[Thumb1] Do not allow Armv6-m XO and PI code

[keith.walker.arm]

Summary

When generating armv6-m (Thumb1) Position Independent (PI) code
there are currently some code sequences that are not compatible
with eXecute-Only (XO) code.

For example, this simple code sequence when compiler for XO & PI:

extern int x;
int fn() { return x; }

is a problem as the address of x is currently loaded by:

  ldr r0, .L0
:
:
.L0:
  .long   x

which is not XO compiant as this involves reading the value at
.L0 which is in the code section. Generating correct code is
currently hindered by lack of suitable relocations.

Disallow the generation of armv6-m PI code together with XO code
until they can be made to work together.

[john.brawn]

clang/lib/Driver/ToolChains/Arch/ARM.cpp

853

We probably don't need to use a new diagnostic here, as -mexecute-only is already specific to the arm target and we already use err_opt_not_valid_with_opt for clashes between two target-specific options (like we do with -mno-movt just below).

[keith.walker.arm]

clang/lib/Driver/ToolChains/Arch/ARM.cpp

853

I disagree.

In the -mno-movt case, the problem is due to that option being incompatible with the selected target, so the err_opt_not_valid_with_opt is sufficient to know that this option is clashing with the selected target.

In the XO & PI case, that clash is not with the selected target, but rather with another option on the command line when used with the selected target. You really need to know what is the other option that is causing the clash.

[john.brawn]

clang/lib/Driver/ToolChains/Arch/ARM.cpp

853

Ah, I think you're taking 'target' to mean 'the specific target' i.e. armv6-m, whereas I'm taking it to mean the arm target as a whole.

In that case I think it would be better to mention the specific target in the error message, e.g. like in err_target_unsupported_execute_only (and other err_target_unsupported errors).

[olista01]

clang/lib/Driver/ToolChains/Arch/ARM.cpp

852

If we are rejecting -fpic and -fpie, should we also check for -fPIC and -fPIE?

[keith.walker.arm]

Added checks for -FPIE and -f PIC command line options.

Improved error message with additiona of sub-architecture details.

[olista01]

LGTM

clang/include/clang/Basic/DiagnosticCommonKinds.td

 def err_target_unsupported_execute_only : Error<
   "execute only is not supported for the %0 sub-architecture">;
 def err_target_unsupported_tp_hard : Error<
   "hardware TLS register is not supported for the %0 sub-architecture">;
 def err_target_unsupported_mcmse : Error<
   "-mcmse is not supported for %0">;
 def err_opt_not_valid_with_opt : Error<
   "option '%0' cannot be specified with '%1'">;
+def err_opt_not_valid_with_opt_on_target : Error<
+  "option '%0' cannot be specified with '%1' for the %2 sub-architecture">;
 def err_opt_not_valid_without_opt : Error<
   "option '%0' cannot be specified without '%1'">;
 def err_opt_not_valid_on_target : Error<
   "option '%0' cannot be specified on this target">;
 def err_invalid_feature_combination : Error<
   "invalid feature combination: %0">;
 def warn_target_unrecognized_env : Warning<
   "mismatch between architecture and environment in target triple '%0'; did you mean '%1'?">,

clang/lib/Driver/ToolChains/Arch/ARM.cpp

if (!ForAS && !ForMultilib) {
     // Supported only on ARMv6T2 and ARMv7 and above.
     // Cannot be combined with -mno-movt.
     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::ArchKind::ARMV6T2 &&
             llvm::ARM::parseArch(Triple.getArchName()) != llvm::ARM::ArchKind::ARMV6M)
               D.Diag(diag::err_target_unsupported_execute_only) << Triple.getArchName();
-        else if (Arg *B = Args.getLastArg(options::OPT_mno_movt))
+        else if (llvm::ARM::parseArch(Triple.getArchName()) == llvm::ARM::ArchKind::ARMV6M) {
+          if (Arg *PIArg = Args.getLastArg(options::OPT_fropi, options::OPT_frwpi,
+                                           options::OPT_fpic, options::OPT_fpie,
+                                           options::OPT_fPIC, options::OPT_fPIE))
+            D.Diag(diag::err_opt_not_valid_with_opt_on_target)
+                << A->getAsString(Args) << PIArg->getAsString(Args) << 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);
         Features.push_back("+execute-only");
       }
     }
   }
 
   // Kernel code has more strict alignment requirements.

clang/test/Driver/arm-execute-only.c

 
 // -mpure-code flag for GCC compatibility
 // RUN: not %clang -### -c --target=arm-arm-none-eabi -march=armv6 -mpure-code %s 2>&1 | \
 // RUN:   FileCheck --check-prefix CHECK-EXECUTE-ONLY-NOT-SUPPORTED %s
 
 // RUN: not %clang -### --target=arm-arm-none-eabi -march=armv8-m.main -mpure-code -mno-movt %s 2>&1 \
 // RUN:    | FileCheck %s -check-prefix CHECK-PURE-CODE-NO-MOVT
 // CHECK-PURE-CODE-NO-MOVT: error: option '-mpure-code' cannot be specified with '-mno-movt'
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -fropi %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-ROPI
+// CHECK-NO-EXECUTE-ROPI: error: option '-mexecute-only' cannot be specified with '-fropi' for the thumbv6m sub-architecture
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -frwpi %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-RWPI
+// CHECK-NO-EXECUTE-RWPI: error: option '-mexecute-only' cannot be specified with '-frwpi' for the thumbv6m sub-architecture
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -fpic %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-PIC
+// CHECK-NO-EXECUTE-PIC: error: option '-mexecute-only' cannot be specified with '-fpic' for the thumbv6m sub-architecture
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -fpie %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-PIE
+// CHECK-NO-EXECUTE-PIE: error: option '-mexecute-only' cannot be specified with '-fpie' for the thumbv6m sub-architecture
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -fPIC %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-PIC2
+// CHECK-NO-EXECUTE-PIC2: error: option '-mexecute-only' cannot be specified with '-fPIC' for the thumbv6m sub-architecture
+
+// RUN: not %clang -### --target=arm-arm-none-eabi -march=armv6-m -mexecute-only -fPIE %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-EXECUTE-PIE2
+// CHECK-NO-EXECUTE-PIE2: error: option '-mexecute-only' cannot be specified with '-fPIE' for the thumbv6m sub-architecture