Re: [patch] Fix PR tdep/12352: Handle str pc, [Rd, #imm] in displaced stepping

["Ulrich Weigand" <uweigand@de.ibm.com>]

Yao Qi wrote:

> I am afraid they are not equal to each other.  The intention of this
> complicated insn sequence is used to compute the implementation-defined
> constant offset of `str pc'.  See more explanations below.
> 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204g/Cihbjifh.html
> 
> Section "Saving from r15"
> [...]
> If you do save from r15, the value saved is the address of the current
> instruction, plus an implementation-defined constant. The constant is
> always the same for a particular processor.
> If your assembled code might be used on different processors, you can
> find out what the constant is at runtime using code like the following:
> 
>     SUB R1, PC, #4 ; R1 = address of following STR instruction
>     STR PC, [R0]   ; Store address of STR instruction + offset,
>     LDR R0, [R0]   ; then reload it
>     SUB R0, R0, R1 ; Calculate the offset as the difference

Yes, I'm aware of that.  However, my understanding is that this special
definition of STR PC applies to *all* variants of STR, including PUSH
(PUSH { PC } is just another mnemonic for STR PC, [ SP, #-4 ]).

If you look at the formal semantics definition in the ARM reference
manual, all variants of STR (including PUSH) use the pseudo-code macro
"PCStoreValue" to implement storing of the PC, which is defined to
include the implementation-defined constant ...

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com