[PATCH] D54892: [LAA] Avoid generating RT checks for known deps preventing vectorization.

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: lib/Analysis/LoopAccessAnalysis.cpp:1620
+  // Runtime checks are only feasible if only unknown dependences prevent
+  // vectorization.
   MaxSafeDepDistBytes = -1;
----------------
Right, but comment best be placed elsewhere.


================
Comment at: lib/Analysis/LoopAccessAnalysis.cpp:1661
+            // the runtime checks at compile time.
+            RuntimeChecksFeasible &= Type == Dependence::Unknown || DepSafe;
 
----------------
Suggest to add and call `Dependence::isUnsafeForVectorization(Type)` which will determine if there's a dependence that cannot be overcome by runtime checks. Perhaps find a better name - it implies but is not equivalent to `!isSafeForVectorization(Type)`.

Then bail-out as soon as such a dependence is encountered; there's no point in continuing to `RecordDependences`.

This could alternatively be done here as follows, by leveraging `ShouldRetryWithRuntimeCheck`, which essentially already indicates if runtime checks may be useful (but then early bail-out is necessary):


```
            if (!DepSafe && Type != Dependence::Unknown) {
              ShouldRetryWithRuntimeCheck = false;
              RecordDependences = false;
              Dependences.clear();
              LLVM_DEBUG(dbgs() << "Found unsafe dependence\n");
              return false;
            }
```



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  https://reviews.llvm.org/D54892/new/

https://reviews.llvm.org/D54892