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

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.

Differential Revision: https://reviews.llvm.org/D157620

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -338,6 +338,8 @@ 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<

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

@@ -847,7 +847,13 @@ llvm::ARM::FPUKind arm::getARMTargetFeatures(const Driver &D,
             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");

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

@@ -20,3 +20,27 @@
 // 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