Re: [PATCH] [gdb/riscv] Fix test for riscv: zero register.

[Andrew Burgess via Gdb-patches <gdb-patches@sourceware.org>]

Mark Goncharov <mark.goncharov@syntacore.com> writes:

> From: mga-sc <mark.goncharov@syntacore.com>
>
> x0 register in riscv must have permanent value.
> We already have test for that: gdb/testsuite/riscv-reg-aliases.exp.
> This patch fixes 4 test drops.

Thanks for this.

I guess you must be using a target that doesn't like it if GDB tries to
write to x0?  The native Linux target for example is fine with writes to
x0, as the kernel already ignores these.  It's always nice to include
these sorts of details in the commit message.

I also wanted to tweak some of the comments, there's an updated patch
below, are you happy if I push this?  I've kept the text about the
target generic, but if you have any additional details I'd be happy to
add these before committing.

Thanks,
Andrew

---

commit e5b1b4690ba1bab285e5a4341990dcca43343b77
Author: mga-sc <mark.goncharov@syntacore.com>
Date:   Mon Aug 8 16:01:47 2022 +0300

    gdb/riscv: implement cannot_store_register gdbarch method
    
    The x0 (zero) register is read-only on RISC-V.  Implement the
    cannot_store_register gdbarch method to tell GDB this.
    
    Without this method GDB will try to write to x0, and relies on the
    target to ignore such writes.  If you are using a target that
    complains (or throws an error) when writing to x0, this change will
    prevent this from happening.
    
    The gdb.arch/riscv-reg-aliases.exp test exercises writing to x0, and
    will show the errors when using a suitable target.

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 2d41be96b20..b9a51f7ae6a 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -933,6 +933,15 @@ riscv_register_name (struct gdbarch *gdbarch, int regnum)
   return name;
 }
 
+/* Implement the cannot_store_register gdbarch method.  The zero register
+   (x0) is read-only on RISC-V.  */
+
+static int
+riscv_cannot_store_register (struct gdbarch *gdbarch, int regnum)
+{
+  return regnum == RISCV_ZERO_REGNUM;
+}
+
 /* Construct a type for 64-bit FP registers.  */
 
 static struct type *
@@ -3822,6 +3831,9 @@ riscv_gdbarch_init (struct gdbarch_info info,
      registers, no matter what the target description called them.  */
   set_gdbarch_register_name (gdbarch, riscv_register_name);
 
+  /* Tell GDB which RISC-V registers are read-only. */
+  set_gdbarch_cannot_store_register (gdbarch, riscv_cannot_store_register);
+
   /* Override the register group callback setup by the target description
      mechanism.  This allows us to force registers into the groups we
      want, ignoring what the target tells us.  */