1 Also see the Khronos landing page for glslang as a reference front end:
3 https://www.khronos.org/opengles/sdk/tools/Reference-Compiler/
5 The above page includes where to get binaries, and is kept up to date
6 regarding the feature level of glslang.
11 An OpenGL and OpenGL ES shader front end and validator.
13 There are two components:
15 1. A front-end library for programmatic parsing of GLSL/ESSL into an AST.
17 2. A standalone wrapper, `glslangValidator`, that can be used as a shader
20 How to add a feature protected by a version/extension/stage/profile: See the
21 comment in `glslang/MachineIndependent/Versions.cpp`.
23 Things left to do: See `Todo.txt`
25 Execution of Standalone Wrapper
26 -------------------------------
28 There are binaries in the `Install/Windows` and `Install/Linux` directories.
30 To use the standalone binary form, execute `glslangValidator`, and it will print
31 a usage statement. Basic operation is to give it a file containing a shader,
32 and it will print out warnings/errors and optionally an AST.
34 The applied stage-specific rules are based on the file extension:
35 * `.vert` for a vertex shader
36 * `.tesc` for a tessellation control shader
37 * `.tese` for a tessellation evaluation shader
38 * `.geom` for a geometry shader
39 * `.frag` for a fragment shader
40 * `.comp` for a compute shader
42 There is also a non-shader extension
43 * `.conf` for a configuration file of limits, see usage statement for example
48 CMake: The currently maintained and preferred way of building is through CMake.
49 In MSVC, after running CMake, you may need to use the Configuration Manager to
50 check the INSTALL project.
52 Note there are some legacy build methods still intermingled within the directory
53 structure (make, MSVC), but these are no longer maintained, having been
56 Programmatic Interfaces
57 -----------------------
59 Another piece of software can programmatically translate shaders to an AST
60 using one of two different interfaces:
61 * A new C++ class-oriented interface, or
62 * The original C functional interface
64 The `main()` in `StandAlone/StandAlone.cpp` shows examples using both styles.
66 ### C++ Class Interface (new, preferred)
68 This interface is in roughly the last 1/3 of `ShaderLang.h`. It is in the
69 glslang namespace and contains the following.
72 const char* GetEsslVersionString();
73 const char* GetGlslVersionString();
74 bool InitializeProcess();
75 void FinalizeProcess();
80 const char* getInfoLog();
85 const char* getInfoLog();
89 See `ShaderLang.h` and the usage of it in `StandAlone/StandAlone.cpp` for more
92 ### C Functional Interface (orginal)
94 This interface is in roughly the first 2/3 of `ShaderLang.h`, and referred to
95 as the `Sh*()` interface, as all the entry points start `Sh`.
97 The `Sh*()` interface takes a "compiler" call-back object, which it calls after
98 building call back that is passed the AST and can then execute a backend on it.
100 The following is a simplified resulting run-time call stack:
103 ShCompile(shader, compiler) -> compiler(AST) -> <back end>
106 In practice, `ShCompile()` takes shader strings, default version, and
107 warning/error and other options for controling compilation.
112 `Test` is an active test directory that contains test input and a
113 subdirectory `baseResults` that contains the expected results of the
114 tests. Both the tests and `baseResults` are under source-code control.
115 Executing the script `./runtests` will generate current results in
116 the `localResults` directory and `diff` them against the `baseResults`.
117 When you want to update the tracked test results, they need to be
118 copied from `localResults` to `baseResults`.
120 There are some tests borrowed from LunarGLASS. If LunarGLASS is
121 missing, those tests just won't run.
123 Basic Internal Operation
124 ------------------------
126 * Initial lexical analysis is done by the preprocessor in
127 `MachineIndependent/Preprocessor`, and then refined by a GLSL scanner
128 in `MachineIndependent/Scan.cpp`. There is currently no use of flex.
130 * Code is parsed using bison on `MachineIndependent/glslang.y` with the
131 aid of a symbol table and an AST. The symbol table is not passed on to
132 the back-end; the intermediate representation stands on its own.
133 The tree is built by the grammar productions, many of which are
134 offloaded into `ParseHelper.cpp`, and by `Intermediate.cpp`.
136 * The intermediate representation is very high-level, and represented
137 as an in-memory tree. This serves to lose no information from the
138 original program, and to have efficient transfer of the result from
139 parsing to the back-end. In the AST, constants are propogated and
140 folded, and a very small amount of dead code is eliminated.
142 To aid linking and reflection, the last top-level branch in the AST
143 lists all global symbols.
145 * The primary algorithm of the back-end compiler is to traverse the
146 tree (high-level intermediate representation), and create an internal
147 object code representation. There is an example of how to do this
148 in `MachineIndependent/intermOut.cpp`.
150 * Reduction of the tree to a linear byte-code style low-level intermediate
151 representation is likely a good way to generate fully optimized code.
153 * There is currently some dead old-style linker-type code still lying around.
155 * Memory pool: parsing uses types derived from C++ `std` types, using a
156 custom allocator that puts them in a memory pool. This makes allocation
157 of individual container/contents just few cycles and deallocation free.
158 This pool is popped after the AST is made and processed.
160 The use is simple: if you are going to call `new`, there are three cases:
162 - the object comes from the pool (its base class has the macro
163 `POOL_ALLOCATOR_NEW_DELETE` in it) and you do not have to call `delete`
165 - it is a `TString`, in which case call `NewPoolTString()`, which gets
166 it from the pool, and there is no corresponding `delete`
168 - the object does not come from the pool, and you have to do normal
169 C++ memory management of what you `new`