[PATCH] D29833: Improve the API of DILocation::getMergedLocation()

[Adrian Prantl via llvm-commits]

> On Feb 23, 2017, at 10:08 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Thu, Feb 23, 2017 at 9:35 AM Adrian Prantl <aprantl at apple.com <mailto:aprantl at apple.com>> wrote:
>> On Feb 23, 2017, at 9:13 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> You were suggesting that the nice thing about the former was that the user couldn't fail to have a debug location:
>> 
>> "The primary benefit of having the API on Instruction is that we can make it impossible to accidentally create a locationless call instruction in a function with debuginfo (which would crash/assert the backend when creating inline scopes)."
>> 
>> Which I'm a bit fuzzy on - if a new call instruction is created it'd still have no location so I'm not sure this choice of API provides extra defense against that mistake.
>> 
>> If, today, an existing call instruction is moved then the location zero'd out - it's going to need work to replace the zeroing with merging no matter whether the API for that merging is on DebugLoc or Instruction.
> 
> Agreed.
> 
>> 
>> So I guess coming back to your point: could you explain in more detail how having the API on Instruction provides a stronger guarantee/safety than having it elsewhere? I'm not clear on what that's defending against.
> 
> I want to make it harder to accidentally assign a null DebugLoc to a call. Currently getMergedDebugLoc() returns a null location (to save space in the line table) and at each call site we have to write extra code to create a line:0 location if the instruction we are creating the merged location for is a call. See https://reviews.llvm.org/D29833 <https://reviews.llvm.org/D29833>, lib/Transforms/Utils/SimplifyCFG.cpp:1280 for an example — though upon closer inspection the original code was just keeping the original location of the call instead of assigning a line:0 location.
> I though it would be nice to hide this detail in the API, but this really makes it look like we want to decide how to handle call instructions (assign line:0, keep original loc) on a case-by-case basis.
> 
> Oh, right, sorry - sure, seems reasonable to make it part of Instruction so it can Do The Right Thing depending on whether the Instruction is a Call. (maybe a "setMergedLocation(DebugLoc, DebugLoc)" that ignores the current location - I suppose in theory it could be a variadic function to handle more than two locations, but that probably doesn't come up often enough to worry? Not sure))

Yes, but then I realized that it is not obvious what The Right Thing is. When the instruction is a call, should we return a line:0 location, should we keep the original location? Is there one answer that fits all use-cases? I’m afraid now that we would be hiding the fact that the user needs to do make an informed decision here behind the new API.

> You mentioned abandoning this in favor of the other change under review? Seems the discussion is here so we might as well continue it here.

Sure, works for me.
-- adrian

> - Dave
>  
> 
> thanks,
> adrian

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