[RISC-V] Add -msave-restore and -mno-save-restore to clang driver

[lenary]

Summary

The GCC RISC-V toolchain accepts -msave-restore and -mno-save-restore
to control whether libcalls are used for saving and restoring the stack within
prologues and epilogues.

Clang currently errors if someone passes -msave-restore or -mno-save-restore.
This means that people need to change build configurations to use clang. This
patch adds these flags, so that clang invocations can now match gcc.

As the RISC-V backend does not currently have a save-restore target feature,
we emit a warning if someone requests -msave-restore. LLVM does not error if
we pass the (unimplemented) target features +save-restore or -save-restore.

[asb]

Can you add a CHECK line that shows the expected warning is emitted?

[lenary]

• Add CHECK lines for warnings Hopefully these will work regardless of race conditions in the merging of stdout and stderr.

[asb]

LGTM, thanks! There was some discussion about the potential stdout+stderr merging issues http://lists.llvm.org/pipermail/llvm-dev/2017-February/110469.html but I don't think it resulted in any new recommended best practice.

clang/include/clang/Driver/Options.td

def mspeculative_load_hardening : Flag<["-"], "mspeculative-load-hardening">,
   Group<m_Group>, Flags<[CoreOption,CC1Option]>;
 def mno_speculative_load_hardening : Flag<["-"], "mno-speculative-load-hardening">,
   Group<m_Group>, Flags<[CoreOption]>;
 
 def mrelax : Flag<["-"], "mrelax">, Group<m_riscv_Features_Group>,
   HelpText<"Enable linker relaxation">;
 def mno_relax : Flag<["-"], "mno-relax">, Group<m_riscv_Features_Group>,
   HelpText<"Disable linker relaxation">;
+def msave_restore : Flag<["-"], "msave-restore">, Group<m_riscv_Features_Group>,
+  HelpText<"Enable using library calls for save and restore">;
+def mno_save_restore : Flag<["-"], "mno-save-restore">, Group<m_riscv_Features_Group>,
+  HelpText<"Disable using library calls for save and restore">;
 
 def munaligned_access : Flag<["-"], "munaligned-access">, Group<m_arm_Features_Group>,
   HelpText<"Allow memory accesses to be unaligned (AArch32/AArch64 only)">;
 def mno_unaligned_access : Flag<["-"], "mno-unaligned-access">, Group<m_arm_Features_Group>,
   HelpText<"Force all memory accesses to be aligned (AArch32/AArch64 only)">;
 def mstrict_align : Flag<["-"], "mstrict-align">, Alias<mno_unaligned_access>, Flags<[CC1Option,HelpHidden]>,
   HelpText<"Force all memory accesses to be aligned (same as mno-unaligned-access)">;
 def mno_thumb : Flag<["-"], "mno-thumb">, Group<m_arm_Features_Group>;

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

void riscv::getRISCVTargetFeatures(const Driver &D, const ArgList &Args,
   }
 
   // -mrelax is default, unless -mno-relax is specified.
   if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true))
     Features.push_back("+relax");
   else
     Features.push_back("-relax");
 
+  // -mno-save-restore is default, unless -msave-restore is specified.
+  if (Args.hasFlag(options::OPT_msave_restore, options::OPT_mno_save_restore, false)) {
+    Features.push_back("+save-restore");
+    // ... but we don't yet support +save-restore, so issue a warning.
+    D.Diag(diag::warn_drv_clang_unsupported)
+      << Args.getLastArg(options::OPT_msave_restore)->getAsString(Args);
+  } else {
+    Features.push_back("-save-restore");
+  }
+
   // Now add any that the user explicitly requested on the command line,
   // which may override the defaults.
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_riscv_Features_Group);
 }
 
 StringRef riscv::getRISCVABI(const ArgList &Args, const llvm::Triple &Triple) {
   if (Arg *A = Args.getLastArg(options::OPT_mabi_EQ))
     return A->getValue();
 
   return Triple.getArch() == llvm::Triple::riscv32 ? "ilp32" : "lp64";
 }

clang/test/Driver/riscv-features.c

 // RUN: %clang -target riscv32-unknown-elf -### %s -fsyntax-only 2>&1 | FileCheck %s
 // RUN: %clang -target riscv64-unknown-elf -### %s -fsyntax-only 2>&1 | FileCheck %s
 
 // CHECK: fno-signed-char
 
+// RUN: %clang -target riscv32-unknown-elf -### %s 2>&1 | FileCheck %s -check-prefix=DEFAULT
+
 // RUN: %clang -target riscv32-unknown-elf -### %s -mrelax 2>&1 | FileCheck %s -check-prefix=RELAX
 // RUN: %clang -target riscv32-unknown-elf -### %s -mno-relax 2>&1 | FileCheck %s -check-prefix=NO-RELAX
-// RUN: %clang -target riscv32-unknown-elf -### %s 2>&1 | FileCheck %s -check-prefix=DEFAULT
 
 // RELAX: "-target-feature" "+relax"
 // NO-RELAX: "-target-feature" "-relax"
 // DEFAULT: "-target-feature" "+relax"
 // DEFAULT-NOT: "-target-feature" "-relax"
+
+// RUN: %clang -target riscv32-unknown-elf -### %s -msave-restore 2>&1 | FileCheck %s -check-prefix=SAVE-RESTORE
+// RUN: %clang -target riscv32-unknown-elf -### %s -mno-save-restore 2>&1 | FileCheck %s -check-prefix=NO-SAVE-RESTORE
+
+// SAVE-RESTORE: "-target-feature" "+save-restore"
+// NO-SAVE-RESTORE: "-target-feature" "-save-restore"
+// DEFAULT: "-target-feature" "-save-restore"
+// DEFAULT-NOT: "-target-feature" "+save-restore"