[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, 8 insertions, 0 deletions

diff --git a/llvm/tools/llvm-readobj/ELFDumper.cpp b/llvm/tools/llvm-readobj/ELFDumper.cpp
index 801bc8eca2a..e939135b612 100644
--- a/llvm/tools/llvm-readobj/ELFDumper.cpp
+++ b/llvm/tools/llvm-readobj/ELFDumper.cpp
@@ -1069,6 +1069,10 @@ static const EnumEntry<unsigned> ElfAMDGPUSectionFlags[] = {
   LLVM_READOBJ_ENUM_ENT(ELF, SHF_AMDGPU_HSA_AGENT)
 };
 
+static const EnumEntry<unsigned> ElfARMSectionFlags[] = {
+  LLVM_READOBJ_ENUM_ENT(ELF, SHF_ARM_PURECODE)
+};
+
 static const EnumEntry<unsigned> ElfHexagonSectionFlags[] = {
   LLVM_READOBJ_ENUM_ENT(ELF, SHF_HEX_GPREL)
 };
@@ -3596,6 +3600,10 @@ template <class ELFT> void LLVMStyle<ELFT>::printSections(const ELFO *Obj) {
       SectionFlags.insert(SectionFlags.end(), std::begin(ElfAMDGPUSectionFlags),
                           std::end(ElfAMDGPUSectionFlags));
       break;
+    case EM_ARM:
+      SectionFlags.insert(SectionFlags.end(), std::begin(ElfARMSectionFlags),
+                          std::end(ElfARMSectionFlags));
+      break;
     case EM_HEXAGON:
       SectionFlags.insert(SectionFlags.end(),
                           std::begin(ElfHexagonSectionFlags),