From 06f3b094e4ad2624d8376e4312eebbc5214faf94 Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <peter@pcc.me.uk>
Date: Mon, 3 Jun 2019 20:14:25 +0000
Subject: [PATCH] ELF: Introduce a separate bit for tracking whether an output
 section has ever had an input section added to it. NFCI.

We currently (ab)use the Live bit on output sections to track whether
the section has ever had an input section added to it, and then later
use it during orphan placement. This will conflict with one of my upcoming
partition-related changes that will assign all output sections to a partition
(thus marking them as live) so that they can be added to the correct segment
by the code that creates program headers.

Instead of using the Live bit for this purpose, create a new flag and
start using it to track the property explicitly.

Differential Revision: https://reviews.llvm.org/D62348

llvm-svn: 362444
---
 lld/ELF/LinkerScript.cpp   |  7 +++----
 lld/ELF/OutputSections.cpp |  3 ++-
 lld/ELF/OutputSections.h   |  5 +++++
 lld/ELF/Writer.cpp         | 13 +++++++------
 4 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 71c8ff2..24c4fb8 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -889,10 +889,9 @@ void LinkerScript::adjustSectionsBeforeSorting() {
       Sec->Alignment =
           std::max<uint32_t>(Sec->Alignment, Sec->AlignExpr().getValue());
 
-    // A live output section means that some input section was added to it. It
-    // might have been removed (if it was empty synthetic section), but we at
-    // least know the flags.
-    if (Sec->isLive())
+    // The input section might have been removed (if it was an empty synthetic
+    // section), but we at least know the flags.
+    if (Sec->HasInputSections)
       Flags = Sec->Flags;
 
     // We do not want to keep any special flags for output section
diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index 8927b69..8048609 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -84,9 +84,10 @@ static bool canMergeToProgbits(unsigned Type) {
 }
 
 void OutputSection::addSection(InputSection *IS) {
-  if (!isLive()) {
+  if (!HasInputSections) {
     // If IS is the first section to be added to this section,
     // initialize Partition, Type, Entsize and flags from IS.
+    HasInputSections = true;
     Partition = IS->Partition;
     Type = IS->Type;
     Entsize = IS->Entsize;
diff --git a/lld/ELF/OutputSections.h b/lld/ELF/OutputSections.h
index c072f2c..dded729 100644
--- a/lld/ELF/OutputSections.h
+++ b/lld/ELF/OutputSections.h
@@ -93,6 +93,11 @@ public:
   bool UsedInExpression = false;
   bool InOverlay = false;
 
+  // Tracks whether the section has ever had an input section added to it, even
+  // if the section was later removed (e.g. because it is a synthetic section
+  // that wasn't needed). This is needed for orphan placement.
+  bool HasInputSections = false;
+
   void finalize();
   template <class ELFT> void writeTo(uint8_t *Buf);
   template <class ELFT> void maybeCompress();
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index ec5be8c..40d32a8 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -1052,7 +1052,7 @@ static int getRankProximityAux(OutputSection *A, OutputSection *B) {
 
 static int getRankProximity(OutputSection *A, BaseCommand *B) {
   auto *Sec = dyn_cast<OutputSection>(B);
-  return (Sec && Sec->isLive()) ? getRankProximityAux(A, Sec) : -1;
+  return (Sec && Sec->HasInputSections) ? getRankProximityAux(A, Sec) : -1;
 }
 
 // When placing orphan sections, we want to place them after symbol assignments
@@ -1094,19 +1094,20 @@ findOrphanPos(std::vector<BaseCommand *>::iterator B,
   int Proximity = getRankProximity(Sec, *I);
   for (; I != E; ++I) {
     auto *CurSec = dyn_cast<OutputSection>(*I);
-    if (!CurSec || !CurSec->isLive())
+    if (!CurSec || !CurSec->HasInputSections)
       continue;
     if (getRankProximity(Sec, CurSec) != Proximity ||
         Sec->SortRank < CurSec->SortRank)
       break;
   }
 
-  auto IsLiveOutputSec = [](BaseCommand *Cmd) {
+  auto IsOutputSecWithInputSections = [](BaseCommand *Cmd) {
     auto *OS = dyn_cast<OutputSection>(Cmd);
-    return OS && OS->isLive();
+    return OS && OS->HasInputSections;
   };
   auto J = std::find_if(llvm::make_reverse_iterator(I),
-                        llvm::make_reverse_iterator(B), IsLiveOutputSec);
+                        llvm::make_reverse_iterator(B),
+                        IsOutputSecWithInputSections);
   I = J.base();
 
   // As a special case, if the orphan section is the last section, put
@@ -1114,7 +1115,7 @@ findOrphanPos(std::vector<BaseCommand *>::iterator B,
   // This matches bfd's behavior and is convenient when the linker script fully
   // specifies the start of the file, but doesn't care about the end (the non
   // alloc sections for example).
-  auto NextSec = std::find_if(I, E, IsLiveOutputSec);
+  auto NextSec = std::find_if(I, E, IsOutputSecWithInputSections);
   if (NextSec == E)
     return E;
 
-- 
2.7.4