[PATCH] D123386: [AArch64] Add lane moves to PerfectShuffle tables

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

Cheers



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Comment at: llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:9755
+      while (Elt > 0) {
+        ID /= 9;
+        Elt--;
----------------
SjoerdMeijer wrote:
> Don't know what magic number 9 is, or generally what this is doing. But yeah, looking something up in a table, so am guessing something related to the number of items per table entry. There seems to be enough prior art of magic constants, so looks fine. 
Yeah - 9 is `2 * 4 + 1`. i.e each lane of a 2 input v4 shuffle, plus one for undef. It's just how the tables are encoded.


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  https://reviews.llvm.org/D123386/new/

https://reviews.llvm.org/D123386