[AArch64] Pass subtarget feature "+reserve-x18"

[ahatanak]

Summary

This patch changes clang driver to pass subtarget feature "+reserve-x18" instead of passing backend option "-aarch64-reserve-x18". This is needed since backend options do not make it to the backend when doing LTO.

[echristo]

Do we accept the inverse option here? (If not, why not?) If so, this will
add it in places it isn't supposed to go.

-eric

[ahatanak]

In D11462#210805, @echristo wrote:

Do we accept the inverse option here? (If not, why not?) If so, this will
add it in places it isn't supposed to go.

-eric

This part was committed in r227062 for ffixed-x18. I don't think adding an inverse option fno-fixed-x18 was discussed when the original patch was reviewed:

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20150119/121916.html

ffixed-r9 doesn't have an inverse option either.

[ahatanak]

It doesn't look like there was any particular reason for not defining an inverse option that cancels ffixed-x18, but we can add one in Options.td if it's important to do so.

[echristo]

Fine with me either way, just means that if you want to turn it off it'll require some more changes. I don't think it's a problem though.

cfe/trunk/lib/Driver/Tools.cpp

void Clang::AddAArch64TargetArgs(const ArgList &Args,
   if (Arg *A = Args.getLastArg(options::OPT_mglobal_merge,
                                options::OPT_mno_global_merge)) {
     CmdArgs.push_back("-backend-option");
     if (A->getOption().matches(options::OPT_mno_global_merge))
       CmdArgs.push_back("-aarch64-global-merge=false");
     else
       CmdArgs.push_back("-aarch64-global-merge=true");
   }
-
-  if (Args.hasArg(options::OPT_ffixed_x18)) {
-    CmdArgs.push_back("-backend-option");
-    CmdArgs.push_back("-aarch64-reserve-x18");
-  }
 }
 
 // Get CPU and ABI names. They are not independent
 // so we have to calculate them together.
 void mips::getMipsCPUAndABI(const ArgList &Args, const llvm::Triple &Triple,
                             StringRef &CPUName, StringRef &ABIName) {
   const char *DefMips32CPU = "mips32r2";
   const char *DefMips64CPU = "mips64r2";
static void getAArch64TargetFeatures(const Driver &D, const ArgList &Args,
 
   // En/disable crc
   if (Arg *A = Args.getLastArg(options::OPT_mcrc, options::OPT_mnocrc)) {
     if (A->getOption().matches(options::OPT_mcrc))
       Features.push_back("+crc");
     else
       Features.push_back("-crc");
   }
+
+  if (Args.hasArg(options::OPT_ffixed_x18))
+    Features.push_back("+reserve-x18");
 }
 
 static void getTargetFeatures(const Driver &D, const llvm::Triple &Triple,
                               const ArgList &Args, ArgStringList &CmdArgs,
                               bool ForAS) {
   std::vector<const char *> Features;
   switch (Triple.getArch()) {
   default: