[PATCH] D69498: IR: Invert convergent attribute handling

[Matt Arsenault via Phabricator via cfe-commits]

arsenm added a comment.

In D69498#2707100 <https://reviews.llvm.org/D69498#2707100>, @sameerds wrote:

>>> 1. The meaning of the `convergent` attribute has always been target-dependent.
>>> 2. Dependence on TTI is not a real cost at all. We may eventually update every use of isConvergent() to depend on a check for divergence. The check for TTI is only the first step towards that.
>>
>> The core IR semantics must *not* depend on target interpretation. convergent has never been target dependent, and was defined in an abstract way. Only certain targets will really care, but we cannot directly define it to mean what we want it to mean for particular targets
>
> My bad. I should have said "the implementation of convergent" is target dependent. But even that is not precise enough. It is more correct to say that the convergent attribute can be implemented inside LLVM by depending on TTI so that it only impacts targets that have divergence. This dependence is not new; it's merely missing because the current uses of convergent pessimistically assume divergent control flow on targets.

You're still saying the same thing. This needs to be defined generically. Frontends don't *have* to to anything, they'll just get the assumed convergent behavior by default. Either frontends could always add noconvergent to avoid any possible optimization hit, or we could have a pass add noconvergent depending on the target. I don't want to change the interpretation of core attributes based on the target


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