[PATCH] D38113: OpenCL: Assume functions are convergent

[Justin Lebar via Phabricator via cfe-commits]

jlebar added a comment.

> The problem of adding this attribute conservatively for all functions is that it prevents some optimizations to happen.

function-attrs removes the convergent attribute from anything it can prove does not call a convergent function.

I agree this is a nonoptimal solution.  A better way would be to assume that any cuda/opencl function is convergent and then figure out what isn't.  This would let you generate correct cuda/opencl code in a front-end without worrying about this attribute.

One problem with this approach is, suppose you call an external function, whose body llvm cannot see.  We need some way to mark this function as not-convergent, so that its callers can also be inferred to be not convergent.  LLVM currently only has a "convergent" attribute.  In the absence of a new "not-convergent" attribute, the only way we can tell LLVM that this external function is not convergent is to leave off the attribute.  But then this means we assume all functions without the convergent attribute are not convergent, and thus we have to add the attribute everywhere, as this patch does.

OTOH if we added a not-convergent attribute, we'd have to have rules about what happens if both attributes are on a function, and everywhere that checked whether a function was convergent would become significantly more complicated.  I'm not sure that's worthwhile.


https://reviews.llvm.org/D38113