[RISCV] Support experimental v extensions.

[HsiangKai]

Summary

This follows the design as discussed on the mailing lists in the
following RFC: http://lists.llvm.org/pipermail/llvm-dev/2020-January/138364.html

Support for the vector 'v' extension v0.8.

[MaskRay]

clang/test/Driver/riscv-arch.c

364

For the RUN lines, I'd prefer the suggestion in https://reviews.llvm.org/D80838?id=267971#inline-744878

You replace -fsyntax-only to -c if it is too long.

379

Should there be a p7 test? Along with a p8 test, we can know that p8 is the split line.

[HsiangKai]

Address @MaskRay's comments.

[MaskRay]

LGTM.

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

 // If the extension is supported as experimental, return the version of that
 // extension that the compiler currently supports.
 static Optional<RISCVExtensionVersion>
 isExperimentalExtension(StringRef Ext) {
   if (Ext == "b" || Ext == "zbb" || Ext == "zbc" || Ext == "zbe" ||
       Ext == "zbf" || Ext == "zbm" || Ext == "zbp" || Ext == "zbr" ||
       Ext == "zbs" || Ext == "zbt" || Ext == "zbproposedc")
     return RISCVExtensionVersion{"0", "92"};
+  if (Ext == "v")
+    return RISCVExtensionVersion{"0", "8"};
   return None;
 }
 
 static bool isSupportedExtension(StringRef Ext) {
   // LLVM supports "z" extensions which are marked as experimental.
   if (isExperimentalExtension(Ext))
     return true;
 
case 'd':
       HasD = true;
       break;
     case 'c':
       Features.push_back("+c");
       break;
     case 'b':
       Features.push_back("+experimental-b");
       break;
+    case 'v':
+      Features.push_back("+experimental-v");
+      break;
     }
 
     // Consume full extension name and version, including any optional '_'
     // between this extension and the next
     ++I;
     I += Major.size();
     if (Minor.size())
       I += Minor.size() + 1 /*'p'*/;

clang/test/Driver/riscv-arch.c

 // RUN: %clang -target riscv32-unknown-elf -march=rv32izbb0p92 -menable-experimental-extensions -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-ZBB %s
 // RV32-EXPERIMENTAL-ZBB: "-target-feature" "+experimental-zbb"
 
 // RUN: %clang -target riscv32-unknown-elf -march=rv32izbb0p92_zbp0p92 -menable-experimental-extensions -### %s \
 // RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-ZBB-ZBP %s
 // RV32-EXPERIMENTAL-ZBB-ZBP: "-target-feature" "+experimental-zbb"
 // RV32-EXPERIMENTAL-ZBB-ZBP: "-target-feature" "+experimental-zbp"
+
+// RUN: %clang -target riscv32-unknown-elf -march=rv32iv -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-V-NOFLAG %s
+// RV32-EXPERIMENTAL-V-NOFLAG: error: invalid arch name 'rv32iv'
+// RV32-EXPERIMENTAL-V-NOFLAG: requires '-menable-experimental-extensions'
+
+// RUN: %clang -target riscv32-unknown-elf -march=rv32iv -menable-experimental-extensions -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-V-NOVERS %s
+// RV32-EXPERIMENTAL-V-NOVERS: error: invalid arch name 'rv32iv'
+// RV32-EXPERIMENTAL-V-NOVERS: experimental extension requires explicit version number
+
+// RUN: %clang -target riscv32-unknown-elf -march=rv32iv0p1 -menable-experimental-extensions -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-V-BADVERS %s
+// RV32-EXPERIMENTAL-V-BADVERS: error: invalid arch name 'rv32iv0p1'
+// RV32-EXPERIMENTAL-V-BADVERS: unsupported version number 0.1 for experimental extension
+
+// RUN: %clang -target riscv32-unknown-elf -march=rv32iv0p8 -menable-experimental-extensions -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-V-GOODVERS %s
+// RV32-EXPERIMENTAL-V-GOODVERS: "-target-feature" "+experimental-v"