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Note there are some legacy build methods still intermingled within the directory structure (make, MSVC), but these are no longer maintained, having been replaced with CMake.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

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:

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