[cfe-commits] RFC: Half floating point support

[Anton Korobeynikov]

Hello Everyone

Attached is a preliminary patch for storage-only half floating point
support in clang. Basically, lack of such support is a regression now,
because llvm-gcc supported it.

The key idea of the patch is the following: when building AST for new
'half' type implicit conversion to float is performed (the opposite
conversion thus will be done automatically). Such a conversion,
fortunately, should be done only in few places:

1. During lvalue-to-rvalue conversion
2. During building initializer sequence
3. For some unary ops where no lvalue-to-rvalue conversion is
performed (e.g. ++foo).

Later these promotions can be turned off depending on whether the
target wants half fp to be native or storage only type.

During the codegen I just turn such float<->half conversion into
appropriate intrinsic calls plus some other minor stuff for
initializers.

What is missed in the patch:

1. Sema checks.
2. Testsuite (I have extensive one, but I don't have for sema checks)
3. (Maybe) Some C++ bits.... Most probably RTTI?

So, I just wanted thoughts and comments on this patch.

Answering the very first question: why this is not done entirely in
codegen phase? Answer is pretty simple: there are a lot of EmitFoo()
methods, so I'd need to modify all of them and this looked pretty
fragile and inconvenient. Also, I thought it's pretty important to
have everything in AST (implicit conversions to float) instead of
codegen phase magic.

Thanks!
-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University
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