Halide
tutorial/lesson_14_types.cpp
// Halide tutorial lesson 14: The Halide type system
// This lesson more precisely describes Halide's type system.
// On linux, you can compile and run it like so:
// g++ lesson_14*.cpp -g -I ../include -L ../bin -lHalide -lpthread -ldl -o lesson_14 -std=c++11
// LD_LIBRARY_PATH=../bin ./lesson_14
// On os x:
// g++ lesson_14*.cpp -g -I ../include -L ../bin -lHalide -o lesson_14 -std=c++11
// DYLD_LIBRARY_PATH=../bin ./lesson_14
// If you have the entire Halide source tree, you can also build it by
// running:
// make tutorial_lesson_14_types
// in a shell with the current directory at the top of the halide
// source tree.
#include "Halide.h"
#include <stdio.h>
using namespace Halide;
// This function is used to demonstrate generic code at the end of
// this lesson.
Expr average(Expr a, Expr b);
int main(int argc, char **argv) {
// All Exprs have a scalar type, and all Funcs evaluate to one or
// more scalar types. The scalar types in Halide are unsigned
// integers of various bit widths, signed integers of the same set
// of bit widths, floating point numbers in single and double
// precision, and opaque handles (equivalent to void *). The
// following array contains all the legal types.
Type valid_halide_types[] = {
UInt(8), UInt(16), UInt(32), UInt(64),
Int(8), Int(16), Int(32), Int(64),
Float(32), Float(64), Handle()
};
// Constructing and inspecting types.
{
// You can programmatically examine the properties of a Halide
// type. This is useful when you write a C++ function that has
// Expr arguments and you wish to check their types:
assert(UInt(8).bits() == 8);
assert(Int(8).is_int());
// You can also programmatically construct Types as a function of other Types.
Type t = UInt(8);
t = t.with_bits(t.bits() * 2);
assert(t == UInt(16));
// Or construct a Type from a C++ scalar type
assert(type_of<float>() == Float(32));
// The Type struct is also capable of representing vector types,
// but this is reserved for Halide's internal use. You should
// vectorize code by using Func::vectorize, not by attempting to
// construct vector expressions directly. You may encounter vector
// types if you programmatically manipulate lowered Halide code,
// but this is an advanced topic (see Func::add_custom_lowering_pass).
// You can query any Halide Expr for its type. An Expr
// representing a Var has type Int(32):
Var x;
assert(Expr(x).type() == Int(32));
// Most transcendental functions in Halide cast their inputs to a
// Float(32) and return a Float(32):
assert(sin(x).type() == Float(32));
// You can cast an Expr from one Type to another using the cast operator:
assert(cast(UInt(8), x).type() == UInt(8));
// This also comes in a template form that takes a C++ type.
assert(cast<uint8_t>(x).type() == UInt(8));
// You can also query any defined Func for the types it produces.
Func f1;
f1(x) = cast<uint8_t>(x);
assert(f1.output_types()[0] == UInt(8));
Func f2;
f2(x) = {x, sin(x)};
assert(f2.output_types()[0] == Int(32) &&
f2.output_types()[1] == Float(32));
}
// Type promotion rules.
{
// When you combine Exprs of different types (e.g. using '+',
// '*', etc), Halide uses a system of type promotion
// rules. These differ to C's rules. To demonstrate these
// we'll make some Exprs of each type.
Var x;
Expr u8 = cast<uint8_t>(x);
Expr u16 = cast<uint16_t>(x);
Expr u32 = cast<uint32_t>(x);
Expr u64 = cast<uint64_t>(x);
Expr s8 = cast<int8_t>(x);
Expr s16 = cast<int16_t>(x);
Expr s32 = cast<int32_t>(x);
Expr s64 = cast<int64_t>(x);
Expr f32 = cast<float>(x);
Expr f64 = cast<double>(x);
// The rules are as follows, and are applied in the order they are
// written below.
// 1) It is an error to cast or use arithmetic operators on Exprs of type Handle().
// 2) If the types are the same, then no type conversions occur.
for (Type t : valid_halide_types) {
// Skip the handle type.
if (t.is_handle()) continue;
Expr e = cast(t, x);
assert((e + e).type() == e.type());
}
// 3) If one type is a float but the other is not, then the
// non-float argument is promoted to a float (possibly causing a
// loss of precision for large integers).
assert((u8 + f32).type() == Float(32));
assert((f32 + s64).type() == Float(32));
assert((u16 + f64).type() == Float(64));
assert((f64 + s32).type() == Float(64));
// 4) If both types are float, then the narrower argument is
// promoted to the wider bit-width.
assert((f64 + f32).type() == Float(64));
// The rules above handle all the floating-point cases. The
// following three rules handle the integer cases.
// 5) If one of the arguments is an C++ int, and the other is
// a Halide::Expr, then the int is coerced to the type of the
// expression.
assert((u32 + 3).type() == UInt(32));
assert((3 + s16).type() == Int(16));
// If this rule would cause the integer to overflow, then Halide
// will trigger an error, e.g. uncommenting the following line
// will cause this program to terminate with an error.
// Expr bad = u8 + 257;
// 6) If both types are unsigned integers, or both types are
// signed integers, then the narrower argument is promoted to
// wider type.
assert((u32 + u8).type() == UInt(32));
assert((s16 + s64).type() == Int(64));
// 7) If one type is signed and the other is unsigned, both
// arguments are promoted to a signed integer with the greater of
// the two bit widths.
assert((u8 + s32).type() == Int(32));
assert((u32 + s8).type() == Int(32));
// Note that this may silently overflow the unsigned type in the
// case where the bit widths are the same.
assert((u32 + s32).type() == Int(32));
// When an unsigned Expr is converted to a wider signed type in
// this way, it is first widened to a wider unsigned type
// (zero-extended), and then reinterpreted as a signed
// integer. I.e. casting the UInt(8) value 255 to an Int(32)
// produces 255, not -1.
int32_t result32 = evaluate<int>(cast<int32_t>(cast<uint8_t>(255)));
assert(result32 == 255);
// When a signed type is explicitly converted to a wider unsigned
// type with the cast operator (the type promotion rules will
// never do this automatically), it is first converted to the
// wider signed type (sign-extended), and then reinterpreted as
// an unsigned integer. I.e. casting the Int(8) value -1 to a
// UInt(16) produces 65535, not 255.
uint16_t result16 = evaluate<uint16_t>(cast<uint16_t>(cast<int8_t>(-1)));
assert(result16 == 65535);
}
// The type Handle().
{
// Handle is used to represent opaque pointers. Applying
// type_of to any pointer type will return Handle()
assert(type_of<void *>() == Handle());
assert(type_of<const char * const **>() == Handle());
// Handles are always stored as 64-bit, regardless of the compilation
// target.
assert(Handle().bits() == 64);
// The main use of an Expr of type Handle is to pass
// it through Halide to other external code.
}
// Generic code.
{
// The main explicit use of Type in Halide is to write Halide
// code parameterized by a Type. In C++ you'd do this with
// templates. In Halide there's no need - you can inspect and
// modify the types dynamically at C++ runtime instead. The
// function defined below averages two expressions of any
// equal numeric type.
Var x;
assert(average(cast<float>(x), 3.0f).type() == Float(32));
assert(average(x, 3).type() == Int(32));
assert(average(cast<uint8_t>(x), cast<uint8_t>(3)).type() == UInt(8));
}
printf("Success!\n");
return 0;
}
Expr average(Expr a, Expr b) {
// Types must match.
assert(a.type() == b.type());
// For floating point types:
if (a.type().is_float()) {
// The '2' will be promoted to the floating point type due to
// rule 3 above.
return (a + b)/2;
}
// For integer types, we must compute the intermediate value in a
// wider type to avoid overflow.
Type narrow = a.type();
Type wider = narrow.with_bits(narrow.bits() * 2);
a = cast(wider, a);
b = cast(wider, b);
return cast(narrow, (a + b)/2);
}