Add an OpenGL program binary disk cache

Introduce a glProgramBinary-based disk cache in QOpenGLShaderProgram.

By switching the typical

program->addShaderFromSourceCode(QOpenGLShader::Vertex, ...)
program->addShaderFromSourceCode(QOpenGLShader::Fragment, ...)

invocations to

program->addCacheableShaderFromSourceCode(QOpenGLShader::Vertex, ...)
program->addCacheableShaderFromSourceCode(QOpenGLShader::Fragment, ...)

the compilation may be skipped via gl(Get)ProgramBinary and a disk
cache, when supported.  Such QOpenGLShaderProgram instances will have
no QOpenGLShader instances attached. Instead, the entire program
binary (which is driver-specific) is loaded as-is.

Support means OpenGL ES 3.0 or the presence of
GL_ARB_get_program_binary, in combination with >= 1 supported binary
formats. Note that some drivers claim program binary support but
expose no formats. This amounts to no support in practice.

When support is not present, calling the new functions is equivalent
to the non-cacheable variants. If the OpenGL driver changes (vendor,
renderer, version strings), recompilation and storage of the new,
potentially incompatible binary program will happen transparently.

The cache can always be disabled by setting
QT_DISABLE_SHADER_DISK_CACHE=1 or the new application attribute
Qt::AA_DisableShaderDiskCache.

Location-wise the primary choice is the shared cache
(GenericCacheLocation). If this is not available or is not writable,
the per-process one (CacheLocation) is used instead.

In addition to the new public APIs in QOpenGLShaderProgram, the main
shader users in QtGui are migrated as well. (OpenGL paint engine,
glyph cache, blitter, eglfs mouse cursor).  This means that any
application using QPainter on OpenGL or widgets with eglfs will
benefit from the improved startup times. Qt Quick will follow suit as
well.

[ChangeLog][QtGui][OpenGL] QOpenGLShaderProgram offers a built-in
program binary disk cache for systems with OpenGL ES 3.x or
GL_ARB_get_program_binary. This can lead to significant increases in
performance when it comes to application startup times for
example. Usage is opt-in for direct C++ users of the class, however
Qt's own main users of shaders, including Qt Quick and QPainter's
OpenGL engine, are migrated to use the new, cache-enabled APIs.
Opting out on application level is always possible via
Qt::AA_DisableShaderDiskCache.

Task-number: QTBUG-55496
Change-Id: I556f053d258bfa6887b1d5238c9f6396914c5421
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/corelib/global/qnamespace.qdoc b/src/corelib/global/qnamespace.qdoc
index 250195a512..64f0b5e1da 100644
--- a/src/corelib/global/qnamespace.qdoc
+++ b/src/corelib/global/qnamespace.qdoc
@@ -257,6 +257,15 @@
            \l{QOpenGLContext::makeCurrent}{makeCurrent()}. This value has been
            added in Qt 5.8.
 
+    \value AA_DisableShaderDiskCache Disables caching of shader program binaries
+           on disk. By default Qt Quick, QPainter's OpenGL backend, and any
+           application using QOpenGLShaderProgram with one of its
+           \e addCacheableShaderFromSource overloads will employ a disk-based
+           \l{Caching Program Binaries}{program binary cache} in either the shared
+           or per-process cache storage location, on systems that support
+           \e glProgramBinary(). In the unlikely event of this being problematic,
+           set this attribute to disable all disk-based caching of shaders.
+
     The following values are obsolete:
 
     \value AA_ImmediateWidgetCreation This attribute is no longer fully