[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

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index 8298aff..4798991 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -368,7 +368,9 @@ MCSectionELF *MCContext::getELFSection(const Twine &Section, unsigned Type,
   StringRef CachedName = Entry.first.SectionName;
 
   SectionKind Kind;
-  if (Flags & ELF::SHF_EXECINSTR)
+  if (Flags & ELF::SHF_ARM_PURECODE)
+    Kind = SectionKind::getExecuteOnly();
+  else if (Flags & ELF::SHF_EXECINSTR)
     Kind = SectionKind::getText();
   else
     Kind = SectionKind::getReadOnly();