From 21a08f751bd641f9658455cbf44ecadbf895f5b2 Mon Sep 17 00:00:00 2001
From: Max Graey <maxgraey@gmail.com>
Date: Tue, 25 Aug 2020 00:06:06 +0300
Subject: Avoid wasted work when there are no locals (#3060)

Adds early returns to local optimizations passes in cases where there are no locals to optimize.
---
 src/passes/RedundantSetElimination.cpp | 3 +++
 src/passes/ReorderLocals.cpp           | 9 +++++----
 src/passes/SimplifyLocals.cpp          | 3 +++
 3 files changed, 11 insertions(+), 4 deletions(-)

(limited to 'src')

diff --git a/src/passes/RedundantSetElimination.cpp b/src/passes/RedundantSetElimination.cpp
index a891653df..e36cefc9e 100644
--- a/src/passes/RedundantSetElimination.cpp
+++ b/src/passes/RedundantSetElimination.cpp
@@ -81,6 +81,9 @@ struct RedundantSetElimination
 
   void doWalkFunction(Function* func) {
     numLocals = func->getNumLocals();
+    if (numLocals == 0) {
+      return; // nothing to do
+    }
     // create the CFG by walking the IR
     CFGWalker<RedundantSetElimination, Visitor<RedundantSetElimination>, Info>::
       doWalkFunction(func);
diff --git a/src/passes/ReorderLocals.cpp b/src/passes/ReorderLocals.cpp
index 7b4c99e78..8d7c0f1d9 100644
--- a/src/passes/ReorderLocals.cpp
+++ b/src/passes/ReorderLocals.cpp
@@ -46,6 +46,9 @@ struct ReorderLocals : public WalkerPass<PostWalker<ReorderLocals>> {
   enum { Unseen = 0 };
 
   void doWalkFunction(Function* curr) {
+    if (curr->getNumVars() == 0) {
+      return; // nothing to do. All locals are parameters
+    }
     Index num = curr->getNumLocals();
     counts.resize(num);
     std::fill(counts.begin(), counts.end(), 0);
@@ -54,9 +57,9 @@ struct ReorderLocals : public WalkerPass<PostWalker<ReorderLocals>> {
     // Gather information about local usages.
     walk(curr->body);
     // Use the information about local usages.
-    std::vector<Index> newToOld;
+    std::vector<Index> newToOld(num);
     for (size_t i = 0; i < num; i++) {
-      newToOld.push_back(i);
+      newToOld[i] = i;
     }
     // sort, keeping params in front (where they will not be moved)
     sort(
@@ -82,8 +85,6 @@ struct ReorderLocals : public WalkerPass<PostWalker<ReorderLocals>> {
     size_t numParams = curr->sig.params.size();
     for (size_t i = 0; i < numParams; i++) {
       assert(newToOld[i] < numParams);
-    }
-    for (size_t i = 0; i < numParams; i++) {
       newToOld[i] = i;
     }
     // sort vars, and drop unused ones
diff --git a/src/passes/SimplifyLocals.cpp b/src/passes/SimplifyLocals.cpp
index ca3d714f4..9c831a551 100644
--- a/src/passes/SimplifyLocals.cpp
+++ b/src/passes/SimplifyLocals.cpp
@@ -811,6 +811,9 @@ struct SimplifyLocals
   }
 
   void doWalkFunction(Function* func) {
+    if (func->getNumLocals() == 0) {
+      return; // nothing to do
+    }
     // scan local.gets
     getCounter.analyze(func);
     // multiple passes may be required per function, consider this:
-- 
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