[llvm] r344852 - [InstCombine] add explanatory comment for strange vector logic; NFC

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Sat Oct 20 09:25:55 2018
New Revision: 344852

URL: http://llvm.org/viewvc/llvm-project?rev=344852&view=rev
Log:
[InstCombine] add explanatory comment for strange vector logic; NFC

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp?rev=344852&r1=344851&r2=344852&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp Sat Oct 20 09:25:55 2018
@@ -900,6 +900,22 @@ Instruction *InstCombiner::visitInsertEl
 
       // If this insertelement isn't used by some other insertelement, turn it
       // (and any insertelements it points to), into one big shuffle.
+
+      // TODO: Looking at the user(s) to determine if this insert is a
+      // fold-to-shuffle opportunity does not match the usual instcombine
+      // constraints. We should decide if the transform is worthy based only
+      // on this instruction and its operands, but that may not work currently.
+      //
+      // Here, we are trying to avoid creating shuffles before reaching
+      // the end of a chain of extract-insert pairs. This is complicated because
+      // we do not generally form arbitrary shuffle masks in instcombine
+      // (because those may codegen poorly), but collectShuffleElements() does
+      // exactly that.
+      //
+      // The rules for determining what is an acceptable target-independent
+      // shuffle mask are fuzzy because they evolve based on the backend's
+      // capabilities and real-world impact.
+
       if (!IE.hasOneUse() || !isa<InsertElementInst>(IE.user_back())) {
         SmallVector<Constant*, 16> Mask;
         ShuffleOps LR = collectShuffleElements(&IE, Mask, nullptr, *this);