[cfe-dev] AST Writer

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Jul 10 15:17:08 PDT 2018


On Tue, Jul 10, 2018 at 2:49 PM Matthieu Brucher <matthieu.brucher at gmail.com>
wrote:

> That's my use case, it's different than the OP, probably.
>
> In my case, I want to generate a first pass, with a JIT (the code is
> generated from another description), but the generated code could be
> changed by the user in a subsequent pass.
>

Curious. As much as possible, I'd encourage you to find ways to not have
users work with generated code (by abstracting that generated code away
from them - giving them a higher level representation to write, places
where the generated code calls back into the user code, etc). But I don't
know your domain, etc, and wouldn't suggest what is or isn't right for you
and your users.

But the main takeaway is that modifying the AST and generating code from
that is discouraged in favor of generating source code edits.


> Modifying directly the AST is not an option, try generating equations with
> thousands of parameters that are solved in real time. Just no way someone
> can write them efficiently in IR (that's why you have the AST to IR
> generator!).
>
> I don't understand your last paragraph. If clang-format can cleanup
> rewrites, why can't it reformat code from the AST? If the AST printer
> writes any kind of code, why couldn't clang-format reformat it?
>

clang-format could format AST generated source too - I was commenting on
that in answer to your question "Easier to generate correctly formatted
code from the AST?" - that it's not easier to generate correctly formatted
code from the AST than it is from a textual edit. In both cases you'd use
something like clang-format to tidy up the result. The AST itself doesn't
have fancy formatting support so it's no better than a textual edit in
terms of getting nicely formatted results.


>
>
>
> Le mar. 10 juil. 2018 à 22:41, David Blaikie <dblaikie at gmail.com> a
> écrit :
>
>> Hmm, not sure I follow.
>>
>> Did the user write this source code? Are they going to want to change it
>> later? Does it make sense for them to see the edits you're suggesting, or
>> are those edits really compiler optimizations/transformations? If they're
>> more the latter, then perhaps caching the LLVM IR (with these
>> optimizations/transformations applied) rather than modifying the source
>> would be more suitable.
>>
>> Easier to generate correctly formatted code from the AST? Not really -
>> the AST printing doesn't have any particularly nuanced formatted printing.
>> That's what clang-format is for (it was specifically built for doing code
>> rewrites based on ASTs - where the rewrite is expressed as a textual change
>> to the original source (not an AST modification) & that change is applied,
>> then clang-format is used to tidy it up).
>>
>> On Tue, Jul 10, 2018 at 2:11 PM Matthieu Brucher <
>> matthieu.brucher at gmail.com> wrote:
>>
>>> It's odd though, because generating code on the fly would be easier on
>>> the AST than on the IR tree, if the goal is JIT and also saving the code at
>>> the same time.
>>> It's probably also easier also to generate properly formatted code?
>>>
>>> Regards,
>>>
>>> Matthieu
>>>
>>> Le mar. 10 juil. 2018 à 16:21, David Blaikie via cfe-dev <
>>> cfe-dev at lists.llvm.org> a écrit :
>>>
>>>> It's generally considered that the AST invariants are too
>>>> subtle/complex to use AST modification and AST->source conversion reliably.
>>>> Refactoring/source code modification is generally encouraged to be done via
>>>> textual edits generated from source location information in the AST.
>>>>
>>>> On Mon, Jul 9, 2018 at 8:36 PM Ridwan Shariffdeen via cfe-dev <
>>>> cfe-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I am trying to build a tool which can insert new AST nodes to a AST
>>>>> tree obtained from a source code and generate the modified source code. For
>>>>> example add an if condition to a given location.
>>>>>
>>>>> I have seen examples on ReWriter which can insert text, but I want to
>>>>> insert a proper AST node and generate the source code from the modified
>>>>> AST.
>>>>>
>>>>> For this purpose, I think I should be using ASTWriter and not
>>>>> ReWriter. Is there any documentation I can refer on how to implement this?
>>>>>
>>>>> Any help in this regard is highly appreciated.
>>>>>
>>>>> Thanks!
>>>>> Ridwan
>>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>
>>>> _______________________________________________
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>>>
>>>
>>> --
>>> Quantitative analyst, Ph.D.
>>> Blog: http://blog.audio-tk.com/
>>> LinkedIn: http://www.linkedin.com/in/matthieubrucher
>>>
>>
>
> --
> Quantitative analyst, Ph.D.
> Blog: http://blog.audio-tk.com/
> LinkedIn: http://www.linkedin.com/in/matthieubrucher
>
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