[ARM] Implement execute-only support in CodeGen

[prakhar]

Summary

[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 SubtargetFeature "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 feature.
• 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.

[rengolin]

lib/Driver/Tools.cpp

1145

In theory, ARMv6T2+ supports it. I just want to make sure we're blocking it for a reason.

1149

Please, add curly brackets, for clarity.

test/Driver/arm-execute-only.c

64

Why not long-calls? Where is this being disabled / guaranteed?

[prakhar]

lib/Driver/Tools.cpp

1145

Yes, there's no real reason to arbitrarily block ARMv6T2 here, so I'll explicitly allow it here.

test/Driver/arm-execute-only.c

64

Long calls generates constant pool entries in several cases, and the work required to make long calls play nicely with execute-only has not yet been done. I'll add in a diagnostic to prevent the two being used together for now.

[prakhar]

Allow ARMv6T2, and provide diagnostics if combined with -mno-movt or -mlong-calls

[prakhar]

Fixes for test and commit message

[rengolin]

In D27450#620895, @prakhar wrote:

Allow ARMv6T2, and provide diagnostics if combined with -mno-movt or -mlong-calls

Can you add a comment to the long-calls if block saying why they're not compatible yet?

Otherwise, LGTM. Thanks!

[rengolin]

Just to be clear, this should be committed after the LLVM change, once it's approved.

[prakhar]

Make execute-only a codegen option instead of a subtarget feature

[prakhar]

Message about why long calls are not supported in execute-only

include/clang/Basic/DiagnosticCommonKinds.td

 def err_target_unsupported_abi : Error<"ABI '%0' is not supported on CPU '%1'">;
 def err_target_unsupported_abi_for_triple : Error<
   "ABI '%0' is not supported for '%1'">;
 def err_target_unknown_fpmath : Error<"unknown FP unit '%0'">;
 def err_target_unsupported_fpmath : Error<
     "the '%0' unit is not supported with this instruction set">;
 def err_target_unsupported_unaligned : Error<
   "the %0 sub-architecture does not support unaligned accesses">;
+def err_target_unsupported_execute_only : Error<
+  "execute only is not supported for the %0 sub-architecture">;
 def err_opt_not_valid_with_opt : Error<
   "option '%0' cannot be specified with '%1'">;
 
 // Source manager
 def err_cannot_open_file : Error<"cannot open file '%0': %1">, DefaultFatal;
 def err_file_modified : Error<
   "file '%0' modified since it was first processed">, DefaultFatal;
 def err_unsupported_bom : Error<"%0 byte order mark detected in '%1', but "

include/clang/Driver/Options.td

 def malign_functions_EQ : Joined<["-"], "malign-functions=">, Group<clang_ignored_m_Group>;
 def malign_loops_EQ : Joined<["-"], "malign-loops=">, Group<clang_ignored_m_Group>;
 def malign_jumps_EQ : Joined<["-"], "malign-jumps=">, Group<clang_ignored_m_Group>;
 def mfancy_math_387 : Flag<["-"], "mfancy-math-387">, Group<clang_ignored_m_Group>;
 def mlong_calls : Flag<["-"], "mlong-calls">, Group<m_Group>,
   HelpText<"Generate branches with extended addressability, usually via indirect jumps.">;
 def mno_long_calls : Flag<["-"], "mno-long-calls">, Group<m_Group>,
   HelpText<"Restore the default behaviour of not generating long calls">;
+def mexecute_only : Flag<["-"], "mexecute-only">, Group<m_arm_Features_Group>,
+  HelpText<"Disallow generation of data access to code sections (ARM only)">;
+def mno_execute_only : Flag<["-"], "mno-execute-only">, Group<m_arm_Features_Group>,
+  HelpText<"Allow generation of data access to code sections (ARM only)">;
+def mpure_code : Flag<["-"], "mpure-code">, Alias<mexecute_only>; // Alias for GCC compatibility
+def mno_pure_code : Flag<["-"], "mno-pure-code">, Alias<mno_execute_only>;
 def mtvos_version_min_EQ : Joined<["-"], "mtvos-version-min=">, Group<m_Group>;
 def mappletvos_version_min_EQ : Joined<["-"], "mappletvos-version-min=">, Alias<mtvos_version_min_EQ>;
 def mtvos_simulator_version_min_EQ : Joined<["-"], "mtvos-simulator-version-min=">, Alias<mtvos_version_min_EQ>;
 def mappletvsimulator_version_min_EQ : Joined<["-"], "mappletvsimulator-version-min=">, Alias<mtvos_version_min_EQ>;
 def mwatchos_version_min_EQ : Joined<["-"], "mwatchos-version-min=">, Group<m_Group>;
 def mwatchos_simulator_version_min_EQ : Joined<["-"], "mwatchos-simulator-version-min=">, Alias<mwatchos_version_min_EQ>;
 def mwatchsimulator_version_min_EQ : Joined<["-"], "mwatchsimulator-version-min=">, Alias<mwatchos_version_min_EQ>;
 def march_EQ : Joined<["-"], "march=">, Group<m_Group>;

lib/Driver/Tools.cpp

if (Arg *A = Args.getLastArg(options::OPT_mlong_calls,
                                options::OPT_mno_long_calls)) {
     if (A->getOption().matches(options::OPT_mlong_calls))
       Features.push_back("+long-calls");
   } else if (KernelOrKext && (!Triple.isiOS() || Triple.isOSVersionLT(6)) &&
              !Triple.isWatchOS()) {
       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);
+      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);
+      }
+      else
+        Features.push_back("+execute-only");
+    } else {
+      Features.push_back("-execute-only");
+    }
+  }
+
   // Kernel code has more strict alignment requirements.
   if (KernelOrKext)
     Features.push_back("+strict-align");
   else if (Arg *A = Args.getLastArg(options::OPT_mno_unaligned_access,
                                     options::OPT_munaligned_access)) {
     if (A->getOption().matches(options::OPT_munaligned_access)) {
       // No v6M core supports unaligned memory access (v6M ARM ARM A3.2).
       if (Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)

test/Driver/arm-execute-only.c

+// RUN: %clang -target armv6t2-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv6t2-eabi -### -mexecute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv6t2-eabi -### -mexecute-only -mno-execute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### -mexecute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### -mexecute-only -mno-execute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### -mexecute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### -mexecute-only -mno-execute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### -mexecute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### -mexecute-only -mno-execute-only %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: not %clang -c -target thumbv6m-eabi -mexecute-only %s 2>&1 | \
+// RUN:   FileCheck --check-prefix CHECK-EXECUTE-ONLY-NOT-SUPPORTED %s
+
+// RUN: %clang -target armv8m.main-eabi -mexecute-only -mno-movt %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY-NO-MOVT
+
+// RUN: %clang -target armv8m.main-eabi -mexecute-only -mlong-calls %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY-LONG-CALLS
+
+
+// -mpure-code flag for GCC compatibility
+// RUN: %clang -target armv6t2-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv6t2-eabi -### -mpure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv6t2-eabi -### -mpure-code -mno-pure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### -mpure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv7m-eabi -### -mpure-code -mno-pure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### -mpure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.base-eabi -### -mpure-code -mno-pure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### -mpure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY
+
+// RUN: %clang -target armv8m.main-eabi -### -mpure-code -mno-pure-code %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ONLY
+
+// RUN: not %clang -c -target thumbv6m-eabi -mpure-code %s 2>&1 | \
+// RUN:   FileCheck --check-prefix CHECK-EXECUTE-ONLY-NOT-SUPPORTED %s
+
+// RUN: %clang -target armv8m.main-eabi -mpure-code -mno-movt %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY-NO-MOVT
+
+// RUN: %clang -target armv8m.main-eabi -mpure-code -mlong-calls %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-EXECUTE-ONLY-LONG-CALLS
+
+//
+// CHECK-NO-EXECUTE-ONLY-NOT: "-target-feature" "+execute-only"
+// CHECK-EXECUTE-ONLY: "-target-feature" "+execute-only"
+
+// CHECK-EXECUTE-ONLY-NOT-SUPPORTED: error: execute only is not supported for the thumbv6m sub-architecture
+// CHECK-EXECUTE-ONLY-NO-MOVT: error: option '-mexecute-only' cannot be specified with '-mno-movt'
+// CHECK-EXECUTE-ONLY-LONG-CALLS: error: option '-mexecute-only' cannot be specified with '-mlong-calls'