Re: Fix ARM stepping over Thumb-mode "bx pc" or "blx pc"

[Yao Qi <yao at codesourcery dot com>]

On 08/11/2012 01:06 AM, Joseph S. Myers wrote:
> arm-tdep.c has code to determine the next instruction for use in
> single stepping.  This code fails to handle a Thumb-mode "bx pc" or
> "blx pc" correctly; it acts as if the branch target (four bytes after
> the current instruction) should be in Thumb mode, when actually these

Yes, as we can see, at the beginning of thumb_get_next_pc_raw, the
'pc_val' is converted to THUMB_ADDR,

static CORE_ADDR
thumb_get_next_pc_raw (struct frame_info *frame, CORE_ADDR pc)
{
[...]
  pc_val = MAKE_THUMB_ADDR (pc_val)

> Index: gdb/arm-tdep.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/arm-tdep.c,v
> retrieving revision 1.365
> diff -u -r1.365 arm-tdep.c
> --- gdb/arm-tdep.c	25 Jun 2012 12:32:45 -0000	1.365
> +++ gdb/arm-tdep.c	10 Aug 2012 15:18:37 -0000
> @@ -4541,7 +4541,7 @@
>    else if ((inst1 & 0xff00) == 0x4700)	/* bx REG, blx REG */
>      {
>        if (bits (inst1, 3, 6) == 0x0f)
> -	nextpc = pc_val;
> +	nextpc = pc_val & 0xfffffffc;
>        else
>  	nextpc = get_frame_register_unsigned (frame, bits (inst1, 3, 6));
>      }

I don't have any preference on clearing either the last one bit in
address or the last two bits.  Looks like two ways coexist in arm-tdep.c
nowadays.  As 'pc_val' is set by MAKE_THUMB_ADDR at the beginning, it is
better to revert its change by using UNMAKE_THUMB_ADDR (which only
clears the last one bit of address).

-- 
Yao (éå)