glslang/README.md

glslang
=======

An OpenGL and OpenGL ES shader front end and validator.

There are two components:

1. A front-end library for programmatic parsing of GLSL/ESSL into an AST.

2. A standalone wrapper, `glslangValidator`, that can be used as a shader
   validation tool.

How to add a feature protected by a version/extension/stage/profile:  See the
comment in `glslang/MachineIndependent/Versions.cpp`.

Things left to do:  See `Todo.txt`

Execution of Standalone Wrapper
-------------------------------

There are binaries in the `Install/Windows` and `Install/Linux` directories.

To use the standalone binary form, execute `glslangValidator`, and it will print
a usage statement.  Basic operation is to give it a file containing a shader,
and it will print out warnings/errors and optionally an AST.

The applied stage-specific rules are based on the file extension:
* `.vert` for a vertex shader
* `.tesc` for a tessellation control shader
* `.tese` for a tessellation evaluation shader
* `.geom` for a geometry shader
* `.frag` for a fragment shader
* `.comp` for a compute shader

There is also a non-shader extension
* `.conf` for a configuration file of limits, see usage statement for example

Building
--------

CMake: The currently maintained and preferred way of building is through CMake.
In MSVC, after running CMake, you may need to use the Configuration Manager to
check the INSTALL project.

Note there are some legacy build methods still intermingled within the directory
structure (make, MSVC), but these are no longer maintained, having been
replaced with CMake.

Programmatic Interfaces
-----------------------

Another piece of software can programmatically translate shaders to an AST
using one of two different interfaces:
* A new C++ class-oriented interface, or
* The original C functional interface

The `main()` in `StandAlone/StandAlone.cpp` shows examples using both styles.

### C++ Class Interface (new, preferred)

This interface is in roughly the last 1/3 of `ShaderLang.h`.  It is in the
glslang namespace and contains the following.

```cxx
const char* GetEsslVersionString();
const char* GetGlslVersionString();
bool InitializeProcess();
void FinalizeProcess();

class TShader
    bool parse(...);
    void setStrings(...);
    const char* getInfoLog();

class TProgram
    void addShader(...);
    bool link(...);
    const char* getInfoLog();
    Reflection queries
```

See `ShaderLang.h` and the usage of it in `StandAlone/StandAlone.cpp` for more
details.

### C Functional Interface (orginal)

This interface is in roughly the first 2/3 of `ShaderLang.h`, and referred to
as the `Sh*()` interface, as all the entry points start `Sh`.

The `Sh*()` interface takes a "compiler" call-back object, which it calls after
building call back that is passed the AST and can then execute a backend on it.

The following is a simplified resulting run-time call stack:

```c
ShCompile(shader, compiler) -> compiler(AST) -> <back end>
```

In practice, `ShCompile()` takes shader strings, default version, and
warning/error and other options for controling compilation.

Testing
-------

`Test` is an active test directory that contains test input and a
subdirectory `baseResults` that contains the expected results of the
tests.  Both the tests and `baseResults` are under source-code control.
Executing the script `./runtests` will generate current results in
the `localResults` directory and `diff` them against the `baseResults`.
When you want to update the tracked test results, they need to be
copied from `localResults` to `baseResults`.

There are some tests borrowed from LunarGLASS.  If LunarGLASS is
missing, those tests just won't run.

Basic Internal Operation
------------------------

* Initial lexical analysis is done by the preprocessor in
  `MachineIndependent/Preprocessor`, and then refined by a GLSL scanner
  in `MachineIndependent/Scan.cpp`.  There is currently no use of flex.

* Code is parsed using bison on `MachineIndependent/glslang.y` with the
  aid of a symbol table and an AST.  The symbol table is not passed on to
  the back-end; the intermediate representation stands on its own.
  The tree is built by the grammar productions, many of which are
  offloaded into `ParseHelper.cpp`, and by `Intermediate.cpp`.

* The intermediate representation is very high-level, and represented
  as an in-memory tree.   This serves to lose no information from the
  original program, and to have efficient transfer of the result from
  parsing to the back-end.  In the AST, constants are propogated and
  folded, and a very small amount of dead code is eliminated.

  To aid linking and reflection, the last top-level branch in the AST
  lists all global symbols.

* The primary algorithm of the back-end compiler is to traverse the
  tree (high-level intermediate representation), and create an internal
  object code representation.  There is an example of how to do this
  in `MachineIndependent/intermOut.cpp`.

* Reduction of the tree to a linear byte-code style low-level intermediate
  representation is likely a good way to generate fully optimized code.

* There is currently some dead old-style linker-type code still lying around.

* Memory pool: parsing uses types derived from C++ `std` types, using a
  custom allocator that puts them in a memory pool.  This makes allocation
  of individual container/contents just few cycles and deallocation free.
  This pool is popped after the AST is made and processed.

  The use is simple: if you are going to call `new`, there are three cases:

  - the object comes from the pool (its base class has the macro
    `POOL_ALLOCATOR_NEW_DELETE` in it) and you do not have to call `delete`

  - it is a `TString`, in which case call `NewPoolTString()`, which gets
    it from the pool, and there is no corresponding `delete`

  - the object does not come from the pool, and you have to do normal
    C++ memory management of what you `new`