[PATCH] D101109: [SLP]Improve multinode analysis.

[Vasileios Porpodas via Phabricator via llvm-commits]

vporpo added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:1459
+            if (NumOpsWithSameOpcodeParent == 0) {
+              NumOpsWithSameOpcodeParent = 1;
+              OpcodeI = I;
----------------
ABataev wrote:
> vporpo wrote:
> > ABataev wrote:
> > > vporpo wrote:
> > > > Why is `NumOpsWithSameOpcodeParent` set to 1 the first time a mismatch is found? Shouldn't it be set to 0 ?
> > > It is a kind of increasing the counter for the first element in the sequence.
> > Yes, it is increasing it, but shouldn't it be decreasing it instead (or letting it remain 0) ? This code block executes when there is a mismatch of opcode or parent (or if it is the first iteration), so shouldn't we be decreasing the value of`NumOpsWithSameOpcodeParent` (like in line 1463)?
> > 
> > What confuses me here is that `NumOpsWithSameOpcodeParent` looks like a normal counter that counts the opcode/parent matches. So I would expect it to increase by one if  the opcode/parents match (like what line 1466 does), and to decrease by one if there is a mismatch. But it seems to be more complicated than that: When it reaches 0 it foced to 1 even when there is an opcode mismatch. I find this a bit counter intuitive.
> > 
> > For example if we have mismatching opcodes in sequence, I would expect it to keep decreasing, or at least be capped to 0. But it seems like the value of  `NumOpsWithSameOpcodeParent` will be 0, then 1, then 0, then 1 like so:
> > ```
> > before the loop: 0
> > iteration 1:  1 (because it was == 0)
> > iteration 2:  0 (because of opcode mismatch)
> > iteration 3:  1 (because it was == 0)
> > ```
> This is how the voting algorithm works. Here is described the main idea https://www.geeksforgeeks.org/boyer-moore-majority-voting-algorithm/
OK that makes sense now, thanks for clarifying! Could you please add a comment saying that this loop is a Boyer-Moore majority voting for finding the majority opcode and the number of times it occurs?


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https://reviews.llvm.org/D101109