[cfe-dev] [RFC] Easier source-to-source transformations with clang tooling

Yitzhak Mandelbaum via cfe-dev cfe-dev at lists.llvm.org
Mon Jan 28 09:54:55 PST 2019


Good question. I didn’t originally intend Stencil to be a stand-alone
library, but as an integral part of Transformer.  It is only as I’ve worked
on it that I realized that it may in fact have value on its own.  So, I’m
still thinking this through.  Here’s my logic:

1. Abstraction level.  The matchers work at the abstraction level of node
identifiers.  Stencil complements the matchers with string
concatenation that operates as the same level.  This allows users to write
simple transformations without ever using the AST API. They need only
understand the matchers and the string-concatenation model of Stencil.

2. AST operations.  For users that need more than simple transformations,
Stencil also provides a (growing) assortment of AST operations.
Ultimately, I’d like to collect all relevant AST codegen operations in the
Stencil library.  We could provide an API that works only on AST nodes, so
we don’t need the string-concat functionality, but the combination makes
the abstraction far more compelling, because you can do significant work
without ever needing to learn how to extract pieces of source text with the

3. clang-query integration.  Stencil defines a mini-DSL, so we can
naturally integrate it into clang-query. This will support writing (some)
refactorings as clang-query scripts, as well as a richer tool for
interactively exploring new refactoring ideas.


The tests are not great examples since they are designed as unit tests for
single features rather than demos.  That said, the test
`MemberOpWithNameOp` combines two features in an interesting way:

TEST_F(StencilTest, MemberOpWithNameOp) {
  const std::string Snippet = R"cc(
    int object;
    int* method = &object;
    return object;
  auto StmtMatch = matchStmt(
      Snippet, declStmt(hasSingleDecl(
  auto Stencil = Stencil::cat(member("e", name("d")));
  EXPECT_THAT(Stencil.addedIncludes(), testing::IsEmpty());
  EXPECT_THAT(Stencil.removedIncludes(), testing::IsEmpty());

A slightly more realistic example involves maps. For a map `s`, change
`s.count(k)` in a boolean context to `s.contains(k)`:

                   hasArgument(0, expr().bind("k")))))

The code gen is:
      Stencil::Cat(Member("s", "contains"), "(", Node("k"), ")")
and this handles both the s.count and s->count cases.

On Fri, Jan 25, 2019 at 5:13 AM Jonas Toth <development at jonas-toth.eu>

> Thanks for clarification. Having it directly in clang-tidy would be nice
> though :)
> Am 24.01.19 um 12:17 schrieb Manuel Klimek:
> On Thu, Jan 24, 2019 at 11:56 AM Jonas Toth <development at jonas-toth.eu>
> wrote:
>> Hi,
>> one question that came to my mind because of Joel's Mail:
>> Is it possible to run in multiple passes?
>> My use-case is adding `const` to variables that could be 'const' but
>> aren't. If you declare multiple variables like `int not_const,
>> const_variable;`
>> the transformation requires splitting these up, first -> `int not_const;
>> int const_variable;` and then do the `const` transformation -> `int
>> not_const; const int const_variable;`.
>> I have implemented both transformations in clang-tidy (only partially
>> landed yet) but there is no easy way I can run them in one check. The
>> current workflow
>> pretty much forces us to run clang-tidy multiple times and converge to
>> the final solution.
>> If your framework could give an improvement in this place, would be
>> awesome! And I think worth to consider anyway If we change the way
>> we do transformations.
> Generally, you can already do this, but it'd be outside clang-tidy. You
> can write a clang tool that
> a) runs over the codebase and stores the "const graph"
> b) load the "const graph" into memory and generate all the replacements
> (there's a way to create yaml replacements)
> c) apply all the replacements
>> Best, Jonas
>> Am 16.11.18 um 16:22 schrieb Yitzhak Mandelbaum via cfe-dev:
>> Hi all,
>> I have a proposal for a framework that makes it easier to write source to
>> source transformations with the clang::Tooling libraries, including
>> clang-tidy checks.
>> The full proposal is in this doc:
>> https://docs.google.com/document/d/1ppw0RhjwsrbBcHYhI85pe6ISDbA6r5d00ot3N8cQWeQ/edit?usp=sharing
>> From the doc:
>> Transformer is a framework that aims to simplify development of
>> clang-based source-to-source transformations.  It focuses on the particular
>> class of transformations that act only locally — that is, use local
>> information about the code and make local changes  (like a syntax-aware
>> “find-and-replace”); and at scale — that is, will be carried out on many
>> source files.  The target audience is users that are comfortable with, or
>> willing to become comfortable with, the clang AST matchers library.
>> I have a working prototype of this library which I've used on small
>> examples inside Google.  I plan to put together a patch for reference
>> next week, although the doc should stand on its own.
>> Thanks!
>> Yitzhak Mandelbaum
>> _______________________________________________
>> cfe-dev mailing listcfe-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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