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 outofrange 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 builtin
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 outofrange behavior and saturated conversions.
Rounding Modes
Conversions to and from floatingpoint type shall conform to IEEE754 rounding rules. Conversions may have an optional rounding mode modifier described in the following table.
Modifier 
Rounding Mode Description 

Round to nearest even 

Round toward zero 

Round toward positive infinity 

Round toward negative infinity 
no modifier specified 
Use the default rounding mode for this destination
type, 
By default, conversions to integer type use the _rtz
(round toward zero)
rounding mode and conversions to floatingpoint type^{10} use the default
rounding mode.
The only default floatingpoint rounding mode supported is round to nearest
even i.e the default rounding mode will be _rte
for floatingpoint types.
[10] For conversions to floatingpoint format, when a finite source value exceeds the maximum representable finite floatingpoint destination value, the rounding mode will affect whether the result is the maximum finite floatingpoint value or infinity of same sign as the source value, per IEEE754 rules for rounding.
OutofRange 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 outofrange. The result of outofrange conversion is determined by the conversion rules specified by section 6.3 of the C99 Specification. When converting from a floatingpoint type to integer type, the behavior is implementationdefined.
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 floatingpoint type shall conform to IEEE754 rounding rules.
The _sat
modifier may not be used for conversions to floatingpoint
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 implementationdefined 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 floatingpoint values
// are rounded to the nearest integer instead of truncated
int4 i3 = convert_int4_rte( f );
// similar to convert_int4_sat, except that floatingpoint 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 floatingpoint type is needed, for example to mask off the sign bit or make use of the result of a vector relational operator on floatingpoint data^{11}. 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)