[mips] Support implicit gpopt with N64 when using -fno-pic

[sdardis]

Summary

As clang defaults to -mno-abicalls when using -fno-pic for N64, implicitly use -mgpopt in that case.

[atanasyan]

LGTM

[sdardis]

Thanks for the review and spotting this bug that lead me to finding the other bug.

cfe/trunk/lib/Driver/ToolChains/Clang.cpp

void Clang::AddMIPSTargetArgs(const ArgList &Args,
 
   // -mabicalls is the default for many MIPS environments, even with -fno-pic.
   // -mgpopt is the default for static, -fno-pic environments but these two
   // options conflict. We want to be certain that -mno-abicalls -mgpopt is
   // the only case where -mllvm -mgpopt is passed.
   // NOTE: We need a warning here or in the backend to warn when -mgpopt is
   //       passed explicitly when compiling something with -mabicalls
   //       (implictly) in affect. Currently the warning is in the backend.
+  //
+  // When the ABI in use is  N64, we also need to determine the PIC mode that
+  // is in use, as -fno-pic for N64 implies -mno-abicalls.
   bool NoABICalls =
       ABICalls && ABICalls->getOption().matches(options::OPT_mno_abicalls);
+
+  llvm::Reloc::Model RelocationModel;
+  unsigned PICLevel;
+  bool IsPIE;
+  std::tie(RelocationModel, PICLevel, IsPIE) =
+      ParsePICArgs(getToolChain(), Args);
+
+  NoABICalls = NoABICalls ||
+               (RelocationModel == llvm::Reloc::Static && ABIName == "n64");
+
   bool WantGPOpt = GPOpt && GPOpt->getOption().matches(options::OPT_mgpopt);
   // We quietly ignore -mno-gpopt as the backend defaults to -mno-gpopt.
   if (NoABICalls && (!GPOpt || WantGPOpt)) {
     CmdArgs.push_back("-mllvm");
     CmdArgs.push_back("-mgpopt");
 
     Arg *LocalSData = Args.getLastArg(options::OPT_mlocal_sdata,
                                       options::OPT_mno_local_sdata);

cfe/trunk/test/Driver/mips-features.c

 // RUN:   | FileCheck --check-prefix=CHECK-MNOEMBEDDEDDATA %s
 // CHECK-MNOEMBEDDEDDATA: "-mllvm" "-membedded-data=0"
 //
 // -mno-abicalls -mgpopt
 // RUN: %clang -target mips-linux-gnu -### -c %s -mno-abicalls -mgpopt 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-MEMBEDDEDDATADEF %s
 // CHECK-MEMBEDDEDDATADEF-NOT: "-mllvm" "-membedded-data"
 //
+// MIPS64 + N64: -fno-pic -> -mno-abicalls -mgpopt
+// RUN: %clang -target mips64-mti-elf -mabi=64 -### -c %s -fno-pic 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-N64-GPOPT %s
+// CHECK-N64-GPOPT: "-target-feature" "+noabicalls"
+// CHECK-N64-GPOPT: "-mllvm" "-mgpopt"
+//
+// MIPS64 + N64: -fno-pic -mno-gpopt
+// RUN: %clang -target mips64-mti-elf -mabi=64 -### -c %s -fno-pic -mno-gpopt 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-N64-MNO-GPOPT %s
+// CHECK-N64-MNO-GPOPT: "-target-feature" "+noabicalls"
+// CHECK-N64-MNO-GPOPT-NOT: "-mllvm" "-mgpopt"
+//
+// MIPS64 + N64: -mgpopt (-fpic is implicit)
+// RUN: %clang -target mips64-mti-linux-gnu -mabi=64 -### -c %s -mgpopt 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-N64-PIC-GPOPT %s
+// CHECK-N64-PIC-GPOPT-NOT: "-mllvm" "-mgpopt"
+// CHECK-N64-PIC-GPOPT: ignoring '-mgpopt' option as it cannot be used with the implicit usage of -mabicalls
+//
 // -mips16
 // RUN: %clang -target mips-linux-gnu -### -c %s \
 // RUN:     -mno-mips16 -mips16 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS16 %s
 // CHECK-MIPS16: "-target-feature" "+mips16"
 //
 // -mno-mips16
 // RUN: %clang -target mips-linux-gnu -### -c %s \