[TableGen] Add a general-purpose JSON backend.

Authored by simon_tatham on Jul 11 2018, 1:40 AM.


[TableGen] Add a general-purpose JSON backend.

The aim of this backend is to output everything TableGen knows about
the record set, similarly to the default -print-records backend. But
where -print-records produces output in TableGen's input syntax
(convenient for humans to read), this backend produces it as
structured JSON data, which is convenient for loading into standard
scripting languages such as Python, in order to extract information
from the data set in an automated way.

The output data contains a JSON representation of the variable
definitions in output 'def' records, and a few pieces of metadata such
as which of those definitions are tagged with the 'field' prefix and
which defs are derived from which classes. It doesn't dump out
absolutely every piece of knowledge it _could_ produce, such as type
information and complicated arithmetic operator nodes in abstract
superclasses; the main aim is to allow consumers of this JSON dump to
essentially act as new backends, and backends don't generally need to
depend on that kind of data.

The new backend is implemented as an EmitJSON() function similar to
all of llvm-tblgen's other EmitFoo functions, except that it lives in
lib/TableGen instead of utils/TableGen on the basis that I'm expecting
to add it to clang-tblgen too in a future patch.

To test it, I've written a Python script that loads the JSON output
and tests properties of it based on comments in the .td source - more
or less like FileCheck, except that the CHECK: lines have Python
expressions after them instead of textual pattern matches.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: arichardson, labath, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46054

llvm-svn: 336771


simon_tathamJul 11 2018, 1:40 AM
Differential Revision
D46054: [TableGen] Add a general-purpose JSON backend.
rG867b0e41fe3f: [WebAssembly] Only call llvm::value::dump() in debug build.