[ARM] Pass subtarget feature "+dont-use-movt" instead of passing backend option "-arm-use-movt"

[ahatanak]

Summary

Currently, clang uses backend-option "-arm-use-movt=0" to disallow emitting movt/movw pairs to materialize 32-bit immediate values. This patch makes changes to pass subtarget feature "+dont-use-movt" instead to enable disallowing movt/movw pairs when doing LTO.

lib/Driver/Tools.cpp

static void getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   // decide whether to pass "+long-calls" based on the OS and its version.
   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))) {
       Features.push_back("+long-calls");
   }
+
+  // The kext linker doesn't know how to deal with movw/movt.
+  if (KernelOrKext)
+    Features.push_back("+dont-use-movt");
 }
 
 void Clang::AddARMTargetArgs(const ArgList &Args, ArgStringList &CmdArgs,
                              bool KernelOrKext) const {
   const Driver &D = getToolChain().getDriver();
   // Get the effective triple, which takes into account the deployment target.
   std::string TripleStr = getToolChain().ComputeEffectiveClangTriple(Args);
   llvm::Triple Triple(TripleStr);
if (FloatABI == "soft") {
     CmdArgs.push_back("-mfloat-abi");
     CmdArgs.push_back("hard");
   }
 
   // Kernel code has more strict alignment requirements.
   if (KernelOrKext) {
     CmdArgs.push_back("-backend-option");
     CmdArgs.push_back("-arm-strict-align");
-
-    // The kext linker doesn't know how to deal with movw/movt.
-    CmdArgs.push_back("-backend-option");
-    CmdArgs.push_back("-arm-use-movt=0");
   }
 
   // -mkernel implies -mstrict-align; don't add the redundant option.
   if (!KernelOrKext) {
     if (Arg *A = Args.getLastArg(options::OPT_mno_unaligned_access,
                                  options::OPT_munaligned_access)) {
       CmdArgs.push_back("-backend-option");
       if (A->getOption().matches(options::OPT_mno_unaligned_access))

test/CodeGen/arm-dont-use-movt.c

+// RUN: %clang_cc1 -triple thumbv7-apple-ios5  -target-feature +dont-use-movt -emit-llvm -o - %s | FileCheck -check-prefix=DONT-USE-MOVT %s
+// RUN: %clang_cc1 -triple thumbv7-apple-ios5 -emit-llvm -o - %s | FileCheck -check-prefix=USE-MOVT %s
+
+// DONT-USE-MOVT: attributes #0 = { {{.*}} "target-features"="+dont-use-movt"
+// USE-MOVT-NOT: attributes #0 = { {{.*}} "target-features"="+dont-use-movt"
+
+int foo1(int a) { return a; }

test/Driver/arm-dont-use-movt.c

+// RUN: %clang -target armv7-apple-darwin -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-DEFAULT
+
+// RUN: %clang -target armv7-apple-darwin -mkernel -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-KERNEL
+
+// CHECK-DEFAULT-NOT: "-target-feature" "+dont-use-movt"
+
+// CHECK-KERNEL: "-target-feature" "+dont-use-movt"