src/compiler/glsl/README

   1 Welcome to Mesa's GLSL compiler.  A brief overview of how things flow:
   2
   3 1) lex and yacc-based preprocessor takes the incoming shader string
   4 and produces a new string containing the preprocessed shader.  This
   5 takes care of things like #if, #ifdef, #define, and preprocessor macro
   6 invocations.  Note that #version, #extension, and some others are
   7 passed straight through.  See glcpp/*
   8
   9 2) lex and yacc-based parser takes the preprocessed string and
  10 generates the AST (abstract syntax tree).  Almost no checking is
  11 performed in this stage.  See glsl_lexer.ll and glsl_parser.yy.
  12
  13 3) The AST is converted to "HIR".  This is the intermediate
  14 representation of the compiler.  Constructors are generated, function
  15 calls are resolved to particular function signatures, and all the
  16 semantic checking is performed.  See ast_*.cpp for the conversion, and
  17 ir.h for the IR structures.
  18
  19 4) The driver (Mesa, or main.cpp for the standalone binary) performs
  20 optimizations.  These include copy propagation, dead code elimination,
  21 constant folding, and others.  Generally the driver will call
  22 optimizations in a loop, as each may open up opportunities for other
  23 optimizations to do additional work.  See most files called ir_*.cpp
  24
  25 5) linking is performed.  This does checking to ensure that the
  26 outputs of the vertex shader match the inputs of the fragment shader,
  27 and assigns locations to uniforms, attributes, and varyings.  See
  28 linker.cpp.
  29
  30 6) The driver may perform additional optimization at this point, as
  31 for example dead code elimination previously couldn't remove functions
  32 or global variable usage when we didn't know what other code would be
  33 linked in.
  34
  35 7) The driver performs code generation out of the IR, taking a linked
  36 shader program and producing a compiled program for each stage.  See
  37 ../mesa/program/ir_to_mesa.cpp for Mesa IR code generation.
  38
  39 FAQ:
  40
  41 Q: What is HIR versus IR versus LIR?
  42
  43 A: The idea behind the naming was that ast_to_hir would produce a
  44 high-level IR ("HIR"), with things like matrix operations, structure
  45 assignments, etc., present.  A series of lowering passes would occur
  46 that do things like break matrix multiplication into a series of dot
  47 products/MADs, make structure assignment be a series of assignment of
  48 components, flatten if statements into conditional moves, and such,
  49 producing a low level IR ("LIR").
  50
  51 However, it now appears that each driver will have different
  52 requirements from a LIR.  A 915-generation chipset wants all functions
  53 inlined, all loops unrolled, all ifs flattened, no variable array
  54 accesses, and matrix multiplication broken down.  The Mesa IR backend
  55 for swrast would like matrices and structure assignment broken down,
  56 but it can support function calls and dynamic branching.  A 965 vertex
  57 shader IR backend could potentially even handle some matrix operations
  58 without breaking them down, but the 965 fragment shader IR backend
  59 would want to break to have (almost) all operations down channel-wise
  60 and perform optimization on that.  As a result, there's no single
  61 low-level IR that will make everyone happy.  So that usage has fallen
  62 out of favor, and each driver will perform a series of lowering passes
  63 to take the HIR down to whatever restrictions it wants to impose
  64 before doing codegen.
  65
  66 Q: How is the IR structured?
  67
  68 A: The best way to get started seeing it would be to run the
  69 standalone compiler against a shader:
  70
  71 ./glsl_compiler --dump-lir \
  72         ~/src/piglit/tests/shaders/glsl-orangebook-ch06-bump.frag
  73
  74 So for example one of the ir_instructions in main() contains:
  75
  76 (assign (constant bool (1)) (var_ref litColor)  (expression vec3 * (var_ref Surf
  77 aceColor) (var_ref __retval) ) )
  78
  79 Or more visually:
  80                      (assign)
  81                  /       |        \
  82         (var_ref)  (expression *)  (constant bool 1)
  83          /          /           \
  84 (litColor)      (var_ref)    (var_ref)
  85                   /                  \
  86            (SurfaceColor)          (__retval)
  87
  88 which came from:
  89
  90 litColor = SurfaceColor * max(dot(normDelta, LightDir), 0.0);
  91
  92 (the max call is not represented in this expression tree, as it was a
  93 function call that got inlined but not brought into this expression
  94 tree)
  95
  96 Each of those nodes is a subclass of ir_instruction.  A particular
  97 ir_instruction instance may only appear once in the whole IR tree with
  98 the exception of ir_variables, which appear once as variable
  99 declarations:
 100
 101 (declare () vec3 normDelta)
 102
 103 and multiple times as the targets of variable dereferences:
 104 ...
 105 (assign (constant bool (1)) (var_ref __retval) (expression float dot
 106  (var_ref normDelta) (var_ref LightDir) ) )
 107 ...
 108 (assign (constant bool (1)) (var_ref __retval) (expression vec3 -
 109  (var_ref LightDir) (expression vec3 * (constant float (2.000000))
 110  (expression vec3 * (expression float dot (var_ref normDelta) (var_ref
 111  LightDir) ) (var_ref normDelta) ) ) ) )
 112 ...
 113
 114 Each node has a type.  Expressions may involve several different types:
 115 (declare (uniform ) mat4 gl_ModelViewMatrix)
 116 ((assign (constant bool (1)) (var_ref constructor_tmp) (expression
 117  vec4 * (var_ref gl_ModelViewMatrix) (var_ref gl_Vertex) ) )
 118
 119 An expression tree can be arbitrarily deep, and the compiler tries to
 120 keep them structured like that so that things like algebraic
 121 optimizations ((color * 1.0 == color) and ((mat1 * mat2) * vec == mat1
 122 * (mat2 * vec))) or recognizing operation patterns for code generation
 123 (vec1 * vec2 + vec3 == mad(vec1, vec2, vec3)) are easier.  This comes
 124 at the expense of additional trickery in implementing some
 125 optimizations like CSE where one must navigate an expression tree.
 126
 127 Q: Why no SSA representation?
 128
 129 A: Converting an IR tree to SSA form makes dead code elimination,
 130 common subexpression elimination, and many other optimizations much
 131 easier.  However, in our primarily vector-based language, there's some
 132 major questions as to how it would work.  Do we do SSA on the scalar
 133 or vector level?  If we do it at the vector level, we're going to end
 134 up with many different versions of the variable when encountering code
 135 like:
 136
 137 (assign (constant bool (1)) (swiz x (var_ref __retval) ) (var_ref a) )
 138 (assign (constant bool (1)) (swiz y (var_ref __retval) ) (var_ref b) )
 139 (assign (constant bool (1)) (swiz z (var_ref __retval) ) (var_ref c) )
 140
 141 If every masked update of a component relies on the previous value of
 142 the variable, then we're probably going to be quite limited in our
 143 dead code elimination wins, and recognizing common expressions may
 144 just not happen.  On the other hand, if we operate channel-wise, then
 145 we'll be prone to optimizing the operation on one of the channels at
 146 the expense of making its instruction flow different from the other
 147 channels, and a vector-based GPU would end up with worse code than if
 148 we didn't optimize operations on that channel!
 149
 150 Once again, it appears that our optimization requirements are driven
 151 significantly by the target architecture.  For now, targeting the Mesa
 152 IR backend, SSA does not appear to be that important to producing
 153 excellent code, but we do expect to do some SSA-based optimizations
 154 for the 965 fragment shader backend when that is developed.
 155
 156 Q: How should I expand instructions that take multiple backend instructions?
 157
 158 Sometimes you'll have to do the expansion in your code generation --
 159 see, for example, ir_to_mesa.cpp's handling of ir_unop_sqrt.  However,
 160 in many cases you'll want to do a pass over the IR to convert
 161 non-native instructions to a series of native instructions.  For
 162 example, for the Mesa backend we have ir_div_to_mul_rcp.cpp because
 163 Mesa IR (and many hardware backends) only have a reciprocal
 164 instruction, not a divide.  Implementing non-native instructions this
 165 way gives the chance for constant folding to occur, so (a / 2.0)
 166 becomes (a * 0.5) after codegen instead of (a * (1.0 / 2.0))
 167
 168 Q: How shoud I handle my special hardware instructions with respect to IR?
 169
 170 Our current theory is that if multiple targets have an instruction for
 171 some operation, then we should probably be able to represent that in
 172 the IR.  Generally this is in the form of an ir_{bin,un}op expression
 173 type.  For example, we initially implemented fract() using (a -
 174 floor(a)), but both 945 and 965 have instructions to give that result,
 175 and it would also simplify the implementation of mod(), so
 176 ir_unop_fract was added.  The following areas need updating to add a
 177 new expression type:
 178
 179 ir.h (new enum)
 180 ir.cpp:operator_strs (used for ir_reader)
 181 ir_constant_expression.cpp (you probably want to be able to constant fold)
 182 ir_validate.cpp (check users have the right types)
 183
 184 You may also need to update the backends if they will see the new expr type:
 185
 186 ../mesa/program/ir_to_mesa.cpp
 187
 188 You can then use the new expression from builtins (if all backends
 189 would rather see it), or scan the IR and convert to use your new
 190 expression type (see ir_mod_to_floor, for example).
 191
 192 Q: How is memory management handled in the compiler?
 193
 194 The hierarchical memory allocator "talloc" developed for the Samba
 195 project is used, so that things like optimization passes don't have to
 196 worry about their garbage collection so much.  It has a few nice
 197 features, including low performance overhead and good debugging
 198 support that's trivially available.
 199
 200 Generally, each stage of the compile creates a talloc context and
 201 allocates its memory out of that or children of it.  At the end of the
 202 stage, the pieces still live are stolen to a new context and the old
 203 one freed, or the whole context is kept for use by the next stage.
 204
 205 For IR transformations, a temporary context is used, then at the end
 206 of all transformations, reparent_ir reparents all live nodes under the
 207 shader's IR list, and the old context full of dead nodes is freed.
 208 When developing a single IR transformation pass, this means that you
 209 want to allocate instruction nodes out of the temporary context, so if
 210 it becomes dead it doesn't live on as the child of a live node.  At
 211 the moment, optimization passes aren't passed that temporary context,
 212 so they find it by calling talloc_parent() on a nearby IR node.  The
 213 talloc_parent() call is expensive, so many passes will cache the
 214 result of the first talloc_parent().  Cleaning up all the optimization
 215 passes to take a context argument and not call talloc_parent() is left
 216 as an exercise.
 217
 218 Q: What is the file naming convention in this directory?
 219
 220 Initially, there really wasn't one.  We have since adopted one:
 221
 222  - Files that implement code lowering passes should be named lower_*
 223    (e.g., lower_noise.cpp).
 224  - Files that implement optimization passes should be named opt_*.
 225  - Files that implement a class that is used throught the code should
 226    take the name of that class (e.g., ir_hierarchical_visitor.cpp).
 227  - Files that contain code not fitting in one of the previous
 228    categories should have a sensible name (e.g., glsl_parser.yy).