[PATCH] gdb/riscv: Remove redundant code, and catch more errors when accessing MISA

[Andrew Burgess <andrew dot burgess at embecosm dot com>]

This is a cleanup of a patch I proposed here:

   https://sourceware.org/ml/gdb-patches/2018-09/msg00749.html

I think Jim said he was happy with this, but this is a final chance to
review.

Thanks,
Andrew

---

When reading the MISA register, the RISC-V specification says that, if
MISA can't be found then a default value of 0 should be assumed.

As such, this patch ensures that GDB ignores errors when accessing
both the new and old locations for the MISA register.

Additionally, this patch removes an unneeded flag parameter which
didn't provide any additional functionality beyond checking the MISA
for the default value of 0.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_read_misa_reg): Update comment, remove
	READ_P parameter, catch and ignore register access errors from
	either the old or new MISA location.
	(riscv_has_feature): Update call to riscv_read_misa_reg.
---
 gdb/ChangeLog    |  7 +++++++
 gdb/riscv-tdep.c | 46 ++++++++++++++++++++++++----------------------
 2 files changed, 31 insertions(+), 22 deletions(-)

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 48ca2accb4a..a3c3e833e93 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -290,34 +290,39 @@ static unsigned int riscv_debug_infcall = 0;
 
 static unsigned int riscv_debug_unwinder = 0;
 
-/* Read the MISA register from the target.  The register will only be read
-   once, and the value read will be cached.  If the register can't be read
-   from the target then a default value (0) will be returned.  If the
-   pointer READ_P is not null, then the bool pointed to is updated  to
-   indicate if the value returned was read from the target (true) or is the
-   default (false).  */
+/* Read the MISA register from the target.  There are a couple of locations
+   that the register might be found, these are all tried.  If the MISA
+   register can't be found at all then the default value of 0 is returned,
+   this is inline with the RISC-V specification.  */
 
 static uint32_t
-riscv_read_misa_reg (bool *read_p)
+riscv_read_misa_reg ()
 {
   uint32_t value = 0;
 
   if (target_has_registers)
     {
+      /* Old cores might have MISA located at a different offset.  */
+      static int misa_regs[] =
+	{ RISCV_CSR_MISA_REGNUM, RISCV_CSR_LEGACY_MISA_REGNUM };
+
       struct frame_info *frame = get_current_frame ();
 
-      TRY
-	{
-	  value = get_frame_register_unsigned (frame,
-					       RISCV_CSR_MISA_REGNUM);
-	}
-      CATCH (ex, RETURN_MASK_ERROR)
+      for (int i = 0; i < ARRAY_SIZE (misa_regs); ++i)
 	{
-	  /* Old cores might have MISA located at a different offset.  */
-	  value = get_frame_register_unsigned (frame,
-					       RISCV_CSR_LEGACY_MISA_REGNUM);
+	  TRY
+	    {
+	      value = get_frame_register_unsigned (frame, misa_regs[i]);
+	      break;
+	    }
+	  CATCH (ex, RETURN_MASK_ERROR)
+	    {
+	      /* Ignore errors, it is acceptable for a target to not
+		 provide a MISA register, in which case the default value
+		 of 0 should be used.  */
+	    }
+	  END_CATCH
 	}
-      END_CATCH
     }
 
   return value;
@@ -330,13 +335,10 @@ riscv_read_misa_reg (bool *read_p)
 static bool
 riscv_has_feature (struct gdbarch *gdbarch, char feature)
 {
-  bool have_read_misa = false;
-  uint32_t misa;
-
   gdb_assert (feature >= 'A' && feature <= 'Z');
 
-  misa = riscv_read_misa_reg (&have_read_misa);
-  if (!have_read_misa || misa == 0)
+  uint32_t misa = riscv_read_misa_reg ();
+  if (misa == 0)
     misa = gdbarch_tdep (gdbarch)->core_features;
 
   return (misa & (1 << (feature - 'A'))) != 0;
-- 
2.14.5