diff --git a/clang/lib/Driver/ToolChains/Arch/RISCV.cpp b/clang/lib/Driver/ToolChains/Arch/RISCV.cpp
--- a/clang/lib/Driver/ToolChains/Arch/RISCV.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/RISCV.cpp
@@ -89,7 +89,7 @@
 
   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()) {
@@ -101,6 +101,16 @@
     }
   }
 
+  // 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)) {
diff --git a/clang/test/Driver/riscv-arch.c b/clang/test/Driver/riscv-arch.c
--- a/clang/test/Driver/riscv-arch.c
+++ b/clang/test/Driver/riscv-arch.c
@@ -361,6 +361,10 @@
 // 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'