[ARM] Implement execute-only support in CodeGen

Summary:
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.

Reviewers: t.p.northover, rengolin

Subscribers: llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D27450

llvm-svn: 289786

1 files changed, 24 insertions, 1 deletions

diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 3b20805..8f1f341 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -1001,6 +1001,7 @@ arm::FloatABI arm::getARMFloatABI(const ToolChain &TC, const ArgList &Args) {
 static void getARMTargetFeatures(const ToolChain &TC,
                                  const llvm::Triple &Triple,
                                  const ArgList &Args,
+                                 ArgStringList &CmdArgs,
                                  std::vector<StringRef> &Features,
                                  bool ForAS) {
   const Driver &D = TC.getDriver();
@@ -1139,6 +1140,28 @@ static void getARMTargetFeatures(const ToolChain &TC,
       Features.push_back("+long-calls");
   }
 
+  // Generate execute-only output (no data access to code sections).
+  // Supported only on ARMv6T2 and ARMv7 and above.
+  // Cannot be combined with -mno-movt or -mlong-calls
+  if (Arg *A = Args.getLastArg(options::OPT_mexecute_only, options::OPT_mno_execute_only)) {
+    if (A->getOption().matches(options::OPT_mexecute_only)) {
+      if (getARMSubArchVersionNumber(Triple) < 7 &&
+          llvm::ARM::parseArch(Triple.getArchName()) != llvm::ARM::AK_ARMV6T2)
+            D.Diag(diag::err_target_unsupported_execute_only) << Triple.getArchName();
+      else if (Arg *B = Args.getLastArg(options::OPT_mno_movt))
+        D.Diag(diag::err_opt_not_valid_with_opt) << A->getAsString(Args) << B->getAsString(Args);
+      // Long calls create constant pool entries and have not yet been fixed up
+      // to play nicely with execute-only. Hence, they cannot be used in
+      // execute-only code for now
+      else if (Arg *B = Args.getLastArg(options::OPT_mlong_calls, options::OPT_mno_long_calls)) {
+        if (B->getOption().matches(options::OPT_mlong_calls))
+          D.Diag(diag::err_opt_not_valid_with_opt) << A->getAsString(Args) << B->getAsString(Args);
+      }
+
+      CmdArgs.push_back("-arm-execute-only");
+    }
+  }
+
   // Kernel code has more strict alignment requirements.
   if (KernelOrKext)
     Features.push_back("+strict-align");
@@ -2703,7 +2726,7 @@ static void getTargetFeatures(const ToolChain &TC, const llvm::Triple &Triple,
   case llvm::Triple::armeb:
   case llvm::Triple::thumb:
   case llvm::Triple::thumbeb:
-    getARMTargetFeatures(TC, Triple, Args, Features, ForAS);
+    getARMTargetFeatures(TC, Triple, Args, CmdArgs, Features, ForAS);
     break;
 
   case llvm::Triple::ppc: