C Specification

convert_destType(sourceType)
destType convert_destType<_sat><roundingMode> (sourceType)
destTypen convert_destTypen<_sat><roundingMode> (sourceType)

Description

Explicit conversions may be performed using the

convert_destType(sourceType)

suite of functions. These provide a full set of type conversions between supported scalar, vector, and other data types except for the following types: bool, half, size_t, ptrdiff_t, intptr_t, uintptr_t, and void.

The number of elements in the source and destination vectors must match.

In the example below:

uchar4 u;
int4 c = convert_int4(u);

convert_int4 converts a uchar4 vector u to an int4 vector c.

float f;
int i = convert_int(f);

convert_int converts a float scalar f to an int scalar i.

The behavior of the conversion may be modified by one or two optional modifiers that specify saturation for out-of-range inputs and rounding behavior.

The full form of the scalar convert function is:

destType *convert_destType<_sat><_roundingMode>*(sourceType)

The full form of the vector convert function is:

destType__n__ *convert_destType__n__<_sat><_roundingMode>*(sourceType__n__)

Data Types

Conversions are available for the following scalar types: char, uchar, short, ushort, int, uint, long, ulong, float, and built-in vector types derived therefrom. The operand and result type must have the same number of elements. The operand and result type may be the same type in which case the conversion has no effect on the type or value of an expression.

Conversions between integer types follow the conversion rules specified in sections 6.3.1.1 and 6.3.1.3 of the C99 Specification except for out-of-range behavior and saturated conversions.

Rounding Modes

Conversions to and from floating-point type shall conform to IEEE-754 rounding rules. Conversions may have an optional rounding mode modifier described in the following table.

Table 1. Rounding Modes

Modifier

Rounding Mode Description

_rte

Round to nearest even

_rtz

Round toward zero

_rtp

Round toward positive infinity

_rtn

Round toward negative infinity

no modifier specified

Use the default rounding mode for this destination type, _rtz for conversion to integers or the default rounding mode for conversion to floating-point types.

By default, conversions to integer type use the _rtz (round toward zero) rounding mode and conversions to floating-point type10 use the default rounding mode. The only default floating-point rounding mode supported is round to nearest even i.e the default rounding mode will be _rte for floating-point types.

[10] For conversions to floating-point format, when a finite source value exceeds the maximum representable finite floating-point destination value, the rounding mode will affect whether the result is the maximum finite floating-point value or infinity of same sign as the source value, per IEEE-754 rules for rounding.

Out-of-Range Behavior and Saturated Conversions

When the conversion operand is either greater than the greatest representable destination value or less than the least representable destination value, it is said to be out-of-range. The result of out-of-range conversion is determined by the conversion rules specified by section 6.3 of the C99 Specification. When converting from a floating-point type to integer type, the behavior is implementation-defined.

Conversions to integer type may opt to convert using the optional saturated mode by appending the _sat modifier to the conversion function name. When in saturated mode, values that are outside the representable range shall clamp to the nearest representable value in the destination format. (NaN should be converted to 0).

Conversions to floating-point type shall conform to IEEE-754 rounding rules. The _sat modifier may not be used for conversions to floating-point formats.

Explicit Conversion Examples

Example 1:

short4 s;

// negative values clamped to 0
ushort4 u = convert_ushort4_sat( s );

// values > CHAR_MAX converted to CHAR_MAX
// values < CHAR_MIN converted to CHAR_MIN
char4 c = convert_char4_sat( s );

Example 2:

float4 f;

// values implementation defined for
// f > INT_MAX, f < INT_MIN or NaN
int4 i = convert_int4( f );

// values > INT_MAX clamp to INT_MAX, values < INT_MIN clamp
// to INT_MIN. NaN should produce 0.
// The _rtz_ rounding mode is used to produce the integer values.
int4 i2 = convert_int4_sat( f );

// similar to convert_int4, except that floating-point values
// are rounded to the nearest integer instead of truncated
int4 i3 = convert_int4_rte( f );

// similar to convert_int4_sat, except that floating-point values
// are rounded to the nearest integer instead of truncated
int4 i4 = convert_int4_sat_rte( f );

Example 3:

int4 i;

// convert ints to floats using the default rounding mode.
float4 f = convert_float4( i );

// convert ints to floats. integer values that cannot
// be exactly represented as floats should round up to the
// next representable float.
float4 f = convert_float4_rtp( i );

Reinterpreting Data As Another Type

It is frequently necessary to reinterpret bits in a data type as another data type in OpenCL. This is typically required when direct access to the bits in a floating-point type is needed, for example to mask off the sign bit or make use of the result of a vector relational operator on floating-point data11. Several methods to achieve this (non-) conversion are frequently practiced in C, including pointer aliasing, unions and memcpy. Of these, only memcpy is strictly correct in C99. Since OpenCL does not provide memcpy, other methods are needed.

[11] In addition, some other extensions to the C language designed to support particular vector ISA (e.g. AltiVecā„¢, CELL Broadband Engineā„¢ Architecture) use such conversions in conjunction with swizzle operators to achieve type unconversion. So as to support legacy code of this type, as_typen() allows conversions between vectors of the same size but different numbers of elements, even though the behavior of this sort of conversion is not likely to be portable except to other OpenCL implementations for the same hardware architecture. AltiVec is a trademark of Motorola Inc. Cell Broadband Engine is a trademark of Sony Computer Entertainment, Inc.

Reinterpreting Types Using Unions

The OpenCL language extends the union to allow the program to access a member of a union object using a member of a different type. The relevant bytes of the representation of the object are treated as an object of the type used for the access. If the type used for access is larger than the representation of the object, then the value of the additional bytes is undefined.

Examples:

// d only if double precision is supported
union { float f; uint u; double d; } u;

u.u = 1;    // u.f contains 2**-149.  u.d is undefined --
            // depending on endianness the low or high half
            // of d is unknown

u.f = 1.0f; // u.u contains 0x3f800000, u.d contains an
            // undefined value -- depending on endianness
            // the low or high half of d is unknown

u.d = 1.0;  // u.u contains 0x3ff00000 (big endian) or 0
            // (little endian). u.f contains either 0x1.ep0f
            // (big endian) or 0.0f (little endian)

See Also

Document Notes

For more information, see the OpenCL C Specification at URL

This page is extracted from the OpenCL C Specification. Fixes and changes should be made to the Specification, not directly.

Copyright (c) 2014-2020 Khronos Group. This work is licensed under a Creative Commons Attribution 4.0 International License.