[cfe-dev] How to create a new builtin in clang?
andyg1001 at hotmail.co.uk
Fri May 18 09:12:42 PDT 2012
I'd like to learn a little about how clang works on the inside, and would
like to have a go at implementing a new "builtin" function which forces an
expression to be folded to a constant. The idea would be that it would
either compile-time-evaluate the expression or halt compilation with an
error. I believe there isn't anything currently that does this for
arbitrary types, only integers.
What I envisage is something like...
auto x = __builtin_fold(<expression>)
which returns a constant value of appropriate type for the expression.
I'd envisage it as a replacement for an already-possible two-step approach
static constexpr auto value = <expression>;
auto x = __fold_impl_1::value;
The obvious down-side of the two-step approach is requiring a special macro
for generating __fold_impl_xxx for each folded expression, plus the
inconvenience and possible scope problems of having to use such a macro!
I'd also like it to work in non-C++11 code.
I could imagine that it should be very possible to get clang to generate
such a struct on-the-fly which avoided the scope problems and substitute the
expression for a reference to the value.
The questions I have are:
- how does one go about getting clang to build such substitutiary code in
- is this the best approach, or is there a better/simpler way?
I've been looking into how __builtin_constant_p and __extension__ work, as
well as the Expr::Evaluate* functions and
CodeGenFunction::tryEmitAsConstant, but to little gain I'm afraid! ;o)
Many thanks for any and all suggestions
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