From 402d0d9c83df14c2c11a6fcd15a494ad6cd6c645 Mon Sep 17 00:00:00 2001
From: Tyler McMullen <tyler@fastly.com>
Date: Thu, 3 Jan 2019 11:56:36 -0800
Subject: [PATCH] Add comments to branch folding and reordering functions in
 simple_preopt.

---
 cranelift/codegen/src/simple_preopt.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/cranelift/codegen/src/simple_preopt.rs b/cranelift/codegen/src/simple_preopt.rs
index bee1d6e209..ece05e474b 100644
--- a/cranelift/codegen/src/simple_preopt.rs
+++ b/cranelift/codegen/src/simple_preopt.rs
@@ -555,6 +555,11 @@ enum BranchOptKind {
     NotEqualZero,
 }
 
+/// Fold comparisons into branch operations when possible.
+///
+/// This matches against operations which compare against zero, then use the
+/// result in a `brz` or `brnz` branch. It folds those two operations into a
+/// single `brz` or `brnz`.
 fn branch_opt(pos: &mut FuncCursor, inst: Inst) {
     let info = match pos.func.dfg[inst] {
         InstructionData::Branch {
@@ -648,6 +653,11 @@ enum BranchOrderKind {
     InvertIcmpCond(IntCC, Value, Value),
 }
 
+/// Reorder branches to encourage fallthroughs.
+///
+/// When an ebb ends with a conditional branch followed by an unconditional
+/// branch, this will reorder them if one of them is branching to the next Ebb
+/// layout-wise. The unconditional jump can then become a fallthrough.
 fn branch_order(pos: &mut FuncCursor, cfg: &mut ControlFlowGraph, ebb: Ebb, inst: Inst) {
     let info = match pos.func.dfg[inst] {
         InstructionData::Jump {