[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, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetObjectFile.h b/llvm/lib/Target/ARM/ARMTargetObjectFile.h
index 7a1f3e39b1d..24e755ddac2 100644
--- a/llvm/lib/Target/ARM/ARMTargetObjectFile.h
+++ b/llvm/lib/Target/ARM/ARMTargetObjectFile.h
@@ -18,6 +18,7 @@ class MCContext;
 class TargetMachine;
 
 class ARMElfTargetObjectFile : public TargetLoweringObjectFileELF {
+  mutable bool genExecuteOnly = false;
 protected:
   const MCSection *AttributesSection;
 public:
@@ -36,6 +37,12 @@ public:
 
   /// \brief Describe a TLS variable address within debug info.
   const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
+
+  MCSection *getExplicitSectionGlobal(const GlobalObject *GO, SectionKind Kind,
+                                      const TargetMachine &TM) const override;
+
+  MCSection *SelectSectionForGlobal(const GlobalObject *GO, SectionKind Kind,
+                                    const TargetMachine &TM) const override;
 };
 
 } // end namespace llvm