[LLVMdev] argument registers and -g

Peter Cooper
Wed, 11 Jul 2012 02:57:30 -0000

On Jul 10, 2012, at 7:37 PM, Eric Christopher <echristo at apple.com>
wrote:

> Hi Guys,
>
> Pete's comment made Reed's question make sense to me :)
>
> Pete is essentially correct, with some complications. You need to track the
location of the arguments as they're going through code generation. Look at
the llvm.dbg.value intrinsic as a guideline on how to track a variable through
compilation and any saves should be marked.
>
> We do support location lists in general (optimized code or code with many
variables are impossible without it), but in the case you mention here:
>
> void foo(int i) { // i is passed in a0 register
>   int j = i * 370;
>   // i is never used again
>   //// lots of deep expression code so that the a0 gets used again because
it's a scratch register.
>  // this is okay, because we don't need it, but now in the debugger
>  // we can't display i because it has been clobbered.
>
> you can treat it as "optimized" out. I can't think of a
target that has this problem at the moment so you may need to look into what it
would take to save the value out to the stack and then track it that way.
I'm not a fan of that since that would mean that debug code gen is different
from normal code gen. In general, this is a hard problem and there aren't
any obvious solutions that I know about. One thing you'll want to make sure
is that a variable has a proper address tag so that when it is clobbered you can
express the lack of location to the user.
Only thing i can think here is to always insert the saves from arguments into
the stack then have an optimization to remove those saves if the values are
unused (MachineDCE might already handle this?).  That optimization wouldn't
run at O0 so you'd get the same behavior with and without -g, but at the
expense of slightly worse code in non-debug -O0 builds.

Pete
>
> -eric
>
>
> On Jul 10, 2012, at 7:14 PM, Peter Cooper <peter_cooper at apple.com>
wrote:
>
>> Hi Reed
>>
>> I think what you need is DWARF location lists.  See 2.6.4 here
http://www.dwarfstd.org/doc/040408.1.html
>>
>> I think you'd need to mark the values as being in a register
location from the function start address, then at the point where they are saved
to the stack add a new location to the list for the stack location.
>>
>> Unfortunately i don't know if we support this or how to go about
adding it, but i'm pretty sure thats the DWARF feature you need.
>>
>> Thanks,
>> Pete
>> On Jul 9, 2012, at 3:56 PM, reed kotler <rkotler at mips.com>
wrote:
>>
>>> On Mips, we have argument registers which are treated as scratch
>>> registers with a live in.
>>>
>>> This will not work with -g -O0 because the arguments to the
function
>>> will not be displayable after they are clobbered.
>>>
>>> I'm guess others have solved this problem.
>>>
>>> Is there a preferred way?
>>>
>>> Reed
>>>
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