQUIRED) to:

That doesn't help; there's a ton of references to atomics, which are not currently a part of WASI.

That said, the WASI build is done by this script.

Not only that, but in the entropy store has a mutex lock here: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/hashtablez_sampler.h#L87

So I don't think the bazel rules will help here, as fundamentally the Swiss table just assumes a multi-core system. Unfortunate :(