[RISCV] Add error checking for extensions missing separating underscores

[simoncook]

Summary

Currently if two multi-letter extensions are provided in a -march=
string, the verification code checks the version of the first and
consumes the second, resulting in that part of the architecture
string being ignored. This adds a test that when a version number has
been parsed for an extension, there are no subsequent characters.

[asb]

LGTM, thanks!

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

 static bool getExtensionVersion(const Driver &D, const ArgList &Args,
                                 StringRef MArch, StringRef Ext, StringRef In,
                                 std::string &Major, std::string &Minor) {
   Major = std::string(In.take_while(isDigit));
   In = In.substr(Major.size());
 
   if (Major.size() && In.consume_front("p")) {
     Minor = std::string(In.take_while(isDigit));
-    In = In.substr(Major.size());
+    In = In.substr(Major.size() + 1);
 
     // Expected 'p' to be followed by minor version number.
     if (Minor.empty()) {
       std::string Error =
         "minor version number missing after 'p' for extension";
       D.Diag(diag::err_drv_invalid_riscv_ext_arch_name)
         << MArch << Error << Ext;
       return false;
     }
   }
 
+  // Expected multi-character extension with version number to have no
+  // subsequent characters (i.e. must either end string or be followed by
+  // an underscore).
+  if (Ext.size() > 1 && In.size()) {
+    std::string Error =
+        "multi-character extensions must be separated by underscores";
+    D.Diag(diag::err_drv_invalid_riscv_ext_arch_name) << MArch << Error << In;
+    return false;
+  }
+
   // If experimental extension, require use of current version number number
   if (auto ExperimentalExtension = isExperimentalExtension(Ext)) {
     if (!Args.hasArg(options::OPT_menable_experimental_extensions)) {
       std::string Error =
           "requires '-menable-experimental-extensions' for experimental extension";
       D.Diag(diag::err_drv_invalid_riscv_ext_arch_name)
           << MArch << Error << Ext;
       return false;

clang/test/Driver/riscv-arch.c

 // 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=rv32izbb0p92zbp0p92 -menable-experimental-extensions -### %s \
+// RUN: -fsyntax-only 2>&1 | FileCheck -check-prefix=RV32-EXPERIMENTAL-ZBB-ZBP-UNDERSCORE %s
+// RV32-EXPERIMENTAL-ZBB-ZBP-UNDERSCORE: error: invalid arch name 'rv32izbb0p92zbp0p92', multi-character extensions must be separated by underscores
+
 // RUN: %clang -target riscv32-unknown-elf -march=rv32iv -### %s -c 2>&1 | \
 // RUN:   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 -c 2>&1 | \
 // RUN:   FileCheck -check-prefix=RV32-EXPERIMENTAL-V-NOVERS %s
 // RV32-EXPERIMENTAL-V-NOVERS: error: invalid arch name 'rv32iv'