From 2eccde4a2beb85eb10faf5184987bab180567e88 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Mon, 9 Nov 2020 09:55:09 -0800
Subject: [PATCH] [ELF] Make InputSection smaller

On LP64/Windows platforms, this decreases sizeof(InputSection) from 208 (larger
on Windows) to 184.

For a large executable (7.6GiB, inputSections.size()=5105122,
make<InputSection> called 4835760 times), this decreases cgroup
memory.max_usage_in_bytes by 0.6%

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D91018
---
 lld/ELF/InputSection.h | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/lld/ELF/InputSection.h b/lld/ELF/InputSection.h
index 00024a6f05d7e..f0cc592d13285 100644
--- a/lld/ELF/InputSection.h
+++ b/lld/ELF/InputSection.h
@@ -52,15 +52,15 @@ class SectionBase {
   // this but instead this->Repl.
   SectionBase *repl;
 
-  unsigned sectionKind : 3;
+  uint8_t sectionKind : 3;
 
   // The next two bit fields are only used by InputSectionBase, but we
   // put them here so the struct packs better.
 
-  unsigned bss : 1;
+  uint8_t bss : 1;
 
   // Set for sections that should not be folded by ICF.
-  unsigned keepUnique : 1;
+  uint8_t keepUnique : 1;
 
   // The 1-indexed partition that this section is assigned to by the garbage
   // collector, or 0 if this section is dead. Normally there is only one
@@ -134,6 +134,10 @@ class InputSectionBase : public SectionBase {
   // and shrinking a section.
   unsigned bytesDropped = 0;
 
+  // Whether the section needs to be padded with a NOP filler due to
+  // deleteFallThruJmpInsn.
+  bool nopFiller = false;
+
   void drop_back(uint64_t num) { bytesDropped += num; }
 
   void push_back(uint64_t num) {
@@ -210,17 +214,13 @@ class InputSectionBase : public SectionBase {
   // The native ELF reloc data type is not very convenient to handle.
   // So we convert ELF reloc records to our own records in Relocations.cpp.
   // This vector contains such "cooked" relocations.
-  std::vector<Relocation> relocations;
-
-  // Indicates that this section needs to be padded with a NOP filler if set to
-  // true.
-  bool nopFiller = false;
+  SmallVector<Relocation, 0> relocations;
 
   // These are modifiers to jump instructions that are necessary when basic
   // block sections are enabled.  Basic block sections creates opportunities to
   // relax jump instructions at basic block boundaries after reordering the
   // basic blocks.
-  std::vector<JumpInstrMod> jumpInstrMods;
+  SmallVector<JumpInstrMod, 0> jumpInstrMods;
 
   // A function compiled with -fsplit-stack calling a function
   // compiled without -fsplit-stack needs its prologue adjusted. Find
@@ -390,6 +390,8 @@ class InputSection : public InputSectionBase {
   template <class ELFT> void copyShtGroup(uint8_t *buf);
 };
 
+static_assert(sizeof(InputSection) <= 184, "InputSection is too big");
+
 inline bool isDebugSection(const InputSectionBase &sec) {
   return sec.name.startswith(".debug") || sec.name.startswith(".zdebug");
 }