[llvm] [LoopVectorize] Improve Vectorization of Low Trip Count Loops (PR #195823)

[Florian Hahn via llvm-commits]

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@@ -3066,6 +3071,36 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
       }
     }
 
+    // Allow cases where the ExactTC == (VF * IC) or ExactTC == (VF * IC) + 1.
+    //
+    // This produces at most 1 vector iteration, and at most 1 scalar iteration
+    // with no remainder. Later passes will eliminate the loop and leave
+    // straight-line code as the both iteration counts are statically known.
+    //
+    // If a function is marked as minsize/optsize or OptForSize is set, do not
+    // allow this form of transformation as this will increase CodeSize.
+    //
+    // For loops with small bodies, the cost model is not reliable enough to
+    // accurately determine if vectorization is beneficial.
+    // LowTripCountLoopBodySizeLimit means only loops with larger bodies will
+    // be considered.
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fhahn wrote:

I think this hsould be clear from the help text of the variable.
```suggestion
```

https://github.com/llvm/llvm-project/pull/195823