[InstCombine] add explanatory comment for strange vector logic; NFC
llvm-svn: 344852
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index bdd8fe3..bcf2a25 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -900,6 +900,22 @@
// 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);