Minimal support for __builtin_unreachable and for __builtin_expect
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Not yet used for optimizations.
Not yet used for optimizations. Actually, __builtin_expect is removed during C2C conversion, otherwise the conversion to type "long" produces inefficient code on 64-bit platforms.
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These two builtins are used by GCC and Clang as optimization hints. They occur frequently in open-source code.
This PR adds minimal support for both builtins. They are not taken into account by optimizations yet.
__builtin_unreachableis transported all the way to assembly, where it generates no code. In the future, it could be used during the Linearize pass to remove more unreachable code.__builtin_expectis currently removed during C2C elaboration, so it does not even enter the verified part of CompCert. If we keep it as a "no op" builtin, inefficient code is generated for comparisons on 64-bit platforms. (Because__builtin_expectforces a conversion of its argument to typelong.) More work is needed to take full advantage of this builtin.