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