[PATCH] D26420: Encode duplication factor from loop vectorization and loop unrolling to discriminator.

[Hal Finkel via llvm-commits]

hfinkel added inline comments.


================
Comment at: lib/Transforms/Vectorize/LoopVectorize.cpp:836
+    if (DIL && EncodeDuplicationInDiscriminators)
+      B.SetCurrentDebugLocation(DIL->cloneWithDuplicationFactor(UF * VF));
+    else
----------------
I apologize for rehashing this, but now I'm confused again. We have two situations:

 1. Loop is unrolled (or code is otherwise duplicated). In this case, the discriminator value must be different (so that the relevant counts are summed)
 2. Instruction is vectorized. In this case we need a discriminator value with a duplication factor (so that the counts are multiplied by the duplication factor because each vector instruction represents DF scalar instructions).

And both situations can obviously be combined. It seems like, in general, we're trying to take a shortcut here: instead of giving each instruction from an unrolled loop a different discriminator (so that all of the relevant counts will be summed), we're giving them all the same discriminator with a duplication factor. This will work in some cases, but not if some of the loop iterations (or instructions therein) are executed conditionally. I don't see why this is worthwhile. Can you please explain?


https://reviews.llvm.org/D26420