fixup! fixup! [SandboxVec] Move seed collection into its own separate pass

Add comments to BottomUpVec and SeedColleciton classes.

Author: Vasileios Porpodas

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/Passes/BottomUpVec.h

@@ -22,6 +22,16 @@
 
 namespace llvm::sandboxir {
 
+/// This is a simple bottom-up vectorizer Region pass.
+/// It expects a "seed slice" as an input in the Region's Aux vector.
+/// The "seed slice" is a vector of instructions that can be used as a starting
+/// point for vectorization, like stores to consecutive memory addresses.
+/// Starting from the seed instructions, it walks up the def-use chain looking
+/// for more instructions that can be vectorized. This pass will generate vector
+/// code if it can legally vectorize the code, regardless of whether it is
+/// profitable or not. For now profitability is checked at the end of the region
+/// pass pipeline by a dedicated pass that accepts or rejects the IR
+/// transaction, depending on the cost.
 class BottomUpVec final : public RegionPass {
   bool Change = false;
   std::unique_ptr<LegalityAnalysis> Legality;

llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/Passes/SeedCollection.h

@@ -17,6 +17,10 @@
 
 namespace llvm::sandboxir {
 
+/// This pass collects the instructions that can become vectorization "seeds",
+/// like stores to consecutive memory addresses. It then goes over the collected
+/// seeds, slicing them into appropriately sized chunks, creating a Region with
+/// the seed slice as the Auxiliary vector and runs the region pass pipeline.
 class SeedCollection final : public FunctionPass {
 
   /// The PM containing the pipeline of region passes.