[ELF] Don't sort non reorderable sections with --symbol-ordering-file

[Bigcheese]

Summary

Don't sort known non-reorderable sections such as .init with --symbol-ordering-file.

[grimar]

Can we exclude such sections from Order in buildSectionOrder. With that after landing D42475 we could
warn/error on out such file entries too.

ELF/Writer.cpp

for (Symbol *Sym : File->getSymbols()) {
         continue;
       int &Priority = SectionOrder[D->Section];
       Priority = std::min(Priority, SymbolOrder.lookup(D->getName()));
     }
   }
   return SectionOrder;
 }
 
+static bool isKnownNonreorderableSection(const OutputSection *OS) {
+  return llvm::StringSwitch<bool>(OS->Name)
+      .Cases(".init", ".fini", ".init_array", ".fini_array", ".ctors",
+             ".dtors", true)
+      .Default(false);
+}
+
 // If no layout was provided by linker script, we want to apply default
 // sorting for special input sections. This also handles --symbol-ordering-file.
 template <class ELFT> void Writer<ELFT>::sortInputSections() {
   // Sort input sections by priority using the list provided
   // by --symbol-ordering-file.
   DenseMap<SectionBase *, int> Order = buildSectionOrder();
   if (!Order.empty())
     for (BaseCommand *Base : Script->SectionCommands)
       if (auto *Sec = dyn_cast<OutputSection>(Base))
-        if (Sec->Live)
+        if (Sec->Live && !isKnownNonreorderableSection(Sec))
           Sec->sort([&](InputSectionBase *S) { return Order.lookup(S); });
 
   if (Script->HasSectionsCommand)
     return;
 
   // Sort input sections by section name suffixes for
   // __attribute__((init_priority(N))).
   if (OutputSection *Sec = findSection(".init_array"))

test/ELF/symbol-ordering-file.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t.out
 # RUN: llvm-objdump -s %t.out| FileCheck %s --check-prefix=BEFORE
 
 # BEFORE:      Contents of section .foo:
 # BEFORE-NEXT:  201000 11223344 5566
+# BEFORE:      Contents of section .init:
+# BEFORE-NEXT:  201006 1122
 
 # RUN: echo "_foo4  " > %t_order.txt
 # RUN: echo "  _foo3" >> %t_order.txt
 # RUN: echo "_foo5" >> %t_order.txt
 # RUN: echo "_foo2" >> %t_order.txt
 # RUN: echo " " >> %t_order.txt
 # RUN: echo "_foo4" >> %t_order.txt
 # RUN: echo "_bar1" >> %t_order.txt
 # RUN: echo "_foo1" >> %t_order.txt
+# RUN: echo "_init2" >> %t_order.txt
+# RUN: echo "_init1" >> %t_order.txt
 
 # RUN: ld.lld --symbol-ordering-file %t_order.txt %t.o -o %t2.out
 # RUN: llvm-objdump -s %t2.out| FileCheck %s --check-prefix=AFTER
 
 # AFTER:      Contents of section .foo:
 # AFTER-NEXT:  201000 44335566 2211
+# AFTER:      Contents of section .init:
+# AFTER-NEXT:  201006 1122
 
 .section .foo,"ax",@progbits,unique,1
 _foo1:
  .byte 0x11
 
 .section .foo,"ax",@progbits,unique,2
 _foo2:
  .byte 0x22
 
 .section .foo,"ax",@progbits,unique,3
 _foo3:
  .byte 0x33
 
 .section .foo,"ax",@progbits,unique,4
 _foo4:
  .byte 0x44
 
 .section .foo,"ax",@progbits,unique,5
 _foo5:
  .byte 0x55
 _bar1:
  .byte 0x66
+
+.section .init,"ax",@progbits,unique,1
+_init1:
+ .byte 0x11
+
+.section .init,"ax",@progbits,unique,2
+_init2:
+ .byte 0x22