[LLVMdev] Emit code for 'unreachable'

[Yaron Keren]

Created patch in Phabricator with Vadim patch from the other thread.

  http://reviews.llvm.org/D3417



2014-04-17 11:26 GMT+03:00 David Chisnall <David.Chisnall at cl.cam.ac.uk>:

> On 15 Apr 2014, at 22:22, Reid Kleckner <rnk at google.com> wrote:
>
> > Personally I'm in favor, but others may disagree.  Right now clang
> inserts calls to llvm.trap(), and that's how we get ud2's when falling off
> the end of a function with a return type.  That may be the preferred
> approach for __builtin_unreachable(), since other clients of LLVM at -O0
> may actually want it to produce code quickly without inserting any
> debugging aids.
>
> It would be trivial to write a pre-codegen pass that would turn all
> unreachable instructions into calls to llvm.trap and then unreachable.  If
> a front end wants to guarantee a trap, then it is also free to insert the
> trap (and it will be deleted later if the basic block becomes truly
> unreachable).
>
> I'd be somewhat hesitant about making it the default behaviour, because
> machine instruction passes may want to do some reordering of instructions
> and once they've got an instruction that is guaranteed to trap then it
> enforces hard constraints on flow control.  To preserve this, you'd need to
> keep it as a pseudo right up until the very last stage of compilation,
> which adds some complexity to back ends.
>
> David
>
>
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