[Bug tree-optimization/58359] __builtin_unreachable prevents vectorization

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58359

--- Comment #11 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #10)
> That is still wrong, __builtin_unreachable is still very much useful even at
> the RTL level (where we expand it as basic blocks without successors).
> Perhaps if-conversion (for LOOP_VERSIONED loop only) can just drop the
> __builtin_unreachable () from the to be vectorized loop?

It isn't just vectorization, or even just loop optimizations that are hindered
by spurious control flow. Most of the __builtin_unreachable I see are of the
form:

if(x<0)__builtin_unreachable();

(often hidden below a macro called ASSUME or a similar name)

For those, it is tempting to say that replacing the GIMPLE_COND, leading to a
block that contains only a call to __builtin_unreachable, with an ASSERT_EXPR,
or range information on this SSA_NAME (now that we store it), or anything
without control flow, wouldn't lose any information. In reality, it would still
lose it sometimes, but I don't know how often/bad that is.

Just thinking of lowering *some* of the calls earlier... (though it wouldn't
help with comment #2, where tree-tailcall.c should probably be taught about
__builtin_unreachable)