[PATCH 2/2] gdb: LoongArch: Support reg aliases in info reg command

[Hui Li <lihui@loongson.cn>]

According to LoongArch ELF ABI specification [1], support the register
aliases in "info register" command.

Without this patch:
```
(gdb) info reg a0
Invalid register `a0'

```
With this patch:

```
(gdb) info reg a0

a0             0x1                 1

```
[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html#_register_convention

Signed-off-by: Hui Li <lihui@loongson.cn>
---
 gdb/loongarch-tdep.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/gdb/loongarch-tdep.c b/gdb/loongarch-tdep.c
index 67ea5494d90..f5ddad0ba65 100644
--- a/gdb/loongarch-tdep.c
+++ b/gdb/loongarch-tdep.c
@@ -387,6 +387,14 @@ loongarch_software_single_step (struct regcache *regcache)
   return {next_pc};
 }
 
+/* Callback function for user_reg_add.  */
+
+static struct value *
+value_of_loongarch_user_reg (frame_info_ptr frame, const void *baton)
+{
+  return value_of_register ((long long) baton, frame);
+}
+
 /* Implement the frame_align gdbarch method.  */
 
 static CORE_ADDR
@@ -1589,6 +1597,19 @@ loongarch_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   info.target_desc = tdesc;
   info.tdesc_data = tdesc_data.get ();
 
+  for (int i = 0; i < ARRAY_SIZE (loongarch_r_lp64_name); ++i)
+    if (loongarch_r_lp64_name[i][0] != '\0')
+      user_reg_add (gdbarch, loongarch_r_lp64_name[i] + 1,
+	value_of_loongarch_user_reg, (void *) (size_t) i);
+
+  for (int i = 0; i < ARRAY_SIZE (loongarch_f_lp64_name); ++i)
+    {
+      if (loongarch_f_lp64_name[i][0] != '\0')
+	user_reg_add (gdbarch, loongarch_f_lp64_name[i] + 1,
+		      value_of_loongarch_user_reg,
+		      (void *) (size_t) (LOONGARCH_FIRST_FP_REGNUM + i));
+    }
+
   /* Information about registers.  */
   set_gdbarch_num_regs (gdbarch, regnum);
   set_gdbarch_sp_regnum (gdbarch, LOONGARCH_SP_REGNUM);
-- 
2.38.0