[PATCH] D148120: [InstCombine] Remove scalable get_active_lane_mask calls which are always false

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

Do I understand correctly that this is basically optimizing get.active.lane.mask for the case where we can compute the range of op0 and show that it is always >= op1? And it implements that range calculation for this special case of a post-inc IV?

I don't think this is quite right in that it does not account for addition overflow. That's not actually possible in your specific test cases, but I don't think your implementation has sufficient preconditions to prove this.

Why does the loop vectorizer generate this code in the first place? Given that it involves reasoning about IVs, it might be more straightforward to handle this in SCEV/LV.



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Comment at: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp:3087
+    auto PFalse = Constant::getNullValue(II->getType());
+    PFalse->takeName(II);
+    return replaceInstUsesWith(*II, PFalse);
----------------
takeName on a constant doesn't make sense.


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https://reviews.llvm.org/D148120