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280 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html lang="en">
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<head>
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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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<title>GL Dispatch in Mesa</title>
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<link rel="stylesheet" type="text/css" href="mesa.css">
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</head>
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<body>
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<div class="header">
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<h1>The Mesa 3D Graphics Library</h1>
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</div>
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<iframe src="contents.html"></iframe>
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<div class="content">
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<h1>GL Dispatch in Mesa</h1>
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<p>Several factors combine to make efficient dispatch of OpenGL functions
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fairly complicated. This document attempts to explain some of the issues
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and introduce the reader to Mesa's implementation. Readers already familiar
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with the issues around GL dispatch can safely skip ahead to the <a
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href="#overview">overview of Mesa's implementation</a>.</p>
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<h2>1. Complexity of GL Dispatch</h2>
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<p>Every GL application has at least one object called a GL <em>context</em>.
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This object, which is an implicit parameter to ever GL function, stores all
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of the GL related state for the application. Every texture, every buffer
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object, every enable, and much, much more is stored in the context. Since
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an application can have more than one context, the context to be used is
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selected by a window-system dependent function such as
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<tt>glXMakeContextCurrent</tt>.</p>
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<p>In environments that implement OpenGL with X-Windows using GLX, every GL
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function, including the pointers returned by <tt>glXGetProcAddress</tt>, are
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<em>context independent</em>. This means that no matter what context is
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currently active, the same <tt>glVertex3fv</tt> function is used.</p>
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<p>This creates the first bit of dispatch complexity. An application can
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have two GL contexts. One context is a direct rendering context where
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function calls are routed directly to a driver loaded within the
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application's address space. The other context is an indirect rendering
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context where function calls are converted to GLX protocol and sent to a
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server. The same <tt>glVertex3fv</tt> has to do the right thing depending
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on which context is current.</p>
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<p>Highly optimized drivers or GLX protocol implementations may want to
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change the behavior of GL functions depending on current state. For
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example, <tt>glFogCoordf</tt> may operate differently depending on whether
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or not fog is enabled.</p>
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<p>In multi-threaded environments, it is possible for each thread to have a
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differnt GL context current. This means that poor old <tt>glVertex3fv</tt>
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has to know which GL context is current in the thread where it is being
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called.</p>
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<h2 id="overview">2. Overview of Mesa's Implementation</h2>
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<p>Mesa uses two per-thread pointers. The first pointer stores the address
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of the context current in the thread, and the second pointer stores the
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address of the <em>dispatch table</em> associated with that context. The
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dispatch table stores pointers to functions that actually implement
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specific GL functions. Each time a new context is made current in a thread,
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these pointers a updated.</p>
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<p>The implementation of functions such as <tt>glVertex3fv</tt> becomes
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conceptually simple:</p>
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<ul>
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<li>Fetch the current dispatch table pointer.</li>
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<li>Fetch the pointer to the real <tt>glVertex3fv</tt> function from the
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table.</li>
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<li>Call the real function.</li>
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</ul>
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<p>This can be implemented in just a few lines of C code. The file
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<tt>src/mesa/glapi/glapitemp.h</tt> contains code very similar to this.</p>
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<blockquote>
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<table border="1">
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<tr><td><pre>
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void glVertex3f(GLfloat x, GLfloat y, GLfloat z)
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{
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const struct _glapi_table * const dispatch = GET_DISPATCH();
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(*dispatch->Vertex3f)(x, y, z);
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}</pre></td></tr>
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<tr><td>Sample dispatch function</td></tr></table>
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</blockquote>
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<p>The problem with this simple implementation is the large amount of
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overhead that it adds to every GL function call.</p>
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<p>In a multithreaded environment, a naive implementation of
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<tt>GET_DISPATCH</tt> involves a call to <tt>pthread_getspecific</tt> or a
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similar function. Mesa provides a wrapper function called
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<tt>_glapi_get_dispatch</tt> that is used by default.</p>
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<h2>3. Optimizations</h2>
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<p>A number of optimizations have been made over the years to diminish the
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performance hit imposed by GL dispatch. This section describes these
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optimizations. The benefits of each optimization and the situations where
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each can or cannot be used are listed.</p>
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<h3>3.1. Dual dispatch table pointers</h3>
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<p>The vast majority of OpenGL applications use the API in a single threaded
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manner. That is, the application has only one thread that makes calls into
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the GL. In these cases, not only do the calls to
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<tt>pthread_getspecific</tt> hurt performance, but they are completely
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unnecessary! It is possible to detect this common case and avoid these
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calls.</p>
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<p>Each time a new dispatch table is set, Mesa examines and records the ID
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of the executing thread. If the same thread ID is always seen, Mesa knows
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that the application is, from OpenGL's point of view, single threaded.</p>
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<p>As long as an application is single threaded, Mesa stores a pointer to
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the dispatch table in a global variable called <tt>_glapi_Dispatch</tt>.
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The pointer is also stored in a per-thread location via
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<tt>pthread_setspecific</tt>. When Mesa detects that an application has
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become multithreaded, <tt>NULL</tt> is stored in <tt>_glapi_Dispatch</tt>.</p>
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<p>Using this simple mechanism the dispatch functions can detect the
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multithreaded case by comparing <tt>_glapi_Dispatch</tt> to <tt>NULL</tt>.
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The resulting implementation of <tt>GET_DISPATCH</tt> is slightly more
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complex, but it avoids the expensive <tt>pthread_getspecific</tt> call in
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the common case.</p>
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<blockquote>
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<table border="1">
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<tr><td><pre>
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#define GET_DISPATCH() \
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(_glapi_Dispatch != NULL) \
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? _glapi_Dispatch : pthread_getspecific(&_glapi_Dispatch_key)
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</pre></td></tr>
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<tr><td>Improved <tt>GET_DISPATCH</tt> Implementation</td></tr></table>
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</blockquote>
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<h3>3.2. ELF TLS</h3>
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<p>Starting with the 2.4.20 Linux kernel, each thread is allocated an area
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of per-thread, global storage. Variables can be put in this area using some
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extensions to GCC. By storing the dispatch table pointer in this area, the
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expensive call to <tt>pthread_getspecific</tt> and the test of
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<tt>_glapi_Dispatch</tt> can be avoided.</p>
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<p>The dispatch table pointer is stored in a new variable called
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<tt>_glapi_tls_Dispatch</tt>. A new variable name is used so that a single
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libGL can implement both interfaces. This allows the libGL to operate with
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direct rendering drivers that use either interface. Once the pointer is
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properly declared, <tt>GET_DISPACH</tt> becomes a simple variable
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reference.</p>
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<blockquote>
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<table border="1">
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<tr><td><pre>
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extern __thread struct _glapi_table *_glapi_tls_Dispatch
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__attribute__((tls_model("initial-exec")));
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#define GET_DISPATCH() _glapi_tls_Dispatch
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</pre></td></tr>
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<tr><td>TLS <tt>GET_DISPATCH</tt> Implementation</td></tr></table>
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</blockquote>
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<p>Use of this path is controlled by the preprocessor define
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<tt>GLX_USE_TLS</tt>. Any platform capable of using TLS should use this as
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the default dispatch method.</p>
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<h3>3.3. Assembly Language Dispatch Stubs</h3>
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<p>Many platforms has difficulty properly optimizing the tail-call in the
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dispatch stubs. Platforms like x86 that pass parameters on the stack seem
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to have even more difficulty optimizing these routines. All of the dispatch
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routines are very short, and it is trivial to create optimal assembly
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language versions. The amount of optimization provided by using assembly
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stubs varies from platform to platform and application to application.
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However, by using the assembly stubs, many platforms can use an additional
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space optimization (see <a href="#fixedsize">below</a>).</p>
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<p>The biggest hurdle to creating assembly stubs is handling the various
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ways that the dispatch table pointer can be accessed. There are four
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different methods that can be used:</p>
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<ol>
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<li>Using <tt>_glapi_Dispatch</tt> directly in builds for non-multithreaded
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environments.</li>
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<li>Using <tt>_glapi_Dispatch</tt> and <tt>_glapi_get_dispatch</tt> in
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multithreaded environments.</li>
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<li>Using <tt>_glapi_Dispatch</tt> and <tt>pthread_getspecific</tt> in
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multithreaded environments.</li>
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<li>Using <tt>_glapi_tls_Dispatch</tt> directly in TLS enabled
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multithreaded environments.</li>
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</ol>
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<p>People wishing to implement assembly stubs for new platforms should focus
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on #4 if the new platform supports TLS. Otherwise, implement #2 followed by
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#3. Environments that do not support multithreading are uncommon and not
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terribly relevant.</p>
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<p>Selection of the dispatch table pointer access method is controlled by a
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few preprocessor defines.</p>
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<ul>
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<li>If <tt>GLX_USE_TLS</tt> is defined, method #4 is used.</li>
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<li>If <tt>HAVE_PTHREAD</tt> is defined, method #3 is used.</li>
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<li>If <tt>WIN32_THREADS</tt> is defined, method #2 is used.</li>
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<li>If none of the preceeding are defined, method #1 is used.</li>
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</ul>
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<p>Two different techniques are used to handle the various different cases.
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On x86 and SPARC, a macro called <tt>GL_STUB</tt> is used. In the preamble
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of the assembly source file different implementations of the macro are
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selected based on the defined preprocessor variables. The assmebly code
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then consists of a series of invocations of the macros such as:
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<blockquote>
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<table border="1">
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<tr><td><pre>
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GL_STUB(Color3fv, _gloffset_Color3fv)
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</pre></td></tr>
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<tr><td>SPARC Assembly Implementation of <tt>glColor3fv</tt></td></tr></table>
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</blockquote>
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<p>The benefit of this technique is that changes to the calling pattern
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(i.e., addition of a new dispatch table pointer access method) require fewer
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changed lines in the assembly code.</p>
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<p>However, this technique can only be used on platforms where the function
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implementation does not change based on the parameters passed to the
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function. For example, since x86 passes all parameters on the stack, no
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additional code is needed to save and restore function parameters around a
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call to <tt>pthread_getspecific</tt>. Since x86-64 passes parameters in
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registers, varying amounts of code needs to be inserted around the call to
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<tt>pthread_getspecific</tt> to save and restore the GL function's
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parameters.</p>
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<p>The other technique, used by platforms like x86-64 that cannot use the
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first technique, is to insert <tt>#ifdef</tt> within the assembly
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implementation of each function. This makes the assembly file considerably
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larger (e.g., 29,332 lines for <tt>glapi_x86-64.S</tt> versus 1,155 lines for
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<tt>glapi_x86.S</tt>) and causes simple changes to the function
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implementation to generate many lines of diffs. Since the assmebly files
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are typically generated by scripts (see <a href="#autogen">below</a>), this
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isn't a significant problem.</p>
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<p>Once a new assembly file is created, it must be inserted in the build
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system. There are two steps to this. The file must first be added to
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<tt>src/mesa/sources</tt>. That gets the file built and linked. The second
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step is to add the correct <tt>#ifdef</tt> magic to
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<tt>src/mesa/glapi/glapi_dispatch.c</tt> to prevent the C version of the
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dispatch functions from being built.</p>
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<h3 id="fixedsize">3.4. Fixed-Length Dispatch Stubs</h3>
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<p>To implement <tt>glXGetProcAddress</tt>, Mesa stores a table that
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associates function names with pointers to those functions. This table is
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stored in <tt>src/mesa/glapi/glprocs.h</tt>. For different reasons on
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different platforms, storing all of those pointers is inefficient. On most
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platforms, including all known platforms that support TLS, we can avoid this
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added overhead.</p>
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<p>If the assembly stubs are all the same size, the pointer need not be
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stored for every function. The location of the function can instead be
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calculated by multiplying the size of the dispatch stub by the offset of the
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function in the table. This value is then added to the address of the first
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dispatch stub.</p>
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<p>This path is activated by adding the correct <tt>#ifdef</tt> magic to
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<tt>src/mesa/glapi/glapi.c</tt> just before <tt>glprocs.h</tt> is
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included.</p>
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<h2 id="autogen">4. Automatic Generation of Dispatch Stubs</h2>
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</div>
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</body>
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</html>
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