[PATCH] D38724: [ELF] - Do not collect SHT_REL[A] sections unconditionally when --gc-sections and --emit-relocs used together.

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
Herald added a subscriber: emaste.

This is "Bug 34836 - --gc-sections remove relocations from --emit-relocs",

When --emit-relocs is used, LLD currently always drops SHT_REL[A] sections from
output if  --gc-sections is present. Patch fixes the issue.


https://reviews.llvm.org/D38724

Files:
  ELF/InputSection.cpp
  ELF/InputSection.h
  ELF/MarkLive.cpp
  test/ELF/emit-relocs-gc.s


Index: test/ELF/emit-relocs-gc.s
===================================================================
--- test/ELF/emit-relocs-gc.s
+++ test/ELF/emit-relocs-gc.s
@@ -6,13 +6,24 @@
 # RUN: llvm-objdump %t -section-headers | FileCheck %s --check-prefix=NOGC
 # NOGC: .rela.bar
 
-## GC collects .bar section and we exclude .rela.bar from output.
+## GC collects .bar section and we exclude .rela.bar from output. We keep
+## .rela.text because we keep .text.
 # RUN: ld.lld --gc-sections --emit-relocs --print-gc-sections %t.o -o %t \
 # RUN:   | FileCheck --check-prefix=MSG %s
 # MSG: removing unused section from '.bar' in file
 # MSG: removing unused section from '.rela.bar' in file
 # RUN: llvm-objdump %t -section-headers | FileCheck %s --check-prefix=GC
 # GC-NOT:  rela.bar
+# GC:      rela.text
+# GC-NOT:  rela.bar
 
 .section .bar,"a"
 .quad .bar
+
+.text
+relocs:
+.quad _start
+
+.global _start
+_start:
+ nop
Index: ELF/MarkLive.cpp
===================================================================
--- ELF/MarkLive.cpp
+++ ELF/MarkLive.cpp
@@ -164,16 +164,17 @@
 
 // We do not garbage-collect two types of sections:
 // 1) Sections used by the loader (.init, .fini, .ctors, .dtors or .jcr)
-// 2) Non-allocatable sections which typically contain debugging information
+// 2) Non-allocatable sections which typically contain debugging information,
+//    except SHT_REL[A] sections.
 template <class ELFT> static bool isReserved(InputSectionBase *Sec) {
   switch (Sec->Type) {
   case SHT_FINI_ARRAY:
   case SHT_INIT_ARRAY:
   case SHT_NOTE:
   case SHT_PREINIT_ARRAY:
     return true;
   default:
-    if (!(Sec->Flags & SHF_ALLOC))
+    if (isLiveByDefault(Sec->Flags, Sec->Type))
       return true;
 
     StringRef S = Sec->Name;
@@ -198,8 +199,8 @@
     if (Sec == &InputSection::Discarded)
       return;
 
-    // We don't gc non alloc sections.
-    if (!(Sec->Flags & SHF_ALLOC))
+    // We don't gc sections that should be live by default.
+    if (isLiveByDefault(Sec->Flags, Sec->Type))
       return;
 
     // Usually, a whole section is marked as live or dead, but in mergeable
Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -359,6 +359,10 @@
 // Builds section order for handling --symbol-ordering-file.
 llvm::DenseMap<SectionBase *, int> buildSectionOrder();
 
+// Returns true if section with given Flags and Type should be
+// live for GC by default.
+bool isLiveByDefault(uint64_t Flags, uint32_t Type);
+
 } // namespace elf
 
 std::string toString(const elf::InputSectionBase *);
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -78,7 +78,7 @@
 
 // Return true if a section with given section flags is live (will never be
 // GCed) by default. If a section can be GCed, this function returns false.
-static bool isLiveByDefault(uint64_t Flags, uint32_t Type) {
+bool elf::isLiveByDefault(uint64_t Flags, uint32_t Type) {
   // If GC is enabled, all memory-mapped sections are subject of GC.
   if (!Config->GcSections)
     return true;


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