From 4b075bb21879f25b24391e669eb4e910262dfff8 Mon Sep 17 00:00:00 2001
From: Rafael Espindola <rafael.espindola@gmail.com>
Date: Wed, 26 Jul 2017 23:39:10 +0000
Subject: [PATCH] Make __start_sec __end_sec handling more precise.

With this we only ask LTO to keep a C named section if there is a
__start_ or __end symbol.

This is not as strict as lld's --gc-sections, but is as good as we can
get without having a far more detailed ir summary.

llvm-svn: 309232
---
 lld/ELF/LTO.cpp                  | 12 ++++++++++--
 lld/ELF/LTO.h                    |  2 ++
 lld/test/ELF/lto/section-name.ll | 18 +++++++++++++++++-
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
index 66f2a2a..bf2dadc 100644
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -11,6 +11,7 @@
 #include "Config.h"
 #include "Error.h"
 #include "InputFiles.h"
+#include "SymbolTable.h"
 #include "Symbols.h"
 #include "lld/Core/TargetOptionsCommandFlags.h"
 #include "llvm/ADT/STLExtras.h"
@@ -107,7 +108,14 @@ static std::unique_ptr<lto::LTO> createLTO() {
                                      Config->LTOPartitions);
 }
 
-BitcodeCompiler::BitcodeCompiler() : LTOObj(createLTO()) {}
+BitcodeCompiler::BitcodeCompiler() : LTOObj(createLTO()) {
+  for (Symbol *Sym : Symtab->getSymbols()) {
+    StringRef Name = Sym->body()->getName();
+    for (StringRef Prefix : {"__start_", "__stop_"})
+      if (Name.startswith(Prefix))
+        UsedStartStop.insert(Name.substr(Prefix.size()));
+  }
+}
 
 BitcodeCompiler::~BitcodeCompiler() = default;
 
@@ -138,7 +146,7 @@ void BitcodeCompiler::add(BitcodeFile &F) {
 
     R.VisibleToRegularObj = Sym->IsUsedInRegularObj ||
                             (R.Prevailing && Sym->includeInDynsym()) ||
-                            isValidCIdentifier(ObjSym.getSectionName());
+                            UsedStartStop.count(ObjSym.getSectionName());
     if (R.Prevailing)
       undefine(Sym);
     R.LinkerRedefined = Config->RenamedSymbols.count(Sym);
diff --git a/lld/ELF/LTO.h b/lld/ELF/LTO.h
index d19923c..79dd232 100644
--- a/lld/ELF/LTO.h
+++ b/lld/ELF/LTO.h
@@ -22,6 +22,7 @@
 #define LLD_ELF_LTO_H
 
 #include "lld/Core/LLVM.h"
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallString.h"
 #include <memory>
 #include <vector>
@@ -50,6 +51,7 @@ private:
   std::unique_ptr<llvm::lto::LTO> LTOObj;
   std::vector<SmallString<0>> Buff;
   std::vector<std::unique_ptr<MemoryBuffer>> Files;
+  llvm::DenseSet<StringRef> UsedStartStop;
 };
 } // namespace elf
 } // namespace lld
diff --git a/lld/test/ELF/lto/section-name.ll b/lld/test/ELF/lto/section-name.ll
index 1e62769..cc5f914 100644
--- a/lld/test/ELF/lto/section-name.ll
+++ b/lld/test/ELF/lto/section-name.ll
@@ -7,5 +7,21 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @foo = hidden global i32 42, section "foo_section"
+@bar = hidden global i32 42, section "bar_section"
+@zed = hidden global i32 42, section "zed_section"
 
-; CHECK: foo_section PROGBITS
+@__start_foo_section = external global i32
+@__stop_bar_section = external global i32
+
+define i32* @use1() {
+  ret i32* @__start_foo_section
+}
+
+define i32* @use2() {
+  ret i32* @__stop_bar_section
+}
+
+; CHECK-NOT: zed_section
+; CHECK:     foo_section PROGBITS
+; CHECK-NEXT:     bar_section PROGBITS
+; CHECK-NOT: zed_section
-- 
2.7.4