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Re: [rfc] frame->frame => frame->addr && frame->base()
- From: Andrew Cagney <ac131313 at cygnus dot com>
- To: Richard dot Earnshaw at arm dot com
- Cc: gdb at sources dot redhat dot com
- Date: Fri, 12 Apr 2002 09:38:57 -0400
- Subject: Re: [rfc] frame->frame => frame->addr && frame->base()
- References: <200204121249.NAA02782@cam-mail2.cambridge.arm.com>
>> // An ISA/ABI specific address within the ``specified frame'' that is
>> constant throughout the lifetime of the frame. This address is used by
>> GDB as a handle to identify this frame. This field must be initialized
>> as part of the creation of a frame object. (see dwarf2 CFA)
>>
>> CORE_ADDR addr;
>
>
> The main advantage of the DWARF CFA is that it is, as I understand it,
> precisely defined on all systems (something like: the value of the stack
> pointer when executing the first instruction of a function). As such, it
> will always be valid, and cannot change while executing the function.
Yes. Remember that a CFA tells you nothing about the location of
variables or arguments on the stack. Also remember that at present, no
one is quite sure what frame->frame means - as KevinB pointed out the
``constant'' property is new. I suspect the property most likely
doesn't hold for some existing ISAs (sigh).
>> // High level language concept of the base address of a frame. Often
>> refered to as ``frame_base'' or ``frame pointer''. This value should
>> only be computed on-demand. It is strongly recommended, though, that
>> implementations cache the computed value in the frame cache. The method
>> is initialized as part of the frame objects creation. The default
>> method returns frame->addr. (see dwarf2 DW_AT_frame_base)
>>
>> CORE_ADDR (*base) (struct frame_info *frame);
>
>
> What would this mean in the context of a function that doesn't use a frame
> pointer? What about a leaf function which doesn't store anything on the
> stack? I can't see how this can have any MI interpretation (other than
> the fact that all functions in a nested chain should have a different
> value).
The value is debug-info dependant. See section 3.3.5 of the dwarf2
spec. For some frames this value may not even be applicable - that is
ok because it isn't a requirement of a frame.
It looks like, in a sense this method is already present, check
FRAME_ARGS_ADDRESS_CORRECT.
enjoy,
Andrew