From 2f997310eecbb945e457ba28766b56869beb4e00 Mon Sep 17 00:00:00 2001
From: Shaunak Kishore <kshaunak@fb.com>
Date: Wed, 24 Jun 2020 19:20:06 -0700
Subject: [PATCH] Shrink block bytecode vectors

Summary:
Shrink BytecodeVec at strategic points: a) when initially creating a Func and b) after each call to optimize. Note that we may lose out on a different memory optimization here: sharing bytecode between Funcs. I believe that because of the first shrink_to_fit, most of the time we'll only do the second when we have mutated (and COWed) the bytecode.

We also pre-size a couple of other vectors - the initializer_list constructor gets it right.

Reviewed By: ricklavoie

Differential Revision: D22214700

fbshipit-source-id: cc73913fc33a1db77072d0934003094887e89326
---
 hphp/hhbbc/optimize.cpp    | 9 +++++++++
 hphp/hhbbc/parse.cpp       | 5 +++--
 hphp/util/compact-vector.h | 4 ++--
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/hphp/hhbbc/optimize.cpp b/hphp/hhbbc/optimize.cpp
index 8ae86ea3528..c42dce332c8 100644
--- a/hphp/hhbbc/optimize.cpp
+++ b/hphp/hhbbc/optimize.cpp
@@ -887,6 +887,15 @@ void do_optimize(const Index& index, FuncAnalysis&& ainfo, bool isFinal) {
                  insert_assertions);
   }
 
+  // NOTE: We shouldn't duplicate blocks that are shared between two Funcs
+  // in this loop. We shrink BytecodeVec at the time we parse the function,
+  // so we only shrink when we've already mutated (and COWed) the bytecode.
+  for (auto& block : func->blocks) {
+    assertx(block->hhbcs.size());
+    if (block->hhbcs.capacity() == block->hhbcs.size()) continue;
+    block.mutate()->hhbcs.shrink_to_fit();
+  }
+
   for (auto& p : func->params) fixTypeConstraint(index, p.typeConstraint);
 
   if (RuntimeOption::EvalCheckReturnTypeHints >= 3) {
diff --git a/hphp/hhbbc/parse.cpp b/hphp/hhbbc/parse.cpp
index 2dae92c9b2d..cd0a99744a4 100644
--- a/hphp/hhbbc/parse.cpp
+++ b/hphp/hhbbc/parse.cpp
@@ -282,6 +282,7 @@ void populate_block(ParseUnitState& puState,
   auto decode_stringvec = [&] {
     auto const vecLen = decode_iva(pc);
     CompactVector<LSString> keys;
+    keys.reserve(vecLen);
     for (auto i = size_t{0}; i < vecLen; ++i) {
       keys.push_back(ue.lookupLitstr(decode<int32_t>(pc)));
     }
@@ -653,6 +654,7 @@ void build_cfg(ParseUnitState& puState,
     auto const id = kv.second.first;
     blk->multiSucc = predSuccCounts[id].second > 1;
     blk->multiPred = predSuccCounts[id].first > 1;
+    blk->hhbcs.shrink_to_fit();
     func.blocks[id] = std::move(kv.second.second);
   }
 }
@@ -768,8 +770,7 @@ std::unique_ptr<php::Func> parse_func(ParseUnitState& puState,
       auto blk         = php::Block{};
       blk.exnNodeId    = NoExnNodeId;
 
-      blk.hhbcs.push_back(gen_constant(it->second));
-      blk.hhbcs.push_back(bc::RetC {});
+      blk.hhbcs = {gen_constant(it->second), bc::RetC {}};
       ret->blocks.emplace_back(std::move(blk));
 
       ret->dvEntries.resize(fe.params.size(), NoBlockId);
diff --git a/hphp/util/compact-vector.h b/hphp/util/compact-vector.h
index 4fcdcd81d5d..65b10a070b6 100644
--- a/hphp/util/compact-vector.h
+++ b/hphp/util/compact-vector.h
@@ -79,7 +79,7 @@ struct CompactVector : private Alloc::template rebind<char>::other {
 
   bool empty() const;
   size_type size() const;
-  size_type capacity();
+  size_type capacity() const;
   void clear();
   void push_back(const T& val);
   void push_back(T&& val);
@@ -252,7 +252,7 @@ typename CompactVector<T, A>::size_type CompactVector<T, A>::size() const {
 }
 
 template <typename T, typename A>
-typename CompactVector<T, A>::size_type CompactVector<T, A>::capacity() {
+typename CompactVector<T, A>::size_type CompactVector<T, A>::capacity() const {
   return m_data ? m_data->m_capacity : 0;
 }
 
-- 
2.11.4.GIT