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New IR, or NIR, is an IR for Mesa intended to sit below GLSL IR and Mesa IR.
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Its design inherits from the various IRs that Mesa has used in the past, as
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well as Direct3D assembly, and it includes a few new ideas as well. It is a
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flat (in terms of using instructions instead of expressions), typeless IR,
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similar to TGSI and Mesa IR. It also supports SSA (although it doesn't require
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NIR includes support for source-level GLSL variables through a structure mostly
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copied from GLSL IR. These will be used for linking and conversion from GLSL IR
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(and later, from an AST), but for the most part, they will be lowered to
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registers (see below) and loads/stores.
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Registers are light-weight; they consist of a structure that only contains its
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size, its index for liveness analysis, and an optional name for debugging. In
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addition, registers can be local to a function or global to the entire shader;
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the latter will be used in ARB_shader_subroutine for passing parameters and
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getting return values from subroutines. Registers can also be an array, in which
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case they can be accessed indirectly. Each ALU instruction (add, subtract, etc.)
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works directly with registers or SSA values (see below).
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Everywhere a register can be loaded/stored, an SSA value can be used instead.
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The only exception is that arrays/indirect addressing are not supported with
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SSA; although research has been done on extensions of SSA to arrays before, it's
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usually for the purpose of parallelization (which we're not interested in), and
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adds some overhead in the form of adding copies or extra arrays (which is much
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more expensive than introducing copies between non-array registers). SSA uses
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point directly to their corresponding definition, which in turn points to the
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instruction it is part of. This creates an implicit use-def chain and avoids the
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need for an external structure for each SSA register.
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Support for function calls is mostly similar to GLSL IR. Each shader contains a
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list of functions, and each function has a list of overloads. Each overload
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contains a list of parameters, and may contain an implementation which specifies
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the variables that correspond to the parameters and return value. Inlining a
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function, assuming it has a single return point, is as simple as copying its
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instructions, registers, and local variables into the target function and then
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inserting copies to and from the new parameters as appropriate. After functions
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are inlined and any non-subroutine functions are deleted, parameters and return
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variables will be converted to global variables and then global registers. We
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don't do this lowering earlier (i.e. the fortranizer idea) for a few reasons:
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- If we want to do optimizations before link time, we need to have the function
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signature available during link-time.
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- If we do any inlining before link time, then we might wind up with the
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inlined function and the non-inlined function using the same global
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variables/registers which would preclude optimization.
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Any operation (other than function calls and textures) which touches a variable
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or is not referentially transparent is represented by an intrinsic. Intrinsics
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are similar to the idea of a "builtin function," i.e. a function declaration
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whose implementation is provided by the backend, except they are more powerful
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in the following ways:
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- They can also load and store registers when appropriate, which limits the
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number of variables needed in later stages of the IR while obviating the need
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for a separate load/store variable instruction.
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- Intrinsics can be marked as side-effect free, which permits them to be
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treated like any other instruction when it comes to optimizations. This allows
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load intrinsics to be represented as intrinsics while still being optimized
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away by dead code elimination, common subexpression elimination, etc.
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Intrinsics are used for:
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- Geometry shader emitVertex and endPrimitive
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- Loading and storing variables (before lowering)
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- Loading and storing uniforms, shader inputs and outputs, etc (after lowering)
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- Copying variables (cases where in GLSL the destination is a structure or
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Unfortunately, there are far too many texture operations to represent each one
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of them with an intrinsic, so there's a special texture instruction similar to
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the GLSL IR one. The biggest difference is that, while the texture instruction
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has a sampler dereference field used just like in GLSL IR, this gets lowered to
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a texture unit index (with a possible indirect offset) while the type
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information of the original sampler is kept around for backends. Also, all the
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non-constant sources are stored in a single array to make it easier for
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optimization passes to iterate over all the sources.
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Like in GLSL IR, control flow consists of a tree of "control flow nodes", which
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include if statements and loops, and jump instructions (break, continue, and
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return). Unlike GLSL IR, though, the leaves of the tree aren't statements but
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basic blocks. Each basic block also keeps track of its successors and
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predecessors, and function implementations keep track of the beginning basic
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block (the first basic block of the function) and the ending basic block (a fake
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basic block that every return statement points to). Together, these elements
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make up the control flow graph, in this case a redundant piece of information on
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top of the control flow tree that will be used by almost all the optimizations.
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There are helper functions to add and remove control flow nodes that also update
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the control flow graph, and so usually it doesn't need to be touched by passes
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that modify control flow nodes.