[PATCH] D18565: Implement an "I'm dtrace, please retain all debug types" option.

[Adrian Prantl via cfe-commits]

> On Mar 31, 2016, at 5:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Wed, Mar 30, 2016 at 10:49 AM, Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
> 
>> On Mar 29, 2016, at 10:06 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Mar 29, 2016 at 12:03 PM, Adrian Prantl via cfe-commits <cfe-commits at lists.llvm.org <mailto:cfe-commits at lists.llvm.org>> wrote:
>> 
>> > On Mar 29, 2016, at 12:00 PM, Joerg Sonnenberger <joerg at britannica.bec.de <mailto:joerg at britannica.bec.de>> wrote:
>> >
>> > On Tue, Mar 29, 2016 at 06:47:24PM +0000, Adrian Prantl via cfe-commits wrote:
>> >> This code in this patch listens to the driver option -gfull, and lowers it to the new cc1 option -debug-retain-types (1).
>> >> When -debug-retain-types is present, CGDebugInfo will retain every(2) type it creates.
>> >
>> > Is there a good reason for calling it -gfull? I would find something
>> > -gall-types or -gretain-all-types to make a lot more sense. This should
>> > be orthogonal to other options like providing only line tables?
>> 
>> My thinking was this:
>> The driver already supports -gfull, but it doesn’t do anything.
>> This patch can be considered a first step towards making -gfull behave as expected.
>> Eventually it should emit debug info for *all* types.
>> 
>> Seems somewhat problematic to half implement it, though. (admittedly we're just silently ignoring it right now)
> 
> I don’t think this is problematic at all. This is incremental development.
> 
> It strikes me as a strange increment. Implementing full -gfull doesn't seem like it would take much time to implement, etc.
>> 
>> & is 'real' -gfull what dtrace really wants? (seems it isn't - since clang's never really implemented it?)
> 
> Admitted, ‘real' -gfull is probably more than it absolutely needs.
> 
> If just retaining referenced types is all it needs, yeah, it seems -gfull would be rather beyond that.
>  
> 
>> Emitting all types referenced by used (even if later optimized away) code seems like the thing? -greferenced? or maybe a -f flag? Not sure.
> 
> I don’t see a compelling case for adding another driver option to the already confusing zoo of existing driver options.
> 
> My point is I think a -gfull that does something other than full would possibly be more confusing than not.

Point taken. Let’s surface this under a separate option instead. We can call it “-greferenced” to fit between -gfull and -gused.

thanks,
adrian
>  
> Note that we currently also accept a -gused option which according to the driver code is supposed to be the opposite of -gfull.
> 
> Are -gused/-gfull meant to toggle each other?
> 
> Huh, seems they're not general GCC flags, they're Darwin things - I didn't know that.
> 
> Looks like GCC usually spells this -f[no-]eliminate-unused-debug-types. But doesn't seem to have an intermediate version that would be what you're going for.
>  
> Adding a -greferenced option IMO will only make this more confusion instead of helping.
> My suggestion is to have -gfull (also) activate -debug-retain-types. In the somewhat hypothetical scenario that someone implements a more comprehensive version of -gfull we should revisit this and analyze whether the resulting debug information is really too large to be practical, and if we conclude that this is a problem, we can still decide to expose -debug-retain-types to the driver with a new separate option.
> 
> I would be concerned about breaking other existing users that may grow once we support the flag. (& perhaps inconsistency between GCC and Clang, but inconsistency already exists there of course)


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