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   8 <H1>GL Dispatch in Mesa</H1>
   9
  10 <p>Several factors combine to make efficient dispatch of OpenGL functions
  11 fairly complicated.  This document attempts to explain some of the issues
  12 and introduce the reader to Mesa's implementation.  Readers already familiar
  13 with the issues around GL dispatch can safely skip ahead to the <A
  14 HREF="#overview">overview of Mesa's implementation</A>.</p>
  15
  16 <H2>1. Complexity of GL Dispatch</H2>
  17
  18 <p>Every GL application has at least one object called a GL <em>context</em>.
  19 This object, which is an implicit parameter to ever GL function, stores all
  20 of the GL related state for the application.  Every texture, every buffer
  21 object, every enable, and much, much more is stored in the context.  Since
  22 an application can have more than one context, the context to be used is
  23 selected by a window-system dependent function such as
  24 <tt>glXMakeContextCurrent</tt>.</p>
  25
  26 <p>In environments that implement OpenGL with X-Windows using GLX, every GL
  27 function, including the pointers returned by <tt>glXGetProcAddress</tt>, are
  28 <em>context independent</em>.  This means that no matter what context is
  29 currently active, the same <tt>glVertex3fv</tt> function is used.</p>
  30
  31 <p>This creates the first bit of dispatch complexity.  An application can
  32 have two GL contexts.  One context is a direct rendering context where
  33 function calls are routed directly to a driver loaded within the
  34 application's address space.  The other context is an indirect rendering
  35 context where function calls are converted to GLX protocol and sent to a
  36 server.  The same <tt>glVertex3fv</tt> has to do the right thing depending
  37 on which context is current.</p>
  38
  39 <p>Highly optimized drivers or GLX protocol implementations may want to
  40 change the behavior of GL functions depending on current state.  For
  41 example, <tt>glFogCoordf</tt> may operate differently depending on whether
  42 or not fog is enabled.</p>
  43
  44 <p>In multi-threaded environments, it is possible for each thread to have a
  45 differnt GL context current.  This means that poor old <tt>glVertex3fv</tt>
  46 has to know which GL context is current in the thread where it is being
  47 called.</p>
  48
  49 <A NAME="overview"/>
  50 <H2>2. Overview of Mesa's Implementation</H2>
  51
  52 <p>Mesa uses two per-thread pointers.  The first pointer stores the address
  53 of the context current in the thread, and the second pointer stores the
  54 address of the <em>dispatch table</em> associated with that context.  The
  55 dispatch table stores pointers to functions that actually implement
  56 specific GL functions.  Each time a new context is made current in a thread,
  57 these pointers a updated.</p>
  58
  59 <p>The implementation of functions such as <tt>glVertex3fv</tt> becomes
  60 conceptually simple:</p>
  61
  62 <ul>
  63 <li>Fetch the current dispatch table pointer.</li>
  64 <li>Fetch the pointer to the real <tt>glVertex3fv</tt> function from the
  65 table.</li>
  66 <li>Call the real function.</li>
  67 </ul>
  68
  69 <p>This can be implemented in just a few lines of C code.  The file
  70 <tt>src/mesa/glapi/glapitemp.h</tt> contains code very similar to this.</p>
  71
  72 <blockquote>
  73 <table border="1">
  74 <tr><td><pre>
  75 void glVertex3f(GLfloat x, GLfloat y, GLfloat z)
  76 {
  77     const struct _glapi_table * const dispatch = GET_DISPATCH();
  78
  79     (*dispatch-&gt;Vertex3f)(x, y, z);
  80 }</pre></td></tr>
  81 <tr><td>Sample dispatch function</td></tr></table>
  82 </blockquote>
  83
  84 <p>The problem with this simple implementation is the large amount of
  85 overhead that it adds to every GL function call.</p>
  86
  87 <p>In a multithreaded environment, a niave implementation of
  88 <tt>GET_DISPATCH</tt> involves a call to <tt>pthread_getspecific</tt> or a
  89 similar function.  Mesa provides a wrapper function called
  90 <tt>_glapi_get_dispatch</tt> that is used by default.</p>
  91
  92 <H2>3. Optimizations</H2>
  93
  94 <p>A number of optimizations have been made over the years to diminish the
  95 performance hit imposed by GL dispatch.  This section describes these
  96 optimizations.  The benefits of each optimization and the situations where
  97 each can or cannot be used are listed.</p>
  98
  99 <H3>3.1. Dual dispatch table pointers</H3>
 100
 101 <p>The vast majority of OpenGL applications use the API in a single threaded
 102 manner.  That is, the application has only one thread that makes calls into
 103 the GL.  In these cases, not only do the calls to
 104 <tt>pthread_getspecific</tt> hurt performance, but they are completely
 105 unnecessary!  It is possible to detect this common case and avoid these
 106 calls.</p>
 107
 108 <p>Each time a new dispatch table is set, Mesa examines and records the ID
 109 of the executing thread.  If the same thread ID is always seen, Mesa knows
 110 that the application is, from OpenGL's point of view, single threaded.</p>
 111
 112 <p>As long as an application is single threaded, Mesa stores a pointer to
 113 the dispatch table in a global variable called <tt>_glapi_Dispatch</tt>.
 114 The pointer is also stored in a per-thread location via
 115 <tt>pthread_setspecific</tt>.  When Mesa detects that an application has
 116 become multithreaded, <tt>NULL</tt> is stored in <tt>_glapi_Dispatch</tt>.</p>
 117
 118 <p>Using this simple mechanism the dispatch functions can detect the
 119 multithreaded case by comparing <tt>_glapi_Dispatch</tt> to <tt>NULL</tt>.
 120 The resulting implementation of <tt>GET_DISPATCH</tt> is slightly more
 121 complex, but it avoids the expensive <tt>pthread_getspecific</tt> call in
 122 the common case.</p>
 123
 124 <blockquote>
 125 <table border="1">
 126 <tr><td><pre>
 127 #define GET_DISPATCH() \
 128     (_glapi_Dispatch != NULL) \
 129         ? _glapi_Dispatch : pthread_getspecific(&_glapi_Dispatch_key)
 130 </pre></td></tr>
 131 <tr><td>Improved <tt>GET_DISPATCH</tt> Implementation</td></tr></table>
 132 </blockquote>
 133
 134 <H3>3.2. ELF TLS</H3>
 135
 136 <p>Starting with the 2.4.20 Linux kernel, each thread is allocated an area
 137 of per-thread, global storage.  Variables can be put in this area using some
 138 extensions to GCC.  By storing the dispatch table pointer in this area, the
 139 expensive call to <tt>pthread_getspecific</tt> and the test of
 140 <tt>_glapi_Dispatch</tt> can be avoided.</p>
 141
 142 <p>The dispatch table pointer is stored in a new variable called
 143 <tt>_glapi_tls_Dispatch</tt>.  A new variable name is used so that a single
 144 libGL can implement both interfaces.  This allows the libGL to operate with
 145 direct rendering drivers that use either interface.  Once the pointer is
 146 properly declared, <tt>GET_DISPACH</tt> becomes a simple variable
 147 reference.</p>
 148
 149 <blockquote>
 150 <table border="1">
 151 <tr><td><pre>
 152 extern __thread struct _glapi_table *_glapi_tls_Dispatch
 153     __attribute__((tls_model("initial-exec")));
 154
 155 #define GET_DISPATCH() _glapi_tls_Dispatch
 156 </pre></td></tr>
 157 <tr><td>TLS <tt>GET_DISPATCH</tt> Implementation</td></tr></table>
 158 </blockquote>
 159
 160 <p>Use of this path is controlled by the preprocessor define
 161 <tt>GLX_USE_TLS</tt>.  Any platform capable of using TLS should use this as
 162 the default dispatch method.</p>
 163
 164 <H3>3.3. Assembly Language Dispatch Stubs</H3>
 165
 166 <p>Many platforms has difficulty properly optimizing the tail-call in the
 167 dispatch stubs.  Platforms like x86 that pass parameters on the stack seem
 168 to have even more difficulty optimizing these routines.  All of the dispatch
 169 routines are very short, and it is trivial to create optimal assembly
 170 language versions.  The amount of optimization provided by using assembly
 171 stubs varies from platform to platform and application to application.
 172 However, by using the assembly stubs, many platforms can use an additional
 173 space optimization (see <A HREF="#fixedsize">below</A>).</p>
 174
 175 <p>The biggest hurdle to creating assembly stubs is handling the various
 176 ways that the dispatch table pointer can be accessed.  There are four
 177 different methods that can be used:</p>
 178
 179 <ol>
 180 <li>Using <tt>_glapi_Dispatch</tt> directly in builds for non-multithreaded
 181 environments.</li>
 182 <li>Using <tt>_glapi_Dispatch</tt> and <tt>_glapi_get_dispatch</tt> in
 183 multithreaded environments.</li>
 184 <li>Using <tt>_glapi_Dispatch</tt> and <tt>pthread_getspecific</tt> in
 185 multithreaded environments.</li>
 186 <li>Using <tt>_glapi_tls_Dispatch</tt> directly in TLS enabled
 187 multithreaded environments.</li>
 188 </ol>
 189
 190 <p>People wishing to implement assembly stubs for new platforms should focus
 191 on #4 if the new platform supports TLS.  Otherwise, implement #2 followed by
 192 #3.  Environments that do not support multithreading are uncommon and not
 193 terribly relevant.</p>
 194
 195 <p>Selection of the dispatch table pointer access method is controlled by a
 196 few preprocessor defines.</p>
 197
 198 <ul>
 199 <li>If <tt>GLX_USE_TLS</tt> is defined, method #4 is used.</li>
 200 <li>If <tt>PTHREADS</tt> is defined, method #3 is used.</li>
 201 <li>If any of <tt>PTHREADS</tt>,
 202 <tt>WIN32_THREADS</tt>, or <tt>BEOS_THREADS</tt>
 203 is defined, method #2 is used.</li>
 204 <li>If none of the preceeding are defined, method #1 is used.</li>
 205 </ul>
 206
 207 <p>Two different techniques are used to handle the various different cases.
 208 On x86 and SPARC, a macro called <tt>GL_STUB</tt> is used.  In the preamble
 209 of the assembly source file different implementations of the macro are
 210 selected based on the defined preprocessor variables.  The assmebly code
 211 then consists of a series of invocations of the macros such as:
 212
 213 <blockquote>
 214 <table border="1">
 215 <tr><td><pre>
 216 GL_STUB(Color3fv, _gloffset_Color3fv)
 217 </pre></td></tr>
 218 <tr><td>SPARC Assembly Implementation of <tt>glColor3fv</tt></td></tr></table>
 219 </blockquote>
 220
 221 <p>The benefit of this technique is that changes to the calling pattern
 222 (i.e., addition of a new dispatch table pointer access method) require fewer
 223 changed lines in the assembly code.</p>
 224
 225 <p>However, this technique can only be used on platforms where the function
 226 implementation does not change based on the parameters passed to the
 227 function.  For example, since x86 passes all parameters on the stack, no
 228 additional code is needed to save and restore function parameters around a
 229 call to <tt>pthread_getspecific</tt>.  Since x86-64 passes parameters in
 230 registers, varying amounts of code needs to be inserted around the call to
 231 <tt>pthread_getspecific</tt> to save and restore the GL function's
 232 parameters.</p>
 233
 234 <p>The other technique, used by platforms like x86-64 that cannot use the
 235 first technique, is to insert <tt>#ifdef</tt> within the assembly
 236 implementation of each function.  This makes the assembly file considerably
 237 larger (e.g., 29,332 lines for <tt>glapi_x86-64.S</tt> versus 1,155 lines for
 238 <tt>glapi_x86.S</tt>) and causes simple changes to the function
 239 implementation to generate many lines of diffs.  Since the assmebly files
 240 are typically generated by scripts (see <A HREF="#autogen">below</A>), this
 241 isn't a significant problem.</p>
 242
 243 <p>Once a new assembly file is created, it must be inserted in the build
 244 system.  There are two steps to this.  The file must first be added to
 245 <tt>src/mesa/sources</tt>.  That gets the file built and linked.  The second
 246 step is to add the correct <tt>#ifdef</tt> magic to
 247 <tt>src/mesa/glapi/glapi_dispatch.c</tt> to prevent the C version of the
 248 dispatch functions from being built.</p>
 249
 250 <A NAME="fixedsize"/>
 251 <H3>3.4. Fixed-Length Dispatch Stubs</H3>
 252
 253 <p>To implement <tt>glXGetProcAddress</tt>, Mesa stores a table that
 254 associates function names with pointers to those functions.  This table is
 255 stored in <tt>src/mesa/glapi/glprocs.h</tt>.  For different reasons on
 256 different platforms, storing all of those pointers is inefficient.  On most
 257 platforms, including all known platforms that support TLS, we can avoid this
 258 added overhead.</p>
 259
 260 <p>If the assembly stubs are all the same size, the pointer need not be
 261 stored for every function.  The location of the function can instead be
 262 calculated by multiplying the size of the dispatch stub by the offset of the
 263 function in the table.  This value is then added to the address of the first
 264 dispatch stub.</p>
 265
 266 <p>This path is activated by adding the correct <tt>#ifdef</tt> magic to
 267 <tt>src/mesa/glapi/glapi.c</tt> just before <tt>glprocs.h</tt> is
 268 included.</p>
 269
 270 <A NAME="autogen"/>
 271 <H2>4. Automatic Generation of Dispatch Stubs</H2>
 272
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