[ARM] Implement execute-only support in CodeGen

This implements execute-only support for ARM code generation, which
prevents the compiler from generating data accesses to code sections.
The following changes are involved:

* Add the CodeGen option "-arm-execute-only" to the ARM code generator.
* Add the clang flag "-mexecute-only" as well as the GCC-compatible
  alias "-mpure-code" to enable this option.
* When enabled, literal pools are replaced with MOVW/MOVT instructions,
  with VMOV used in addition for floating-point literals. As the MOVT
  instruction is required, execute-only support is only available in
  Thumb mode for targets supporting ARMv8-M baseline or Thumb2.
* Jump tables are placed in data sections when in execute-only mode.
* The execute-only text section is assigned section ID 0, and is
  marked as unreadable with the SHF_ARM_PURECODE flag with symbol 'y'.
  This also overrides selection of ELF sections for globals.

llvm-svn: 289784

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -181,6 +181,9 @@ static unsigned getELFSectionFlags(SectionKind K) {
   if (K.isText())
     Flags |= ELF::SHF_EXECINSTR;
 
+  if (K.isExecuteOnly())
+    Flags |= ELF::SHF_ARM_PURECODE;
+
   if (K.isWriteable())
     Flags |= ELF::SHF_WRITE;
 
@@ -312,6 +315,9 @@ selectELFSectionForGlobal(MCContext &Ctx, const GlobalObject *GO,
     UniqueID = *NextUniqueID;
     (*NextUniqueID)++;
   }
+  // Use 0 as the unique ID for execute-only text
+  if (Kind.isExecuteOnly())
+    UniqueID = 0;
   return Ctx.getELFSection(Name, getELFSectionType(Name, Kind), Flags,
                            EntrySize, Group, UniqueID);
 }