[ELF] - Implemented -no-rosegment.

[grimar]

Summary

This is close to D26888, but implements only -no-rosegment flag.
This allows to reduce the changes.
So D26888 shows full version of functionality and can
be rebased if this one be landed.

[grimar]

• Addressed review comments.

ELF/Config.h

struct Configuration {
   bool FatalWarnings;
   bool GcSections;
   bool GdbIndex;
   bool GnuHash = false;
   bool ICF;
   bool Mips64EL = false;
   bool MipsN32Abi = false;
   bool NoGnuUnique;
+  bool NoRosegment;
   bool NoUndefinedVersion;
   bool Nostdlib;
   bool OFormatBinary;
   bool Pic;
   bool Pie;
   bool PrintGcSections;
   bool Rela;
   bool Relocatable;

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->EhFrameHdr = Args.hasArg(OPT_eh_frame_hdr);
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->FatalWarnings = Args.hasArg(OPT_fatal_warnings);
   Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->GdbIndex = Args.hasArg(OPT_gdb_index);
   Config->ICF = Args.hasArg(OPT_icf);
   Config->NoGnuUnique = Args.hasArg(OPT_no_gnu_unique);
+  Config->NoRosegment = Args.hasArg(OPT_no_rosegment);
   Config->NoUndefinedVersion = Args.hasArg(OPT_no_undefined_version);
   Config->Nostdlib = Args.hasArg(OPT_nostdlib);
   Config->Pie = getArg(Args, OPT_pie, OPT_nopie, false);
   Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->SaveTemps = Args.hasArg(OPT_save_temps);
   Config->Shared = Args.hasArg(OPT_shared);
   Config->Target1Rel = getArg(Args, OPT_target1_rel, OPT_target1_abs, false);

ELF/Options.td

 def no_whole_archive: F<"no-whole-archive">,
   HelpText<"Restores the default behavior of loading archive members">;
 
 def noinhibit_exec: F<"noinhibit-exec">,
   HelpText<"Retain the executable output file whenever it is still usable">;
 
 def nopie: F<"nopie">, HelpText<"Do not create a position independent executable">;
 
+def no_rosegment: F<"no-rosegment">, HelpText<"Do not put read-only non-executable sections in their own segment">;
+
 def no_undefined: F<"no-undefined">,
   HelpText<"Report unresolved symbols even if the linker is creating a shared library">;
 
 def no_undefined_version: F<"no-undefined-version">,
   HelpText<"Report version scripts that refer undefined symbols">;
 
 def o: JoinedOrSeparate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;

ELF/Writer.cpp

 // cannot create a PT_LOAD there.
 template <class ELFT>
 static typename ELFT::uint computeFlags(typename ELFT::uint F) {
   if (ScriptConfig->HasSections && !(F & PF_W))
     return F | PF_X;
   return F;
 }
 
+// We apply -rosegment by default, -no-rosegment is used to avoid placing read
+// only non-executable sections in their own segment.
+template <class ELFT>
+static bool shouldEmitLoad(typename ELFT::uint Old, typename ELFT::uint New) {
+  if (Config->NoRosegment)
+    return (Old & PF_W) != (New & PF_W);
+  return Old != New;
+}
+
 // Decide which program headers to create and which sections to include in each
 // one.
 template <class ELFT> std::vector<PhdrEntry<ELFT>> Writer<ELFT>::createPhdrs() {
   std::vector<Phdr> Ret;
   auto AddHdr = [&](unsigned Type, unsigned Flags) -> Phdr * {
     Ret.emplace_back(Type, Flags);
     return &Ret.back();
   };
for (OutputSectionBase *Sec : OutputSections) {
     if (Sec->Flags & SHF_TLS)
       TlsHdr.add(Sec);
 
     if (!needsPtLoad<ELFT>(Sec))
       continue;
 
     // Segments are contiguous memory regions that has the same attributes
     // (e.g. executable or writable). There is one phdr for each segment.
-    // Therefore, we need to create a new phdr when the next section has
+    // Therefore, usually we need to create a new phdr when the next section has
     // different flags or is loaded at a discontiguous address using AT linker
-    // script command.
+    // script command. This can be changed using -norosegment option.
     uintX_t NewFlags = computeFlags<ELFT>(Sec->getPhdrFlags());
-    if (Script<ELFT>::X->hasLMA(Sec->getName()) || Flags != NewFlags) {
+    if (Script<ELFT>::X->hasLMA(Sec->getName()) ||
+        shouldEmitLoad<ELFT>(Flags, NewFlags)) {
       Load = AddHdr(PT_LOAD, NewFlags);
       Flags = NewFlags;
+    } else {
+      Load->H.p_flags |= NewFlags;
     }
 
     Load->add(Sec);
 
     if (isRelroSection<ELFT>(Sec))
       RelRo.add(Sec);
     if (Sec->Type == SHT_NOTE)
       Note.add(Sec);

test/ELF/segments.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t1
+# RUN: llvm-readobj --program-headers %t1 | FileCheck --check-prefix=ROSEGMENT %s
+
+# ROSEGMENT:      ProgramHeader {
+# ROSEGMENT:        Type: PT_LOAD
+# ROSEGMENT-NEXT:    Offset: 0x0
+# ROSEGMENT-NEXT:    VirtualAddress: 0x10000
+# ROSEGMENT-NEXT:    PhysicalAddress: 0x10000
+# ROSEGMENT-NEXT:    FileSize:
+# ROSEGMENT-NEXT:    MemSize:
+# ROSEGMENT-NEXT:    Flags [
+# ROSEGMENT-NEXT:      PF_R
+# ROSEGMENT-NEXT:    ]
+# ROSEGMENT-NEXT:    Alignment: 4096
+# ROSEGMENT-NEXT:  }
+# ROSEGMENT-NEXT:  ProgramHeader {
+# ROSEGMENT-NEXT:    Type: PT_LOAD
+# ROSEGMENT-NEXT:    Offset: 0x1000
+# ROSEGMENT-NEXT:    VirtualAddress: 0x11000
+# ROSEGMENT-NEXT:    PhysicalAddress: 0x11000
+# ROSEGMENT-NEXT:    FileSize:
+# ROSEGMENT-NEXT:    MemSize:
+# ROSEGMENT-NEXT:    Flags [
+# ROSEGMENT-NEXT:      PF_R
+# ROSEGMENT-NEXT:      PF_X
+# ROSEGMENT-NEXT:    ]
+# ROSEGMENT-NEXT:    Alignment: 4096
+# ROSEGMENT-NEXT:  }
+# ROSEGMENT-NEXT:  ProgramHeader {
+# ROSEGMENT-NEXT:    Type: PT_LOAD
+# ROSEGMENT-NEXT:    Offset: 0x2000
+# ROSEGMENT-NEXT:    VirtualAddress: 0x12000
+# ROSEGMENT-NEXT:    PhysicalAddress: 0x12000
+# ROSEGMENT-NEXT:    FileSize: 1
+# ROSEGMENT-NEXT:    MemSize: 1
+# ROSEGMENT-NEXT:    Flags [
+# ROSEGMENT-NEXT:      PF_R
+# ROSEGMENT-NEXT:      PF_W
+# ROSEGMENT-NEXT:    ]
+# ROSEGMENT-NEXT:    Alignment: 4096
+# ROSEGMENT-NEXT:  }
+
+# RUN: ld.lld -no-rosegment %t -o %t2
+# RUN: llvm-readobj --program-headers %t2 | FileCheck --check-prefix=NOROSEGMENT %s
+
+# NOROSEGMENT:     ProgramHeader {
+# NOROSEGMENT:       Type: PT_LOAD
+# NOROSEGMENT-NEXT:   Offset: 0x0
+# NOROSEGMENT-NEXT:   VirtualAddress: 0x10000
+# NOROSEGMENT-NEXT:   PhysicalAddress: 0x10000
+# NOROSEGMENT-NEXT:   FileSize:
+# NOROSEGMENT-NEXT:   MemSize:
+# NOROSEGMENT-NEXT:   Flags [
+# NOROSEGMENT-NEXT:     PF_R
+# NOROSEGMENT-NEXT:     PF_X
+# NOROSEGMENT-NEXT:   ]
+# NOROSEGMENT-NEXT:   Alignment: 4096
+# NOROSEGMENT-NEXT: }
+# NOROSEGMENT-NEXT: ProgramHeader {
+# NOROSEGMENT-NEXT:   Type: PT_LOAD
+# NOROSEGMENT-NEXT:   Offset: 0x1000
+# NOROSEGMENT-NEXT:   VirtualAddress: 0x11000
+# NOROSEGMENT-NEXT:   PhysicalAddress: 0x11000
+# NOROSEGMENT-NEXT:   FileSize:
+# NOROSEGMENT-NEXT:   MemSize:
+# NOROSEGMENT-NEXT:   Flags [
+# NOROSEGMENT-NEXT:     PF_R
+# NOROSEGMENT-NEXT:     PF_W
+# NOROSEGMENT-NEXT:   ]
+# NOROSEGMENT-NEXT:   Alignment: 4096
+# NOROSEGMENT-NEXT: }
+# NOROSEGMENT-NEXT: ProgramHeader {
+# NOROSEGMENT-NEXT:   Type: PT_GNU_STACK
+
+.global _start
+_start:
+ nop
+
+.section .ro,"a"
+nop
+
+.section .rw,"aw"
+nop
+
+.section .rx,"ax"
+nop