This commit implements the [wide-arithmetic] proposal which has recently
reached phase 2 in the WebAssembly proposals process. The goal here is
to implement support in LLVM for emitting these instructions which are
gated behind a new feature flag by default. A new `wide-arithmetic`
feature flag is introduced which gates these four new instructions from
being emitted.
Emission of each instruction itself is relatively simple given LLVM's
preexisting lowering rules and infrastructure. The main gotcha is that
due to the multi-result nature of all of these instructions it needed
the lowerings to be implemented in C++ rather than in TableGen.
[wide-arithmetic]: https://github.com/WebAssembly/wide-arithmetic
This currently only defines a constant, but in the future will be used
to gate builtins for experimenting and prototyping half-precision
proposal (https://github.com/WebAssembly/half-precision).
The CPU tests in this file are not working as intended. Specifying
`-mcpu=[mvp|generic|bleeding-edge]` in `clang` command line does NOT add
arguments like `-target-feature`, `+feature-name`, ... to `clang-14`
command line. Specifying `-mcpu=[mvp|generic|bleeding-edge]` in `clang`
command line will just add `-target-cpu` `[mvp|generic|bleeding-edge]`
to `clang-14` command line, and individual features are added here
within `clang-14` invocation:
5b780c8c6c/clang/lib/Basic/Targets/WebAssembly.cpp (L150-L163)
The reason these CPU tests are passing is because they only have `-NOT`
checks, and we don't emit `-target-feature` arguments for them anyway,
the test passes, but they don't check what they are supposed to check.
This make CPU tests only check `-target-cpu` lines instead of individual
features, which will not be emitted.
Adding to allow users to get this flag into the target features section for
future use cases.
Reviewed By: tlively, aheejin
Differential Revision: https://reviews.llvm.org/D158409
