gdb/riscv: Improve support for matching against target descriptions

For the RISC-V target it is desirable if the three floating pointer
status CSRs fflags, frm, and fcsr can be placed into either the FPU
feature or the CSR feature.  This allows different targets to build
the features in a way that better reflects their target.

The change to support this within GDB is fairly simple, so this is
done in this commit, and some tests added to check this new
functionality.

gdb/ChangeLog:

	* riscv-tdep.c (value_of_riscv_user_reg): Moved to here from later
	in the file.
	(class riscv_pending_register_alias): Likewise.
	(riscv_register_feature::register_info): Change 'required_p' field
	to 'required', and change its type.  Add 'check' member function.
	(riscv_register_feature::register_info::check): Define new member
	function.
	(riscv_xreg_feature): Change initialisation of 'required' field.
	(riscv_freg_feature): Likewise.
	(riscv_virtual_feature): Likewise.
	(riscv_csr_feature): Likewise.
	(riscv_check_tdesc_feature): Take extra parameter, the csr
	tdesc_feature, rewrite the function to use the new
	riscv_register_feature::register_info::check function.
	(riscv_gdbarch_init): Pass the csr tdesc_feature where needed.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-tdesc-loading-01.xml: New file.
	* gdb.arch/riscv-tdesc-loading-02.xml: New file.
	* gdb.arch/riscv-tdesc-loading-03.xml: New file.
	* gdb.arch/riscv-tdesc-loading-04.xml: New file.
	* gdb.arch/riscv-tdesc-loading.exp: New file.

1 files changed, 193 insertions, 142 deletions

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 86ab3c3..b1dd79f 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -101,6 +101,66 @@ struct riscv_unwind_cache
 
 static reggroup *csr_reggroup = NULL;
 
+/* Callback function for user_reg_add.  */
+
+static struct value *
+value_of_riscv_user_reg (struct frame_info *frame, const void *baton)
+{
+  const int *reg_p = (const int *) baton;
+  return value_of_register (*reg_p, frame);
+}
+
+/* Information about a register alias that needs to be set up for this
+   target.  These are collected when the target's XML description is
+   analysed, and then processed later, once the gdbarch has been created.  */
+
+class riscv_pending_register_alias
+{
+public:
+  /* Constructor.  */
+
+  riscv_pending_register_alias (const char *name, const void *baton)
+    : m_name (name),
+      m_baton (baton)
+  { /* Nothing.  */ }
+
+  /* Convert this into a user register for GDBARCH.  */
+
+  void create (struct gdbarch *gdbarch) const
+  {
+    user_reg_add (gdbarch, m_name, value_of_riscv_user_reg, m_baton);
+  }
+
+private:
+  /* The name for this alias.  */
+  const char *m_name;
+
+  /* The baton value for passing to user_reg_add.  This must point to some
+     data that will live for at least as long as the gdbarch object to
+     which the user register is attached.  */
+  const void *m_baton;
+};
+
+/* Registers in the RISCV_REGISTER_FEATURE lists below are either optional,
+   or required.  For example the $pc register is always going to be a
+   required register, you can't do much debugging without that.  In
+   contrast, most of the CSRs are optional, GDB doesn't require them in
+   order to have a useful debug session.  This enum models the difference
+   between these register types.  */
+
+enum riscv_register_required_status
+{
+  /* This register is optional within this feature.  */
+  RISCV_REG_OPTIONAL,
+
+  /* This register is required within this feature.  */
+  RISCV_REG_REQUIRED,
+
+  /* This register is required, the register must either be in this
+     feature, or it could appear within the CSR feature.  */
+  RISCV_REG_REQUIRED_MAYBE_CSR
+};
+
 /* A set of registers that we expect to find in a tdesc_feature.  These
    are use in RISCV_GDBARCH_INIT when processing the target description.  */
 
@@ -117,8 +177,18 @@ struct riscv_register_feature
        register.  */
     std::vector<const char *> names;
 
-    /* When true this register is required in this feature set.  */
-    bool required_p;
+    /* Is this register required within this feature?  In some cases the
+       register could be required, but might also be in the CSR feature.  */
+    riscv_register_required_status required;
+
+    /* Look in FEATURE for a register with a name from this classes names
+       list.  If the register is found then register its number with
+       TDESC_DATA and add all its aliases to the ALIASES list.  REG_SET is
+       used to help create the aliases.  */
+    bool check (struct tdesc_arch_data *tdesc_data,
+		const struct tdesc_feature *feature,
+		const struct riscv_register_feature *reg_set,
+		std::vector<riscv_pending_register_alias> *aliases) const;
   };
 
   /* The name for this feature.  This is the name used to find this feature
@@ -144,45 +214,79 @@ struct riscv_register_feature
   std::vector<struct register_info> registers;
 };
 
+/* See description in the class declaration above.  */
+
+bool
+riscv_register_feature::register_info::check
+	(struct tdesc_arch_data *tdesc_data,
+	 const struct tdesc_feature *feature,
+	 const struct riscv_register_feature *reg_set,
+	 std::vector<riscv_pending_register_alias> *aliases) const
+{
+  for (const char *name : this->names)
+    {
+      bool found = tdesc_numbered_register (feature, tdesc_data,
+					    this->regnum, name);
+      if (found)
+	{
+	  /* We know that the target description mentions this
+	     register.  In RISCV_REGISTER_NAME we ensure that GDB
+	     always uses the first name for each register, so here we
+	     add aliases for all of the remaining names.  */
+	  bool prefer_first_name = reg_set->prefer_first_name;
+	  int start_index = prefer_first_name ? 1 : 0;
+	  for (int i = start_index; i < this->names.size (); ++i)
+	    {
+	      const char *alias = this->names[i];
+	      if (alias == name && !prefer_first_name)
+		continue;
+	      aliases->emplace_back (alias, (void *) &this->regnum);
+	    }
+	  return true;
+	}
+    }
+  return false;
+}
+
 /* The general x-registers feature set.  */
 
 static const struct riscv_register_feature riscv_xreg_feature =
 {
  "org.gnu.gdb.riscv.cpu", true,
  {
-   { RISCV_ZERO_REGNUM + 0, { "zero", "x0" }, true },
-   { RISCV_ZERO_REGNUM + 1, { "ra", "x1" }, true },
-   { RISCV_ZERO_REGNUM + 2, { "sp", "x2" }, true },
-   { RISCV_ZERO_REGNUM + 3, { "gp", "x3" }, true },
-   { RISCV_ZERO_REGNUM + 4, { "tp", "x4" }, true },
-   { RISCV_ZERO_REGNUM + 5, { "t0", "x5" }, true },
-   { RISCV_ZERO_REGNUM + 6, { "t1", "x6" }, true },
-   { RISCV_ZERO_REGNUM + 7, { "t2", "x7" }, true },
-   { RISCV_ZERO_REGNUM + 8, { "fp", "x8", "s0" }, true },
-   { RISCV_ZERO_REGNUM + 9, { "s1", "x9" }, true },
-   { RISCV_ZERO_REGNUM + 10, { "a0", "x10" }, true },
-   { RISCV_ZERO_REGNUM + 11, { "a1", "x11" }, true },
-   { RISCV_ZERO_REGNUM + 12, { "a2", "x12" }, true },
-   { RISCV_ZERO_REGNUM + 13, { "a3", "x13" }, true },
-   { RISCV_ZERO_REGNUM + 14, { "a4", "x14" }, true },
-   { RISCV_ZERO_REGNUM + 15, { "a5", "x15" }, true },
-   { RISCV_ZERO_REGNUM + 16, { "a6", "x16" }, true },
-   { RISCV_ZERO_REGNUM + 17, { "a7", "x17" }, true },
-   { RISCV_ZERO_REGNUM + 18, { "s2", "x18" }, true },
-   { RISCV_ZERO_REGNUM + 19, { "s3", "x19" }, true },
-   { RISCV_ZERO_REGNUM + 20, { "s4", "x20" }, true },
-   { RISCV_ZERO_REGNUM + 21, { "s5", "x21" }, true },
-   { RISCV_ZERO_REGNUM + 22, { "s6", "x22" }, true },
-   { RISCV_ZERO_REGNUM + 23, { "s7", "x23" }, true },
-   { RISCV_ZERO_REGNUM + 24, { "s8", "x24" }, true },
-   { RISCV_ZERO_REGNUM + 25, { "s9", "x25" }, true },
-   { RISCV_ZERO_REGNUM + 26, { "s10", "x26" }, true },
-   { RISCV_ZERO_REGNUM + 27, { "s11", "x27" }, true },
-   { RISCV_ZERO_REGNUM + 28, { "t3", "x28" }, true },
-   { RISCV_ZERO_REGNUM + 29, { "t4", "x29" }, true },
-   { RISCV_ZERO_REGNUM + 30, { "t5", "x30" }, true },
-   { RISCV_ZERO_REGNUM + 31, { "t6", "x31" }, true },
-   { RISCV_ZERO_REGNUM + 32, { "pc" }, true }
+   { RISCV_ZERO_REGNUM + 0, { "zero", "x0" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 1, { "ra", "x1" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 2, { "sp", "x2" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 3, { "gp", "x3" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 4, { "tp", "x4" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 5, { "t0", "x5" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 6, { "t1", "x6" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 7, { "t2", "x7" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 8, { "fp", "x8", "s0" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 9, { "s1", "x9" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 10, { "a0", "x10" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 11, { "a1", "x11" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 12, { "a2", "x12" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 13, { "a3", "x13" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 14, { "a4", "x14" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 15, { "a5", "x15" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 16, { "a6", "x16" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 17, { "a7", "x17" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 18, { "s2", "x18" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 19, { "s3", "x19" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 20, { "s4", "x20" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 21, { "s5", "x21" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 22, { "s6", "x22" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 23, { "s7", "x23" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 24, { "s8", "x24" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 25, { "s9", "x25" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 26, { "s10", "x26" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 27, { "s11", "x27" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 28, { "t3", "x28" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 29, { "t4", "x29" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 30, { "t5", "x30" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 31, { "t6", "x31" }, RISCV_REG_REQUIRED },
+   { RISCV_ZERO_REGNUM + 32, { "pc" }, RISCV_REG_REQUIRED }
  }
 };
 
@@ -192,42 +296,42 @@ static const struct riscv_register_feature riscv_freg_feature =
 {
  "org.gnu.gdb.riscv.fpu", true,
  {
-   { RISCV_FIRST_FP_REGNUM + 0, { "ft0", "f0" }, true },
-   { RISCV_FIRST_FP_REGNUM + 1, { "ft1", "f1" }, true },
-   { RISCV_FIRST_FP_REGNUM + 2, { "ft2", "f2" }, true },
-   { RISCV_FIRST_FP_REGNUM + 3, { "ft3", "f3" }, true },
-   { RISCV_FIRST_FP_REGNUM + 4, { "ft4", "f4" }, true },
-   { RISCV_FIRST_FP_REGNUM + 5, { "ft5", "f5" }, true },
-   { RISCV_FIRST_FP_REGNUM + 6, { "ft6", "f6" }, true },
-   { RISCV_FIRST_FP_REGNUM + 7, { "ft7", "f7" }, true },
-   { RISCV_FIRST_FP_REGNUM + 8, { "fs0", "f8" }, true },
-   { RISCV_FIRST_FP_REGNUM + 9, { "fs1", "f9" }, true },
-   { RISCV_FIRST_FP_REGNUM + 10, { "fa0", "f10" }, true },
-   { RISCV_FIRST_FP_REGNUM + 11, { "fa1", "f11" }, true },
-   { RISCV_FIRST_FP_REGNUM + 12, { "fa2", "f12" }, true },
-   { RISCV_FIRST_FP_REGNUM + 13, { "fa3", "f13" }, true },
-   { RISCV_FIRST_FP_REGNUM + 14, { "fa4", "f14" }, true },
-   { RISCV_FIRST_FP_REGNUM + 15, { "fa5", "f15" }, true },
-   { RISCV_FIRST_FP_REGNUM + 16, { "fa6", "f16" }, true },
-   { RISCV_FIRST_FP_REGNUM + 17, { "fa7", "f17" }, true },
-   { RISCV_FIRST_FP_REGNUM + 18, { "fs2", "f18" }, true },
-   { RISCV_FIRST_FP_REGNUM + 19, { "fs3", "f19" }, true },
-   { RISCV_FIRST_FP_REGNUM + 20, { "fs4", "f20" }, true },
-   { RISCV_FIRST_FP_REGNUM + 21, { "fs5", "f21" }, true },
-   { RISCV_FIRST_FP_REGNUM + 22, { "fs6", "f22" }, true },
-   { RISCV_FIRST_FP_REGNUM + 23, { "fs7", "f23" }, true },
-   { RISCV_FIRST_FP_REGNUM + 24, { "fs8", "f24" }, true },
-   { RISCV_FIRST_FP_REGNUM + 25, { "fs9", "f25" }, true },
-   { RISCV_FIRST_FP_REGNUM + 26, { "fs10", "f26" }, true },
-   { RISCV_FIRST_FP_REGNUM + 27, { "fs11", "f27" }, true },
-   { RISCV_FIRST_FP_REGNUM + 28, { "ft8", "f28" }, true },
-   { RISCV_FIRST_FP_REGNUM + 29, { "ft9", "f29" }, true },
-   { RISCV_FIRST_FP_REGNUM + 30, { "ft10", "f30" }, true },
-   { RISCV_FIRST_FP_REGNUM + 31, { "ft11", "f31" }, true },
-
-   { RISCV_CSR_FFLAGS_REGNUM, { "fflags" }, true },
-   { RISCV_CSR_FRM_REGNUM, { "frm" }, true },
-   { RISCV_CSR_FCSR_REGNUM, { "fcsr" }, true },
+   { RISCV_FIRST_FP_REGNUM + 0, { "ft0", "f0" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 1, { "ft1", "f1" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 2, { "ft2", "f2" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 3, { "ft3", "f3" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 4, { "ft4", "f4" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 5, { "ft5", "f5" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 6, { "ft6", "f6" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 7, { "ft7", "f7" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 8, { "fs0", "f8" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 9, { "fs1", "f9" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 10, { "fa0", "f10" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 11, { "fa1", "f11" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 12, { "fa2", "f12" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 13, { "fa3", "f13" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 14, { "fa4", "f14" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 15, { "fa5", "f15" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 16, { "fa6", "f16" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 17, { "fa7", "f17" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 18, { "fs2", "f18" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 19, { "fs3", "f19" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 20, { "fs4", "f20" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 21, { "fs5", "f21" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 22, { "fs6", "f22" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 23, { "fs7", "f23" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 24, { "fs8", "f24" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 25, { "fs9", "f25" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 26, { "fs10", "f26" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 27, { "fs11", "f27" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 28, { "ft8", "f28" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 29, { "ft9", "f29" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 30, { "ft10", "f30" }, RISCV_REG_REQUIRED },
+   { RISCV_FIRST_FP_REGNUM + 31, { "ft11", "f31" }, RISCV_REG_REQUIRED },
+
+   { RISCV_CSR_FFLAGS_REGNUM, { "fflags", "csr1" }, RISCV_REG_REQUIRED_MAYBE_CSR },
+   { RISCV_CSR_FRM_REGNUM, { "frm", "csr2" }, RISCV_REG_REQUIRED_MAYBE_CSR },
+   { RISCV_CSR_FCSR_REGNUM, { "fcsr", "csr3" }, RISCV_REG_REQUIRED_MAYBE_CSR },
 
  }
 };
@@ -242,7 +346,7 @@ static const struct riscv_register_feature riscv_virtual_feature =
 {
  "org.gnu.gdb.riscv.virtual", false,
  {
-   { RISCV_PRIV_REGNUM, { "priv" }, false }
+   { RISCV_PRIV_REGNUM, { "priv" }, RISCV_REG_OPTIONAL }
  }
 };
 
@@ -255,7 +359,7 @@ static struct riscv_register_feature riscv_csr_feature =
  "org.gnu.gdb.riscv.csr", false,
  {
 #define DECLARE_CSR(NAME,VALUE,CLASS,DEFINE_VER,ABORT_VER) \
-  { RISCV_ ## VALUE ## _REGNUM, { # NAME }, false },
+  { RISCV_ ## VALUE ## _REGNUM, { # NAME }, RISCV_REG_OPTIONAL },
 #include "opcode/riscv-opc.h"
 #undef DECLARE_CSR
  }
@@ -475,15 +579,6 @@ riscv_sw_breakpoint_from_kind (struct gdbarch *gdbarch, int kind, int *size)
     }
 }
 
-/* Callback function for user_reg_add.  */
-
-static struct value *
-value_of_riscv_user_reg (struct frame_info *frame, const void *baton)
-{
-  const int *reg_p = (const int *) baton;
-  return value_of_register (*reg_p, frame);
-}
-
 /* Implement the register_name gdbarch method.  This is used instead of
    the function supplied by calling TDESC_USE_REGISTERS so that we can
    ensure the preferred names are offered for x-regs and f-regs.  */
@@ -2965,37 +3060,6 @@ riscv_find_default_target_description (const struct gdbarch_info info)
   return riscv_lookup_target_description (features);
 }
 
-/* Information about a register alias that needs to be set up for this
-   target.  These are collected when the target's XML description is
-   analysed, and then processed later, once the gdbarch has been created.  */
-
-class riscv_pending_register_alias
-{
-public:
-  /* Constructor.  */
-
-  riscv_pending_register_alias (const char *name, const void *baton)
-    : m_name (name),
-      m_baton (baton)
-  { /* Nothing.  */ }
-
-  /* Convert this into a user register for GDBARCH.  */
-
-  void create (struct gdbarch *gdbarch) const
-  {
-    user_reg_add (gdbarch, m_name, value_of_riscv_user_reg, m_baton);
-  }
-
-private:
-  /* The name for this alias.  */
-  const char *m_name;
-
-  /* The baton value for passing to user_reg_add.  This must point to some
-     data that will live for at least as long as the gdbarch object to
-     which the user register is attached.  */
-  const void *m_baton;
-};
-
 /* All of the registers in REG_SET are checked for in FEATURE, TDESC_DATA
    is updated with the register numbers for each register as listed in
    REG_SET.  If any register marked as required in REG_SET is not found in
@@ -3003,40 +3067,26 @@ private:
 
 static bool
 riscv_check_tdesc_feature (struct tdesc_arch_data *tdesc_data,
-                           const struct tdesc_feature *feature,
+                           const struct tdesc_feature *main_feature,
+                           const struct tdesc_feature *csr_feature,
                            const struct riscv_register_feature *reg_set,
                            std::vector<riscv_pending_register_alias> *aliases)
 {
   for (const auto &reg : reg_set->registers)
     {
-      bool found = false;
+      bool found = reg.check (tdesc_data, main_feature, reg_set, aliases);
 
-      for (const char *name : reg.names)
+      if (!found && reg.required != RISCV_REG_OPTIONAL)
 	{
-	  found = tdesc_numbered_register (feature, tdesc_data, reg.regnum,
-					   name);
-
-	  if (found)
-            {
-              /* We know that the target description mentions this
-                 register.  In RISCV_REGISTER_NAME we ensure that GDB
-                 always uses the first name for each register, so here we
-                 add aliases for all of the remaining names.  */
-	      bool prefer_first_name = reg_set->prefer_first_name;
-	      int start_index = prefer_first_name ? 1 : 0;
-	      for (int i = start_index; i < reg.names.size (); ++i)
-                {
-		  const char *alias = reg.names[i];
-                  if (alias == name && !prefer_first_name)
-		    continue;
-		  aliases->emplace_back (alias, (void *) &reg.regnum);
-                }
-              break;
-            }
+	  if (reg.required == RISCV_REG_REQUIRED_MAYBE_CSR
+	      && csr_feature != nullptr)
+	    {
+	      gdb_assert (main_feature != csr_feature);
+	      found = reg.check (tdesc_data, csr_feature,  reg_set, aliases);
+	    }
+	  if (!found)
+	    return false;
 	}
-
-      if (!found && reg.required_p)
-	return false;
     }
 
   return true;
@@ -3166,7 +3216,7 @@ riscv_gdbarch_init (struct gdbarch_info info,
   std::vector<riscv_pending_register_alias> pending_aliases;
 
   bool valid_p = riscv_check_tdesc_feature (tdesc_data,
-                                            feature_cpu,
+                                            feature_cpu, feature_csr,
                                             &riscv_xreg_feature,
                                             &pending_aliases);
   if (valid_p)
@@ -3188,6 +3238,7 @@ riscv_gdbarch_init (struct gdbarch_info info,
   if (feature_fpu != NULL)
     {
       valid_p &= riscv_check_tdesc_feature (tdesc_data, feature_fpu,
+					    feature_csr,
                                             &riscv_freg_feature,
                                             &pending_aliases);
 
@@ -3224,12 +3275,12 @@ riscv_gdbarch_init (struct gdbarch_info info,
     }
 
   if (feature_virtual)
-    riscv_check_tdesc_feature (tdesc_data, feature_virtual,
+    riscv_check_tdesc_feature (tdesc_data, feature_virtual, feature_csr,
                                &riscv_virtual_feature,
                                &pending_aliases);
 
   if (feature_csr)
-    riscv_check_tdesc_feature (tdesc_data, feature_csr,
+    riscv_check_tdesc_feature (tdesc_data, feature_csr, nullptr,
                                &riscv_csr_feature,
                                &pending_aliases);