[PATCH] D39082: [IRCE] Smarter detection of empty ranges using SCEV

[Anna Thomas via Phabricator via llvm-commits]

anna added inline comments.


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Comment at: lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp:185
+      else
+        return SE.isKnownPredicate(ICmpInst::ICMP_UGE, Begin, End);
+    }
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SCEV's isKnownPredicate is actually really good in most cases to identify the empty range (BEGIN >= END). 
However, there's a stronger version of isKnownPredicate in `DependenceInfo::isKnownPredicate`. 

Specifically, it first checks SCEV's version of isKnownPredicate and if it fails, it does a further analysis using the Delta between Begin and End and checking the Delta based on the predicate passed in. 

I'm wondering if SCEV's form maybe weaker and hence we may fire the assert added about empty ranges at line 1756?


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Comment at: test/Transforms/IRCE/empty_ranges.ll:9
+; The intersection with safe iteration space is the empty range [60, 10).
+; It is better to eliminate one range check than attempt to eliminate both given
+; that we will never go to the main loop in the latter case and basically
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So, in this case, from now on, only the first range check is removed, i.e. we will not remove range checks where the safe iteration space intersected with the "true" iteration space is empty. By true iteration space, I mean, the set where the guard is always true.


https://reviews.llvm.org/D39082