Loading...
Searching...
No Matches
RDom.h
Go to the documentation of this file.
#ifndef HALIDE_RDOM_H
#define HALIDE_RDOM_H
/** \file
* Defines the front-end syntax for reduction domains and reduction
* variables.
*/
#include <iostream>
#include <string>
#include <utility>
#include <vector>
#include "Expr.h"
#include "Reduction.h"
#include "Util.h"
namespace Halide {
template<typename T, int Dims>
class Buffer;
class OutputImageParam;
/** A reduction variable represents a single dimension of a reduction
* domain (RDom). Don't construct them directly, instead construct an
* RDom, and use RDom::operator[] to get at the variables. For
* single-dimensional reduction domains, you can just cast a
* single-dimensional RDom to an RVar. */
class RVar {
std::string _name;
Internal::ReductionDomain _domain;
int _index = -1;
const Internal::ReductionVariable &_var() const {
const auto &d = _domain.domain();
internal_assert(_index >= 0 && _index < (int)d.size());
return d.at(_index);
}
public:
/** An empty reduction variable. */
RVar()
: _name(Internal::unique_name('r')) {
}
/** Construct an RVar with the given name */
explicit RVar(const std::string &n)
: _name(n) {
}
/** Construct a reduction variable with the given name and
* bounds. Must be a member of the given reduction domain. */
RVar(Internal::ReductionDomain domain, int index)
: _domain(std::move(domain)), _index(index) {
}
/** The minimum value that this variable will take on */
Expr min() const;
/** The number that this variable will take on. The maximum value
* of this variable will be min() + extent() - 1 */
Expr extent() const;
/** The reduction domain this is associated with. */
Internal::ReductionDomain domain() const {
return _domain;
}
/** The name of this reduction variable */
const std::string &name() const;
/** Reduction variables can be used as expressions. */
operator Expr() const;
};
/** A multi-dimensional domain over which to iterate. Used when
* defining functions with update definitions.
*
* An reduction is a function with a two-part definition. It has an
* initial value, which looks much like a pure function, and an update
* definition, which may refer to some RDom. Evaluating such a
* function first initializes it over the required domain (which is
* inferred based on usage), and then runs update rule for all points
* in the RDom. For example:
*
\code
Func f;
Var x;
RDom r(0, 10);
f(x) = x; // the initial value
f(r) = f(r) * 2;
Buffer<int> result = f.realize({10});
\endcode
*
* This function creates a single-dimensional buffer of size 10, in
* which element x contains the value x*2. Internally, first the
* initialization rule fills in x at every site, and then the update
* definition doubles every site.
*
* One use of reductions is to build a function recursively (pure
* functions in halide cannot be recursive). For example, this
* function fills in an array with the first 20 fibonacci numbers:
*
\code
Func f;
Var x;
RDom r(2, 18);
f(x) = 1;
f(r) = f(r-1) + f(r-2);
\endcode
*
* Another use of reductions is to perform scattering operations, as
* unlike a pure function declaration, the left-hand-side of an update
* definition may contain general expressions:
*
\code
ImageParam input(UInt(8), 2);
Func histogram;
Var x;
RDom r(input); // Iterate over all pixels in the input
histogram(x) = 0;
histogram(input(r.x, r.y)) = histogram(input(r.x, r.y)) + 1;
\endcode
*
* An update definition may also be multi-dimensional. This example
* computes a summed-area table by first summing horizontally and then
* vertically:
*
\code
ImageParam input(Float(32), 2);
Func sum_x, sum_y;
Var x, y;
RDom r(input);
sum_x(x, y) = input(x, y);
sum_x(r.x, r.y) = sum_x(r.x, r.y) + sum_x(r.x-1, r.y);
sum_y(x, y) = sum_x(x, y);
sum_y(r.x, r.y) = sum_y(r.x, r.y) + sum_y(r.x, r.y-1);
\endcode
*
* You can also mix pure dimensions with reduction variables. In the
* previous example, note that there's no need for the y coordinate in
* sum_x to be traversed serially. The sum within each row is entirely
* independent. The rows could be computed in parallel, or in a
* different order, without changing the meaning. Therefore, we can
* instead write this definition as follows:
*
\code
ImageParam input(Float(32), 2);
Func sum_x, sum_y;
Var x, y;
RDom r(input);
sum_x(x, y) = input(x, y);
sum_x(r.x, y) = sum_x(r.x, y) + sum_x(r.x-1, y);
sum_y(x, y) = sum_x(x, y);
sum_y(x, r.y) = sum_y(x, r.y) + sum_y(x, r.y-1);
\endcode
*
* This lets us schedule it more flexibly. You can now parallelize the
* update step of sum_x over y by calling:
\code
sum_x.update().parallel(y).
\endcode
*
* Note that calling sum_x.parallel(y) only parallelizes the
* initialization step, and not the update step! Scheduling the update
* step of a reduction must be done using the handle returned by
* \ref Func::update(). This code parallelizes both the initialization
* step and the update step:
*
\code
sum_x.parallel(y);
sum_x.update().parallel(y);
\endcode
*
* When you mix reduction variables and pure dimensions, the reduction
* domain is traversed outermost. That is, for each point in the
* reduction domain, the inferred pure domain is traversed in its
* entirety. For the above example, this means that sum_x walks down
* the columns, and sum_y walks along the rows. This may not be
* cache-coherent. You may try reordering these dimensions using the
* schedule, but Halide will return an error if it decides that this
* risks changing the meaning of your function. The solution lies in
* clever scheduling. If we say:
*
\code
sum_x.compute_at(sum_y, y);
\endcode
*
* Then the sum in x is computed only as necessary for each scanline
* of the sum in y. This not only results in sum_x walking along the
* rows, it also improves the locality of the entire pipeline.
*/
class RDom {
Internal::ReductionDomain dom;
void init_vars(const std::string &name);
void validate_min_extent(const Expr &min, const Expr &extent);
void initialize_from_region(const Region &region, std::string name = "");
template<typename... Args>
HALIDE_NO_USER_CODE_INLINE void initialize_from_region(Region &region, const Expr &min, const Expr &extent, Args &&...args) {
validate_min_extent(min, extent);
region.emplace_back(min, extent);
initialize_from_region(region, std::forward<Args>(args)...);
}
public:
/** Construct an undefined reduction domain. */
RDom() = default;
/** Construct a multi-dimensional reduction domain with the given name. If the name
* is left blank, a unique one is auto-generated. */
// @{
HALIDE_NO_USER_CODE_INLINE RDom(const Region &region, std::string name = "") {
initialize_from_region(region, std::move(name));
}
template<typename... Args>
HALIDE_NO_USER_CODE_INLINE RDom(const Expr &min, const Expr &extent, Args &&...args) {
// This should really just be a delegating constructor, but I couldn't make
// that work with variadic template unpacking in visual studio 2013
Region region;
initialize_from_region(region, min, extent, std::forward<Args>(args)...);
}
// @}
/** Construct a reduction domain that iterates over all points in
* a given Buffer or ImageParam. Has the same dimensionality as
* the argument. */
// @{
RDom(const Buffer<void, -1> &);
RDom(const OutputImageParam &);
template<typename T, int Dims>
HALIDE_NO_USER_CODE_INLINE RDom(const Buffer<T, Dims> &im)
: RDom(Buffer<void, -1>(im)) {
}
// @}
/** Construct a reduction domain that wraps an Internal ReductionDomain object. */
RDom(const Internal::ReductionDomain &d);
/** Get at the internal reduction domain object that this wraps. */
Internal::ReductionDomain domain() const {
return dom;
}
/** Check if this reduction domain is non-null */
bool defined() const {
return dom.defined();
}
/** Compare two reduction domains for equality of reference */
bool same_as(const RDom &other) const {
return dom.same_as(other.dom);
}
/** Get the dimensionality of a reduction domain */
int dimensions() const;
/** Get at one of the dimensions of the reduction domain */
RVar operator[](int) const;
/** Single-dimensional reduction domains can be used as RVars directly. */
operator RVar() const;
/** Single-dimensional reduction domains can be also be used as Exprs directly. */
operator Expr() const;
/** Add a predicate to the RDom. An RDom may have multiple
* predicates associated with it. An update definition that uses
* an RDom only iterates over the subset points in the domain for
* which all of its predicates are true. The predicate expression
* obeys the same rules as the expressions used on the
* right-hand-side of the corresponding update definition. It may
* refer to the RDom's variables and free variables in the Func's
* update definition. It may include calls to other Funcs, or make
* recursive calls to the same Func. This permits iteration over
* non-rectangular domains, or domains with sizes that vary with
* some free variable, or domains with shapes determined by some
* other Func.
*
* Note that once RDom is used in the update definition of some
* Func, no new predicates can be added to the RDom.
*
* Consider a simple example:
\code
RDom r(0, 20, 0, 20);
r.where(r.x < r.y);
r.where(r.x == 10);
r.where(r.y > 13);
f(r.x, r.y) += 1;
\endcode
* This is equivalent to:
\code
for (int r.y = 0; r.y < 20; r.y++) {
if (r.y > 13) {
for (int r.x = 0; r.x < 20; r.x++) {
if (r.x == 10) {
if (r.x < r.y) {
f[r.x, r.y] += 1;
}
}
}
}
}
\endcode
*
* Where possible Halide restricts the range of the containing for
* loops to avoid the cases where the predicate is false so that
* the if statement can be removed entirely. The case above would
* be further simplified into:
*
\code
for (int r.y = 14; r.y < 20; r.y++) {
f[10, r.y] += 1;
}
\endcode
*
* In general, the predicates that we can simplify away by
* restricting loop ranges are inequalities that compare an inner
* Var or RVar to some expression in outer Vars or RVars.
*
* You can also pack multiple conditions into one predicate like so:
*
\code
RDom r(0, 20, 0, 20);
r.where((r.x < r.y) && (r.x == 10) && (r.y > 13));
f(r.x, r.y) += 1;
\endcode
*
*/
void where(Expr predicate);
/** Direct access to the first four dimensions of the reduction
* domain. Some of these variables may be undefined if the
* reduction domain has fewer than four dimensions. */
// @{
RVar x, y, z, w;
// @}
};
/** Emit an RVar in a human-readable form */
std::ostream &operator<<(std::ostream &stream, const RVar &);
/** Emit an RDom in a human-readable form. */
std::ostream &operator<<(std::ostream &stream, const RDom &);
} // namespace Halide
#endif
internal_assert
#define internal_assert(c)
Definition Error.h:218
Expr.h
Base classes for Halide expressions (Halide::Expr) and statements (Halide::Internal::Stmt).
Reduction.h
Defines internal classes related to Reduction Domains.
Util.h
Various utility functions used internally Halide.
HALIDE_NO_USER_CODE_INLINE
#define HALIDE_NO_USER_CODE_INLINE
Definition Util.h:47
Halide::Buffer
A Halide::Buffer is a named shared reference to a Halide::Runtime::Buffer.
Definition Buffer.h:122
Halide::Internal::ReductionDomain
A reference-counted handle on a reduction domain, which is just a vector of ReductionVariable.
Definition Reduction.h:38
Halide::OutputImageParam
A handle on the output buffer of a pipeline.
Definition OutputImageParam.h:19
Halide::RDom
A multi-dimensional domain over which to iterate.
Definition RDom.h:193
Halide::RDom::same_as
bool same_as(const RDom &other) const
Compare two reduction domains for equality of reference.
Definition RDom.h:254
Halide::RDom::defined
bool defined() const
Check if this reduction domain is non-null.
Definition RDom.h:249
Halide::RDom::dimensions
int dimensions() const
Get the dimensionality of a reduction domain.
Halide::RDom::w
RVar w
Definition RDom.h:339
Halide::RDom::RDom
RDom(const Internal::ReductionDomain &d)
Construct a reduction domain that wraps an Internal ReductionDomain object.
Halide::RDom::z
RVar z
Definition RDom.h:339
Halide::RDom::RDom
RDom()=default
Construct an undefined reduction domain.
Halide::RDom::RDom
HALIDE_NO_USER_CODE_INLINE RDom(const Region &region, std::string name="")
Construct a multi-dimensional reduction domain with the given name.
Definition RDom.h:215
Halide::RDom::x
RVar x
Direct access to the first four dimensions of the reduction domain.
Definition RDom.h:339
Halide::RDom::RDom
HALIDE_NO_USER_CODE_INLINE RDom(const Expr &min, const Expr &extent, Args &&...args)
Definition RDom.h:220
Halide::RDom::where
void where(Expr predicate)
Add a predicate to the RDom.
Halide::RDom::RDom
RDom(const Buffer< void, -1 > &)
Construct a reduction domain that iterates over all points in a given Buffer or ImageParam.
Halide::RDom::y
RVar y
Definition RDom.h:339
Halide::RDom::operator[]
RVar operator[](int) const
Get at one of the dimensions of the reduction domain.
Halide::RDom::RDom
RDom(const OutputImageParam &)
Halide::RDom::RDom
HALIDE_NO_USER_CODE_INLINE RDom(const Buffer< T, Dims > &im)
Definition RDom.h:235
Halide::RDom::domain
Internal::ReductionDomain domain() const
Get at the internal reduction domain object that this wraps.
Definition RDom.h:244
Halide::RVar
A reduction variable represents a single dimension of a reduction domain (RDom).
Definition RDom.h:29
Halide::RVar::RVar
RVar(Internal::ReductionDomain domain, int index)
Construct a reduction variable with the given name and bounds.
Definition RDom.h:53
Halide::RVar::name
const std::string & name() const
The name of this reduction variable.
Halide::RVar::RVar
RVar()
An empty reduction variable.
Definition RDom.h:42
Halide::RVar::domain
Internal::ReductionDomain domain() const
The reduction domain this is associated with.
Definition RDom.h:65
Halide::RVar::RVar
RVar(const std::string &n)
Construct an RVar with the given name.
Definition RDom.h:47
Halide::RVar::extent
Expr extent() const
The number that this variable will take on.
Halide::RVar::min
Expr min() const
The minimum value that this variable will take on.
Halide::Internal
Definition AbstractGenerator.h:24
Halide
This file defines the class FunctionDAG, which is our representation of a Halide pipeline,...
Definition AbstractGenerator.h:19
Halide::operator<<
std::ostream & operator<<(std::ostream &stream, const Expr &)
Emit an expression on an output stream (such as std::cout) in human-readable form.
Halide::min
Expr min(const FuncRef &a, const FuncRef &b)
Explicit overloads of min and max for FuncRef.
Definition Func.h:603
Halide::Region
std::vector< Range > Region
A multi-dimensional box.
Definition Expr.h:350
Halide::Expr
A fragment of Halide syntax.
Definition Expr.h:258
Halide::Internal::ReductionVariable
A single named dimension of a reduction domain.
Definition Reduction.h:16