[libunwind][RISCV] Support reading of VLENB CSR register

Support reading of VLENB (vector byte length) control register, that can be
required for correct unwinding of RVV objects on stack.

Differential Revision: https://reviews.llvm.org/D136264

libunwind/include/libunwind.h

@@ -1023,6 +1023,16 @@ enum {
   UNW_RISCV_F29 = 61,
   UNW_RISCV_F30 = 62,
   UNW_RISCV_F31 = 63,
+  // 65-95 -- Reserved for future standard extensions
+  // 96-127 -- v0-v31 (Vector registers)
+  // 128-3071 -- Reserved for future standard extensions
+  // 3072-4095 -- Reserved for custom extensions
+  // 4096-8191 -- CSRs
+  //
+  // VLENB CSR number: 0xC22 -- defined by section 3 of v-spec:
+  // https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#3-vector-extension-programmers-model
+  // VLENB DWARF number: 0x1000 + 0xC22
+  UNW_RISCV_VLENB = 0x1C22,
 };
 
 // VE register numbers

libunwind/src/Registers.hpp

@@ -4085,6 +4085,8 @@ inline bool Registers_riscv::validRegister(int regNum) const {
     return true;
   if (regNum < 0)
     return false;
+  if (regNum == UNW_RISCV_VLENB)
+    return true;
   if (regNum > UNW_RISCV_F31)
     return false;
   return true;
@@ -4099,6 +4101,11 @@ inline reg_t Registers_riscv::getRegister(int regNum) const {
     return 0;
   if ((regNum > 0) && (regNum < 32))
     return _registers[regNum];
+  if (regNum == UNW_RISCV_VLENB) {
+    reg_t vlenb;
+    __asm__("csrr %0, 0xC22" : "=r"(vlenb));
+    return vlenb;
+  }
   _LIBUNWIND_ABORT("unsupported riscv register");
 }
 
@@ -4250,6 +4257,8 @@ inline const char *Registers_riscv::getRegisterName(int regNum) {
     return "ft10";
   case UNW_RISCV_F31:
     return "ft11";
+  case UNW_RISCV_VLENB:
+    return "vlenb";
   default:
     return "unknown register";
   }

libunwind/test/unwind_scalable_vectors.pass.cpp

@@ -0,0 +1,50 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: linux && target={{riscv64-.+}}
+
+#undef NDEBUG
+#include <assert.h>
+#include <libunwind.h>
+
+// Check correct unwinding of frame with VLENB-sized objects (vector registers):
+// 1. Save return address (ra) in temporary register.
+// 2. Load VLENB (vector length in bytes) and substract it from current stack
+//    pointer (sp) - equivalent to one vector register on stack frame.
+// 3. Set DWARF cannonical frame address (CFA) to "sp + vlenb" expresssion so it
+//    can be correctly unwinded.
+// 4. Call stepper() function and check that 2 unwind steps are successful -
+//    from stepper() into foo() and from foo() into main().
+// 5. Restore stack pointer and return address.
+__attribute__((naked)) static void foo() {
+  __asm__(".cfi_startproc\n"
+          "mv s0, ra\n"
+          "csrr  s1, vlenb\n"
+          "sub sp, sp, s1\n"
+          "# .cfi_def_cfa_expression sp + vlenb\n"
+          ".cfi_escape 0x0f, 0x07, 0x72, 0x00, 0x92, 0xa2, 0x38, 0x00, 0x22\n"
+          "call stepper\n"
+          "add sp, sp, s1\n"
+          "mv ra, s0\n"
+          "ret\n"
+          ".cfi_endproc\n");
+}
+
+extern "C" void stepper() {
+  unw_cursor_t cursor;
+  unw_context_t uc;
+  unw_getcontext(&uc);
+  unw_init_local(&cursor, &uc);
+  // Stepping into foo() should succeed.
+  assert(unw_step(&cursor) > 0);
+  // Stepping past foo() should succeed, too.
+  assert(unw_step(&cursor) > 0);
+}
+
+int main() { foo(); }