[PATCH] LowerDbgDeclare - get rid of redundant dbg.values when an alloca survives optimization

[Eric Christopher]

On Mon, Mar 31, 2014 at 9:25 AM, Robinson, Paul
<Paul_Robinson at playstation.sony.com> wrote:
>> -----Original Message-----
>> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
>> bounces at cs.uiuc.edu] On Behalf Of Adrian Prantl
>> Sent: Monday, March 31, 2014 8:58 AM
>> To: David Blaikie
>> Cc: llvm-commits
>> Subject: Re: [PATCH] LowerDbgDeclare - get rid of redundant dbg.values
>> when an alloca survives optimization
>>
>>
>> On Mar 30, 2014, at 12:02 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> > When you say "dropping all dbg.declares" - do you mean we're removing
>> > that intrinsic from LLVM entirely (ie: it won't exist) or that we
>> > "drop" dbg.declares in IR by removing them at some point in the middle
>> > end?
>>
>> I'm not sure if I understand this question entirely, but yes,
>> LowerDbgDeclare will lower a dbg.declare into multiple dbg.values (one
>> for each use) and then delete the dbg.declare.
>
> In terms of information content, it seems like we could eliminate the
> dbg.declare intrinsic entirely by having the frontend attach one more bit
> of metadata to the associated alloca, which already has two of the three
> bits of info that dbg.declare has.
>
> Given the number of -g optimization issues we've found (without really
> trying) it might be worth thinking about how to eliminate dbg.value also.

And keep in mind here that a lot of them were fixed, just years ago
and they've crept back in. I think to ensure debug info not affecting
optimization we're going to need to make it easier for pass writers to
keep debug information up to date.

-eric