[llvm] r246406 - [dsymutil] Fix handling of inlined_subprogram low_pcs

Wed Sep 9 09:52:29 PDT 2015

> On Sep 9, 2015, at 8:36 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Wed, Sep 9, 2015 at 8:16 AM, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> 
>> On Sep 8, 2015, at 10:09 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> 
>> 
>> On Tue, Sep 8, 2015 at 1:10 PM, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
>> 
>>> On Sep 8, 2015, at 12:24 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> 
>>> 
>>> 
>>> On Mon, Aug 31, 2015 at 11:10 AM, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
>>> 
>>>> On Aug 31, 2015, at 9:07 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>>> 
>>>> 
>>>> 
>>>> On Mon, Aug 31, 2015 at 9:05 AM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>>> 
>>>> 
>>>> On Sun, Aug 30, 2015 at 6:43 PM, Frederic Riss via llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>> wrote:
>>>> Author: friss
>>>> Date: Sun Aug 30 20:43:14 2015
>>>> New Revision: 246406
>>>> 
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=246406&view=rev <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_viewvc_llvm-2Dproject-3Frev-3D246406-26view-3Drev&d=BQMFaQ&c=eEvniauFctOgLOKGJOplqw&r=_sX2G1Du1KZyzi5BD4_ddw&m=FkrVlXa3-EdUHEUklJrpTIxLR2zDdr3ysgnj0hyNiNc&s=-yp_b9w-sonxhFICg6npPkz6_FLOw29qR_X8EIzjwWY&e=>
>>>> Log:
>>>> [dsymutil] Fix handling of inlined_subprogram low_pcs
>>>> 
>>>> The value of an inlined subprogram low_pc attribute should not
>>>> get relocated, but it can happen that it matches the enclosing
>>>> function's start address and thus gets the generic treatment.
>>>> Special case it to avoid applying the PC offset twice.
>>>> 
>>>> I'm a tad confused - do you store the low_pcs as offsets relative to the function
>>>> 
>>>> (sorry, bouncy shuttle to work & accidentally sent before I finished that sentence...)
>>>> 
>>>> do you store the low_pcs as offsets relative to the function's low_pc? That's interesting - and perhaps something we should standardize/generalize to reduce relocations in all our DWARF output (but I don't think there's any standard for it yet in DWARF), but I'm not sure why that would require special casing the case where the two low_pcs are equal - wouldn't that just mean the low_pc of the inlined subroutine would be at zero offset from the subprogram's low_pc? (& still not relocated)
>>> 
>>> dsymutil takes the debug map as input that only contains the function (and variables) start addresses. That’s the only thing we can count on being exact. We then do a pass over all the debug_info relocations to find the ones that correspond to those addresses (and the DIEs where we find the ‘interesting’ relocations are the ones that define which part of the DIE tree we keep). Then — once we decided what to keep — we go over the kept DIEs and we clone them, applying the relocations in the process. But note that the relocations we’ve chosen are only for the entry points, thus we need to have the code around to handle the lexical_block/inlined_subroutine, and this code doesn’t use the relocations (it applies an offset that we computed when handling the subprogram DIE).
>>> 
>>> What  happened here is that the generic code that applied the relocations would also patch the inlined_subroutine low_pc because the relocation was the same as the entry point. And then the code handling the low_pc attributes for the inlined_subroutine would apply the offset a second time.
>>> 
>>> OK - what I'm wondering is whether it would work better/as well to generalize this code, rather than two distinct passes/processes. 
>> 
>> I don’t think there’s a way to generalize this code. But I agree that storing the low_/high_pcs as offsets from their enclosing function low_pc would save quite a few relocations.
>> 
>> Sorry, that wasn't what I was trying to describe,
> 
> I must admit that I didn’t really get your ‘2 distinct passes/processes’ so I replied to you original point. But now I think I see what you meant and I hope the rest of my answer did address that.
> 
>> but it's certainly something we've discussed before (actually I made a silly prototype of using dwarf expressions and debug address pool indicies to do reloc sharing (using one reloc per section (macho would use one reloc per function, due to the implied function-sections like behavior) - never did get around to running good numbers on it, though)).
>>  
>> Note that there is precedent for something like this: the ranges are encoded as offsets from the *CU* low_pc. Maybe it would be more natural to use that then?
> 
> Note to myself: I said ‘more natural’ above, but I didn’t really mean it (more in the line of the standard would have been a better expression of my thought). I never understood why the standard used the CU low_pc as a base. It’s hard to use for the compiler (cf the kludge we use by setting the CU low_pc to 0 when we have multiple address ranges). 
> 
> Do we still put the low_pc to 0 when we have DW_AT_ranges on the CU? I guess maybe we do - been a while since I looked. (debuggers should just have "no base" essentially, when the CU has ranges)
>  
> Maybe I’m missing something, but the start of the function would have been much easier.
> 
> Yeah, I was thinking generalizing it a bit "you can use a constant address value which will be interpreted relative to the nearest enclosing low_pc" - so even if you have a split CU, but a contiguous subprogram, you can still share the low_pc of your subprogram. Or if you have a split subprogram but a contiguous CU (as in the hot/cold splitting case) you could still use that, etc. (this could happen further into subprograms too - split CU, split subprogram, but possibly a contiguous lexical block there, etc) - this wouldn't entirely minimize relocations, though - if you had a split subprogram and a similarly split lexical block - the lexical block ranges wouldn't share the base relocs of the subprogram's ranges relocs, for example. (or if you had a split subprogram, split CU, but contiguous lexical block - you still wouldn't get to share whichever subprogram/cu reloc refers to the chunk that the lexical block is in)

Just out of curiosity: we do not do any form of function splitting (eg hot/cold partitioning) AFAIK, so all these ‘problems’ are mostly theoretical for now, right?

> That's why the prototype I did was fission-based, because there's already address pooling implemented there (& we use fission anyway, so it was in the space I was thinking of). It'd still need some extensions for ranges, if I recall correctly, to allow ranges to use addr+offset as well. (& I don't really think using generalized dwarf expressions is the right solution for the addr+offset in DWARF attributes, but it was a fun way to prototype it)
>  
> 
>> If we had a (probably/preferably compact) encoding to describe this, it would probably be ideal.
>> 
>> DWARF4 already has this /sort/ of thing for high_pc (where it can be encoded as a static offset relative to the low_pc - so it's not another relocation). That could possibly be generalized further to allow high_pcs to be a static offset relative to their enclosing high_pc (if one exists, otherwise it would be an unacceptable encoding (this could occur for functions - if the CU isn't a contiguous PC range (non-CU functions in between CU functions, functions in other sections, etc) or if a function itself is discontiguous (hot/cold code splitting)).
>> 
>> Eric & I have bandied that around now & then, which lead to the aforementioned prototype I played around with, but didn't go any further than that - my improvements to Clang's debug info emission had already brought it down to half the size of GCC's, so we didn't have any particular need to push further at the time.
> 
> Interesting to know.
> 
>> 
>>> low_pc should just be a zero-offset relocation, right?
>> 
>> Not on mach-o. Most relocations will be of the form __text+offset. That’s why there is no way for me to differentiate a __text+offset references the end of a range from the exact same relocation that references the beginning of another one (and as the linker can tear apart sections, that distinction is fundamental).
>> 
>> OK, so you search through looking for a subprogram that has a subprogram low_pc at __text+offset? then assume all the other low/high pcs (and ranges) are relative to that function starting point? (this is how you remove the ambiguity of the start/end?)
> 
> Basically yes. It’s a bit more complicated because it’s a multi-phase process, but the end result is that while linking the DIEs we know if we are in a function and we know it’s object file and its linked address. We just apply that same offset to all the other object file addresses within that function.
> 
> OK, I'll see if I can understand this/explain myself:
> 
> It sounds like you search through for the subprogram DIE with the appropriate low_pc matching the debug map entry you received, then you update that low_pc, record the base offset of the subprogram and add that to all the address attributes in the subprogram?
> 
> But you don't search for the low_pc of the subprogram, you just search for any low_pcs - update them all, then do the addition as a second pass.

Not exactly (sorry I should have given a more detailed answer upfront). The phases I was referring to are:
 1/ Scan the relocations (without touching the debug info contents) to look for interesting relics (i.e. matching something in the debug map)
 2/ Scan the debug info a first time to match these interesting relics to subprograms low_pcs or variables locations (and from these ‘seeds’  construct the full tree of DIEs to link)
 3/ Clone/Link the selected DIEs

3/ is where everything happens. 1/ and 2/ are just here to gather information. During 3/ I apply the relocations *and* I use the special casing code to workaround the bad attribute values it would generate.

> Things I'm confused by:
> 
> * Why does the second pass not touch the subprogram (how does the subprogram's high_pc get updated? Is that a special case? Does it need to be?)

The high_pcs are special cased because of Dwarf2 where they hold addresses (and these addresses could correspond to the wrong thing). This patch added special casing for low_pcs very similar to what was already there for high_pcs.

> * Why is the low_pc (or low_pcs) get updated eagerly, rather than deferring it to be handled with the second pass/addition code? (so then it wouldn't need a special case, with another special case on top to workaround it)

To be extra clear, I don’t eagerly update low_pcs. I try to generically apply relocations which happens to be mostly about updating the low_pcs (I do that one every DIE that has a valid relocation, not only subprograms). But there are other cases like for example:

void foo();
template <void (*T)()> struct S {};
S<foo> s;

The debug info for the template value parameter will have a relocation for foo that I need to apply. Also global variable locations might have complex expression that contain a relocations. Instead of special casing all the attributes that might contain a relocation I just apply the relocations and then patch up the places that I know could be wrong.
Doing it this way is forward looking. For example when I once get back to submitting my default argument value debug info patch, we will have blobs in the debug info that might contain relocations. dsymutil shouldn’t need any updating to handle that because of the way it’s done.
dsymutil would need updating if we add a new attribute containing a relocation that might be ambiguous, but my thinking is that this is much less likely that adding relocations that aren’t.

Fred  

> 
>> 
>>> Maybe I'm not understanding/explaining very well, though.
>>>  
>>> We might be able to completely remove any specific handling and just ‘promote’ all the relocations that fall inside a linked function as interesting. At the point we do that triaging relocs, we are not exploring the DIE tree though, just looking at the relocation list, so it would require us to trust the size field of the debug map, and I’m not sure we can do that 100% of the time (I know that this field is not accurate, it’s usually too big because it factors in alignment, but that might not be an issue if nothing gets allocated in the alignment padding).
>>> 
>>> Hmm - not sure I follow this. You're suggesting that if a non-debug-aware tool applied the relocations in the object file/debug info, it would mangle/damage the debug info?
>> 
>> Basically yes. As I explain above a relocation based off the __text section with a constant offset could be replaced by different values depending on the context. I already said that, but I guess the message is hard to get through: dsymutil uses the object file relocations to know what to link, but it doesn’t do relocation processing in the usual sense, because this simply wouldn’t work (More precisely, it tries to do as much standard relocation processing as possible, but it needs some code to workaround the cases where that logic gives the wrong result).
>> 
>> It's slowly sinking in, I appreciate your patience in (repeatedly) explaining it to me.
> 
> I hope I didn’t come through as complaining about that. I was merely acknowledging that it’s very different from other platforms and thus hard to convey to people not working with that platform. I really appreciate your interest.
> 
> Fred 

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