[ELF] -z combreloc: sort dynamic relocations by (!is_relative,symbol_…

…index,r_offset)

We currently sort dynamic relocations by (!is_relative,symbol_index).
Add r_offset as the third key. This makes `readelf -r` debugging easier
(relocations to the same symbol are ordered by r_offset).

Refactor the test combreloc.s (renamed from combrelocs.s) to check
R_X86_64_RELATIVE, and delete --expand-relocs.

The difference from the reverted D61477 is that we keep !is_relative as
the first key. In local dynamic TLS model, DTPMOD (e.g.
R_ARM_TLS_DTPMOD32 R_X86_64_DTPMOD and R_PPC{,64}_DTPMOD) may use 0 as
the symbol index.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D62141

llvm-svn: 361164

lld/ELF/SyntheticSections.cpp

@@ -1488,17 +1488,17 @@ RelocationSection<ELFT>::RelocationSection(StringRef Name, bool Sort)
   this->Entsize = Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
 }
 
-static bool compRelocations(const DynamicReloc &A, const DynamicReloc &B) {
-  bool AIsRel = A.Type == Target->RelativeRel;
-  bool BIsRel = B.Type == Target->RelativeRel;
-  if (AIsRel != BIsRel)
-    return AIsRel;
-  return A.getSymIndex() < B.getSymIndex();
-}
-
 template <class ELFT> void RelocationSection<ELFT>::writeTo(uint8_t *Buf) {
+  // Sort by (!IsRelative,SymIndex,r_offset). DT_REL[A]COUNT requires us to
+  // place R_*_RELATIVE first. SymIndex is to improve locality, while r_offset
+  // is to make results easier to read.
   if (Sort)
-    llvm::stable_sort(Relocs, compRelocations);
+    llvm::stable_sort(Relocs, [](const DynamicReloc &A, const DynamicReloc &B) {
+      return std::make_tuple(A.Type != Target->RelativeRel, A.getSymIndex(),
+                             A.getOffset()) <
+             std::make_tuple(B.Type != Target->RelativeRel, B.getSymIndex(),
+                             B.getOffset());
+    });
 
   for (const DynamicReloc &Rel : Relocs) {
     encodeDynamicReloc<ELFT>(reinterpret_cast<Elf_Rela *>(Buf), Rel);

lld/test/ELF/combreloc.s

@@ -0,0 +1,48 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: echo 'mov aaa@gotpcrel(%rip), %rax' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+
+# RUN: ld.lld -shared %t.o %t1.o -o %t.so
+# RUN: llvm-readobj -r --dynamic-table %t.so | FileCheck %s
+# RUN: ld.lld -shared %t.o %t1.o -o %t.so -z combreloc
+# RUN: llvm-readobj -r --dynamic-table %t.so | FileCheck %s
+
+# -z combreloc is the default: sort relocations by (!IsRelative,SymIndex,r_offset),
+# and emit DT_RELACOUNT (except on MIPS) to indicate the number of relative
+# relocations.
+
+# CHECK:      Relocations [
+# CHECK-NEXT:   Section ({{.*}}) .rela.dyn {
+# CHECK-NEXT:     0x3020 R_X86_64_RELATIVE - 0x3028
+# CHECK-NEXT:     0x20B0 R_X86_64_GLOB_DAT aaa 0x0
+# CHECK-NEXT:     0x3000 R_X86_64_64 aaa 0x0
+# CHECK-NEXT:     0x3018 R_X86_64_64 aaa 0x0
+# CHECK-NEXT:     0x3010 R_X86_64_64 bbb 0x0
+# CHECK-NEXT:     0x3008 R_X86_64_64 ccc 0x0
+# CHECK-NEXT:   }
+# CHECK:      DynamicSection [
+# CHECK:        RELACOUNT 1
+
+# RUN: ld.lld -z nocombreloc -shared %t.o %t1.o -o %t.so
+# RUN: llvm-readobj -r --dynamic-table %t.so | FileCheck --check-prefix=NOCOMB %s
+
+# NOCOMB:      Relocations [
+# NOCOMB-NEXT:   Section ({{.*}}) .rela.dyn {
+# NOCOMB-NEXT:     0x3000 R_X86_64_64 aaa 0x0
+# NOCOMB-NEXT:     0x3008 R_X86_64_64 ccc 0x0
+# NOCOMB-NEXT:     0x3010 R_X86_64_64 bbb 0x0
+# NOCOMB-NEXT:     0x3018 R_X86_64_64 aaa 0x0
+# NOCOMB-NEXT:     0x3020 R_X86_64_RELATIVE - 0x3028
+# NOCOMB-NEXT:     0x20A0 R_X86_64_GLOB_DAT aaa 0x0
+# NOCOMB-NEXT:   }
+# NOCOMB:      DynamicSection [
+# NOCOMB-NOT:    RELACOUNT
+
+.data
+ .quad aaa
+ .quad ccc
+ .quad bbb
+ .quad aaa
+ .quad relative
+relative:

lld/test/ELF/i386-got-and-copy.s

@@ -14,8 +14,8 @@
 
 # CHECK:      Relocations [
 # CHECK-NEXT:   Section (4) .rel.dyn {
-# CHECK-NEXT:     0x{{[0-9A-F]+}} R_386_COPY foo
 # CHECK-NEXT:     0x{{[0-9A-F]+}} R_386_GLOB_DAT foo
+# CHECK-NEXT:     0x{{[0-9A-F]+}} R_386_COPY foo
 # CHECK-NEXT:   }
 # CHECK-NEXT: ]