[PATCH] D43863: [ELF] - Do not remove empty sections that use symbols in expressions.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, espindola, jhenderson.
Herald added subscribers: arichardson, emaste.

This is PR36515.

Currenly if we have script like `.debug_info 0 : { *(.debug_info) }`,
we would not remove this section and keep it in output.
That does not work, because it is common case for
debug sections to have zero address expression.
Patch changes behavior so that we remove only sections
that do not use symbols in its expressions.


https://reviews.llvm.org/D43863

Files:
  ELF/OutputSections.cpp
  ELF/OutputSections.h
  ELF/ScriptParser.cpp
  test/ELF/linkerscript/empty-sections-expressions.s


Index: test/ELF/linkerscript/empty-sections-expressions.s
===================================================================
--- test/ELF/linkerscript/empty-sections-expressions.s
+++ test/ELF/linkerscript/empty-sections-expressions.s
@@ -0,0 +1,19 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+
+## We remove empty sections that do not reference symbols in address,
+## LMA, align and subalign expressions. Here we check that.
+
+# RUN: echo "SECTIONS { .debug_info 0 : { *(.debug_info) } }" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-objdump -section-headers %t | FileCheck %s
+# CHECK-NOT: debug_info
+
+# RUN: echo "SECTIONS { .debug_info foo : { *(.debug_info) } }" > %t2.script
+# RUN: not ld.lld -o %t --script %t2.script %t.o 2>&1 | FileCheck %s --check-prefix=ERR
+# ERR: symbol not found: foo
+
+.text
+.globl _start
+_start:
+ nop
Index: ELF/ScriptParser.cpp
===================================================================
--- ELF/ScriptParser.cpp
+++ ELF/ScriptParser.cpp
@@ -668,6 +668,8 @@
   OutputSection *Cmd =
       Script->createOutputSection(OutSec, getCurrentLocation());
 
+  size_t SymbolsReferenced = Script->ReferencedSymbols.size();
+
   if (peek() != ":")
     readSectionAddressType(Cmd);
   expect(":");
@@ -734,6 +736,8 @@
   // Consume optional comma following output section command.
   consume(",");
 
+  if (Script->ReferencedSymbols.size() > SymbolsReferenced)
+    Cmd->ExpressionsUseSymbols = true;
   return Cmd;
 }
 
Index: ELF/OutputSections.h
===================================================================
--- ELF/OutputSections.h
+++ ELF/OutputSections.h
@@ -103,6 +103,7 @@
   std::string LMARegionName;
   bool NonAlloc = false;
   bool Noload = false;
+  bool ExpressionsUseSymbols = false;
 
   template <class ELFT> void finalize();
   template <class ELFT> void writeTo(uint8_t *Buf);
Index: ELF/OutputSections.cpp
===================================================================
--- ELF/OutputSections.cpp
+++ ELF/OutputSections.cpp
@@ -84,11 +84,10 @@
   if (!Phdrs.empty())
     return false;
 
-  // We do not want to remove sections that have custom address or align
-  // expressions set even if them are empty. We keep them because we
-  // want to be sure that any expressions can be evaluated and report
-  // an error otherwise.
-  if (AddrExpr || AlignExpr || LMAExpr)
+  // We do not want to remove sections that reference symbols in address and
+  // other expressions. We add script symbols as undefined, and want to ensure
+  // all of them are defined in output, hence have to keep them.
+  if (ExpressionsUseSymbols)
     return false;
 
   for (BaseCommand *Base : SectionCommands)


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