[PATCH] D24383: Add addOrMerge interface to tooling::Replacements.

Александр Шапошников via cfe-commits cfe-commits at lists.llvm.org
Sat Sep 10 13:25:41 PDT 2016

regarding B:
i apologize in advance, i am not sure if it's directly related to your
patch, but i'm kinda afraid that the current approach will shadow
the issue (if it's considered to be an issue).
For simplicity let's take a look at the following example:

   struct Point {};

   include "Point.h"

   include "Point.h"

clang-rename rename-all -i -old-name Point -new-name Point2 a.cpp b.cpp
Renaming failed in /Users/Alexshap/PlayRename/srcs/./include/Point.h! New
/Users/Alexshap/PlayRename/srcs/./include/Point.h: 7:+5:"Point2"
conflicts with existing replacement:
/Users/Alexshap/PlayRename/srcs/./include/Point.h: 7:+5:"Point2"

the thing is that in the past Replacements was a typedef on std::set<...>
(if i am not mistaken)
and insert() ignored duplicates. Now Replacements.add(...) will return an
error (the same replacement has already been added).

(Separate observation, I'd like to take advantage of this discussion and
ask about it)

Replacements Replacements::merge(const Replacements &ReplacesToMerge)

The question is the following: with the current implementation we copy
all the replacements even if the merge is "trivial" or if there are only a
few conflicts.
This looks suboptimal, please, correct me if I'm wrong.

3. Having said that - just curious
- is the fallback on merge (in addOrMergeReplacement ) really necessary in
those examples ?
(I mean in change-namespace and include-fixer)


On Sat, Sep 10, 2016 at 9:43 PM, Daniel Jasper <djasper at google.com> wrote:

> There are several things I find particularly confusing about this.
> Fundamentally, Replacements refer to a specific version of the code. Say
> you have a Replacements object R and a Replacement r that you want to
> integrate into it. Further assume, that C0 is the initial version of the
> code whereas C1 is the version after applying R. If you *add* r to R, that
> means that r (specifically the offset and length in it) refer to C0. If you
> *merge* it, that means that r refers to C1. Correct replacements can always
> be *merged*, while *adding* might cause conflicts if they overlap with
> existing ones. Thus, a function "add or merge" fundamentally doesn't make
> sense to me. These are fundamentally different concepts.
> What this function (and the different implementations we already have) is
> fundamentally trying to do is to have a version of *add* that resolves
> (some) conflicts. It does that by assuming that r is currently referring to
> C0, so it first shifts it and then calls *merge*. The fact that you can
> shortcut if there isn't actually a conflict (i.e. the "addOr" part) is just
> an optimization. I am fine with implementing that optimization, but it
> shouldn't be part of the name.
> Now, I don't (yet) have a good name as this has very intricate behavior.
> And I am not sure it is really a good idea to have this magically "resolve"
> all conflicts, because I think there are different classes. I think what we
> want here is to have a way to combine replacements that require a defined
> order, but that don't actually conflict. Lets look at a few cases. Assume R
> contains a single Replacement A and you trying to "addOrMerge" another
> Replacement B.
> 1. A and B are both inserts at the same code location. This seems
> relatively benign and we just want a defined order
> 2. A inserts at offset X and B replaces (X, X+N). Still quite benign, but
> we don't want B to replace anything that A inserted, it should replace the
> original text
> 3. B inserts at offset X and A replaces (X, X+N). Same thing, though
> interesting as we now actually have to switch the order
> 4. A and B actually replace overlapping code ranges. This is really bad
> and I think we should continue to report a conflict instead of doing
> something weird
> I'd think that your existing implementation gets #1 right but possibly
> only one of #2/#3. It will also do something very interesting for #4 and I
> really think what we want is to report an error.
> On Sat, Sep 10, 2016 at 5:53 PM, Eric Liu <ioeric at google.com> wrote:
>> This function has been implemented in several places, so I figure it
>> might be useful to have this function in the library. Although I'm not sure
>> whether this should be a class interface or just a standalone helper
>> function.
>> @alexshap: thanks for the explanation. For your point B,  I'm not sure
>> about the example, could you elaborate a bit? E.g. which use case to be
>> specific? What are the headers?
>> Generally, the way users of `Replacements` class adding new replacements
>> also matters. For example, if you have multiple insertions at the same
>> offset, you could concatenate them and insert as one single replacement, if
>> you care about performance.
>> For most use cases I've seen when converting from old std::set
>> implementation to the current implementation, performance can be improved
>> by changing the way replacements are added. And the most natural way is
>> often the most efficient one.
>> On Sat, Sep 10, 2016, 10:54 Alexander Shaposhnikov <shal1t712 at gmail.com>
>> wrote:
>>> alexshap added a comment.
>>> disclaimer: i don't know if this method is the right thing to add (to be
>>> honest i would prefer a better interface but don't have any good
>>> suggestions on my mind at the moment), i see several issues (IMO, i might
>>> be mistaken, apologize in advance) with the current interface of
>>> Replacements. I will not list all of them, but want to add some context:
>>> A. right now i see the same code (with minor changes) in several places:
>>> 1.
>>> llvm/tools/clang/tools/extra/include-fixer/IncludeFixer.cpp +382
>>>   auto R = tooling::Replacement(
>>>         {FilePath, Info.Range.getOffset(), Info.Range.getLength(),
>>>         Context.getHeaderInfos().front().QualifiedName});
>>>    auto Err = Replaces.add(R);
>>>    if (Err) {
>>>      llvm::consumeError(std::move(Err));
>>>      R = tooling::Replacement(
>>>          R.getFilePath(), Replaces.getShiftedCodePositio
>>> n(R.getOffset()),
>>>          R.getLength(), R.getReplacementText());
>>>      Replaces = Replaces.merge(tooling::Replacements(R));
>>>    }
>>> 2.
>>> llvm/tools/clang/tools/extra/change-namespace/ChangeNamespace.cpp +126
>>> (see https://reviews.llvm.org/D24183)
>>>   void addOrMergeReplacement(const tooling::Replacement &R,
>>>                            tooling::Replacements *Replaces) {
>>>    auto Err = Replaces->add(R);
>>>    if (Err) {
>>>       llvm::consumeError(std::move(Err));
>>>       auto Replace = getReplacementInChangedCode(*Replaces, R);
>>>       *Replaces = Replaces->merge(tooling::Replacements(Replace));
>>>    }
>>>   }
>>> B. For replacements in headers we can get into if(Err) branch quite
>>> often because the same replacement can be generated multiple times (if that
>>> header is included into several *.cpp files).
>>> https://reviews.llvm.org/D24383
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