Also see the Khronos landing page for glslang as a reference front end:
https://www.khronos.org/opengles/sdk/tools/Reference-Compiler/
The above page includes where to get binaries, and is kept up to date regarding the feature level of glslang.
An OpenGL and OpenGL ES shader front end and validator.
There are two components:
A front-end library for programmatic parsing of GLSL/ESSL into an AST.
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
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 shaderThere is also a non-shader extension
.conf
for a configuration file of limits, see usage statement for exampleCMake: 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.
The grammar in glslang/MachineIndependent/glslang.y has to be recompiled with bison if it changes, the output files are committed to the repo to avoid every developer needing to have bison configured to compile the project when grammar changes are quite infrequent. For windows you can get binaries from GnuWin32.
The command to rebuild is:
bison --defines=MachineIndependent/glslang_tab.cpp.h
-t MachineIndependent/glslang.y
-o MachineIndependent/glslang_tab.cpp
Another piece of software can programmatically translate shaders to an AST using one of two different interfaces:
The main()
in StandAlone/StandAlone.cpp
shows examples using both styles.
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.
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.
Test results should always be included with a pull request that modifies functionality. There is a simple process for doing this, described here:
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
. This can be done by
the bump
shell script.
The list of files tested comes from testlist
, and lists input shaders
in this directory, which must all be public for this to work. However,
you can add your own private list of tests, not tracked here, by using
localtestlist
to list non-tracked tests. This is automatically read
by runtests
and included in the diff
and bump
process.
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