Re: [PATCH] Consistent display of "<optimized out>"

["Andrew Burgess" <aburgess@broadcom.com>]

On 06/08/2013 2:18 PM, Mark Kettenis wrote:
>> Date: Tue, 6 Aug 2013 14:08:46 +0100
>> From: "Andrew Burgess" <aburgess@broadcom.com>
>>
>> In some cases we report optimized out registers as "*value not available*"
>> rather than "<optimized out>", the patch below makes this more consistent
>> in the case I've spotted.
>>
>> Here's an example:
>>
>> (gdb) up
>> #1  0x0000000000400800 in first_frame () at dw2-reg-undefined.c:27
>> 27      in dw2-reg-undefined.c
>> (gdb) info registers rax
>> rax            *value not available*
>> (gdb) p/x $rax
>> $1 = <optimized out>
>>
>> After the patch the behaviour is now:
>>
>> (gdb) up
>> #1  0x0000000000400800 in first_frame () at dw2-reg-undefined.c:27
>> 27      in dw2-reg-undefined.c
>> (gdb) info registers rax
>> rax            <optimized out>
>> (gdb) p/x $rax
>> $1 = <optimized out>
>>
>> The behaviour for values that are unavailable is currently unchanged,
>> though I have a follow up patch for this too.
>>
>> OK to apply?
> 
> I'd say no.  There is a difference between "unavailable" and
> "optimized out".  Registers will be unavailable even if you compile
> without any optimization, because the ABI specifies that their
> contents are not saved across function calls.  So "optimized out"
> makes very little sense for registers.

I disagree for 3 reasons.

1. This patch is about consistency, having "print <reg>" report one
thing and "info register <reg>" report another seems like a bad thing to
me.

2. We previously fetched the register by calling
deprecated_frame_register_read, this eventually gets a value object by
called frame_unwind_register_value, we then extract the unavailable and
optimized-out attributes from the value object and for no good reason I
can see create a new value object and mark it as unavailable.  My patch
side-steps this middle ground of calling
deprecated_frame_register_read, and instead works with the value we get
back by reading the register, this is already marked optimized out.  My
interpretation of your concern would be that you don't think it /should/
be marked as optimized out, I believe however, that this is not really
an issue for this patch, if this changes later then we'd revert back to
printing unavailable rather than optimized out, my patch doesn't prevent
that, it just prints the real state of the value object.

3. My understanding was that values lost due to the ABI of a call site
were recorded as optimized out.  For evidence I would present
dwarf2_frame_prev_register, and how DWARF2_FRAME_REG_UNDEFINED is handled.

For these reasons I believe my patch should still be considered, what do
you think?

Thanks,
Andrew