Name
MESA_shader_integer_functions
Name Strings
GL_MESA_shader_integer_functions
Contact
Ian Romanick
Contributors
All the contributors of GL_ARB_gpu_shader5
Status
Supported by all GLSL 1.30 capable drivers in Mesa 12.1 and later
Version
Version 2, July 7, 2016
Number
OpenGL Extension #495
Dependencies
This extension is written against the OpenGL 3.2 (Compatibility Profile)
Specification.
This extension is written against Version 1.50 (Revision 09) of the OpenGL
Shading Language Specification.
GLSL 1.30 is required.
This extension interacts with ARB_gpu_shader5.
This extension interacts with ARB_gpu_shader_fp64.
This extension interacts with NV_gpu_shader5.
Overview
GL_ARB_gpu_shader5 extends GLSL in a number of useful ways. Much of this
added functionality requires significant hardware support. There are many
aspects, however, that can be easily implmented on any GPU with "real"
integer support (as opposed to simulating integers using floating point
calculations).
This extension provides a set of new features to the OpenGL Shading
Language to support capabilities of these GPUs, extending the capabilities
of version 1.30 of the OpenGL Shading Language. Shaders
using the new functionality provided by this extension should enable this
functionality via the construct
#extension GL_MESA_shader_integer_functions : require (or enable)
This extension provides a variety of new features for all shader types,
including:
* support for implicitly converting signed integer types to unsigned
types, as well as more general implicit conversion and function
overloading infrastructure to support new data types introduced by
other extensions;
* new built-in functions supporting:
* splitting a floating-point number into a significand and exponent
(frexp), or building a floating-point number from a significand and
exponent (ldexp);
* integer bitfield manipulation, including functions to find the
position of the most or least significant set bit, count the number
of one bits, and bitfield insertion, extraction, and reversal;
* extended integer precision math, including add with carry, subtract
with borrow, and extenended multiplication;
The resulting extension is a strict subset of GL_ARB_gpu_shader5.
IP Status
No known IP claims.
New Procedures and Functions
None
New Tokens
None
Additions to Chapter 2 of the OpenGL 3.2 (Compatibility Profile) Specification
(OpenGL Operation)
None.
Additions to Chapter 3 of the OpenGL 3.2 (Compatibility Profile) Specification
(Rasterization)
None.
Additions to Chapter 4 of the OpenGL 3.2 (Compatibility Profile) Specification
(Per-Fragment Operations and the Frame Buffer)
None.
Additions to Chapter 5 of the OpenGL 3.2 (Compatibility Profile) Specification
(Special Functions)
None.
Additions to Chapter 6 of the OpenGL 3.2 (Compatibility Profile) Specification
(State and State Requests)
None.
Additions to Appendix A of the OpenGL 3.2 (Compatibility Profile)
Specification (Invariance)
None.
Additions to the AGL/GLX/WGL Specifications
None.
Modifications to The OpenGL Shading Language Specification, Version 1.50
(Revision 09)
Including the following line in a shader can be used to control the
language features described in this extension:
#extension GL_MESA_shader_integer_functions :
where is as specified in section 3.3.
New preprocessor #defines are added to the OpenGL Shading Language:
#define GL_MESA_shader_integer_functions 1
Modify Section 4.1.10, Implicit Conversions, p. 27
(modify table of implicit conversions)
Can be implicitly
Type of expression converted to
--------------------- -----------------
int uint, float
ivec2 uvec2, vec2
ivec3 uvec3, vec3
ivec4 uvec4, vec4
uint float
uvec2 vec2
uvec3 vec3
uvec4 vec4
(modify second paragraph of the section) No implicit conversions are
provided to convert from unsigned to signed integer types or from
floating-point to integer types. There are no implicit array or structure
conversions.
(insert before the final paragraph of the section) When performing
implicit conversion for binary operators, there may be multiple data types
to which the two operands can be converted. For example, when adding an
int value to a uint value, both values can be implicitly converted to uint
and float. In such cases, a floating-point type is chosen if either
operand has a floating-point type. Otherwise, an unsigned integer type is
chosen if either operand has an unsigned integer type. Otherwise, a
signed integer type is chosen.
Modify Section 5.9, Expressions, p. 57
(modify bulleted list as follows, adding support for implicit conversion
between signed and unsigned types)
Expressions in the shading language are built from the following:
* Constants of type bool, int, int64_t, uint, uint64_t, float, all vector
types, and all matrix types.
...
* The operator modulus (%) operates on signed or unsigned integer scalars
or vectors. If the fundamental types of the operands do not match, the
conversions from Section 4.1.10 "Implicit Conversions" are applied to
produce matching types. ...
Modify Section 6.1, Function Definitions, p. 63
(modify description of overloading, beginning at the top of p. 64)
Function names can be overloaded. The same function name can be used for
multiple functions, as long as the parameter types differ. If a function
name is declared twice with the same parameter types, then the return
types and all qualifiers must also match, and it is the same function
being declared. For example,
vec4 f(in vec4 x, out vec4 y); // (A)
vec4 f(in vec4 x, out uvec4 y); // (B) okay, different argument type
vec4 f(in ivec4 x, out uvec4 y); // (C) okay, different argument type
int f(in vec4 x, out ivec4 y); // error, only return type differs
vec4 f(in vec4 x, in vec4 y); // error, only qualifier differs
vec4 f(const in vec4 x, out vec4 y); // error, only qualifier differs
When function calls are resolved, an exact type match for all the
arguments is sought. If an exact match is found, all other functions are
ignored, and the exact match is used. If no exact match is found, then
the implicit conversions in Section 4.1.10 (Implicit Conversions) will be
applied to find a match. Mismatched types on input parameters (in or
inout or default) must have a conversion from the calling argument type
to the formal parameter type. Mismatched types on output parameters (out
or inout) must have a conversion from the formal parameter type to the
calling argument type.
If implicit conversions can be used to find more than one matching
function, a single best-matching function is sought. To determine a best
match, the conversions between calling argument and formal parameter
types are compared for each function argument and pair of matching
functions. After these comparisons are performed, each pair of matching
functions are compared. A function definition A is considered a better
match than function definition B if:
* for at least one function argument, the conversion for that argument
in A is better than the corresponding conversion in B; and
* there is no function argument for which the conversion in B is better
than the corresponding conversion in A.
If a single function definition is considered a better match than every
other matching function definition, it will be used. Otherwise, a
semantic error occurs and the shader will fail to compile.
To determine whether the conversion for a single argument in one match is
better than that for another match, the following rules are applied, in
order:
1. An exact match is better than a match involving any implicit
conversion.
2. A match involving an implicit conversion from float to double is
better than a match involving any other implicit conversion.
3. A match involving an implicit conversion from either int or uint to
float is better than a match involving an implicit conversion from
either int or uint to double.
If none of the rules above apply to a particular pair of conversions,
neither conversion is considered better than the other.
For the function prototypes (A), (B), and (C) above, the following
examples show how the rules apply to different sets of calling argument
types:
f(vec4, vec4); // exact match of vec4 f(in vec4 x, out vec4 y)
f(vec4, uvec4); // exact match of vec4 f(in vec4 x, out ivec4 y)
f(vec4, ivec4); // matched to vec4 f(in vec4 x, out vec4 y)
// (C) not relevant, can't convert vec4 to
// ivec4. (A) better than (B) for 2nd
// argument (rule 2), same on first argument.
f(ivec4, vec4); // NOT matched. All three match by implicit
// conversion. (C) is better than (A) and (B)
// on the first argument. (A) is better than
// (B) and (C).
Modify Section 8.3, Common Functions, p. 84
(add support for single-precision frexp and ldexp functions)
Syntax:
genType frexp(genType x, out genIType exp);
genType ldexp(genType x, in genIType exp);
The function frexp() splits each single-precision floating-point number in
into a binary significand, a floating-point number in the range [0.5,
1.0), and an integral exponent of two, such that:
x = significand * 2 ^ exponent
The significand is returned by the function; the exponent is returned in
the parameter . For a floating-point value of zero, the significant
and exponent are both zero. For a floating-point value that is an
infinity or is not a number, the results of frexp() are undefined.
If the input is a vector, this operation is performed in a
component-wise manner; the value returned by the function and the value
written to are vectors with the same number of components as .
The function ldexp() builds a single-precision floating-point number from
each significand component in and the corresponding integral exponent
of two in , returning:
significand * 2 ^ exponent
If this product is too large to be represented as a single-precision
floating-point value, the result is considered undefined.
If the input is a vector, this operation is performed in a
component-wise manner; the value passed in and returned by the
function are vectors with the same number of components as .
(add support for new integer built-in functions)
Syntax:
genIType bitfieldExtract(genIType value, int offset, int bits);
genUType bitfieldExtract(genUType value, int offset, int bits);
genIType bitfieldInsert(genIType base, genIType insert, int offset,
int bits);
genUType bitfieldInsert(genUType base, genUType insert, int offset,
int bits);
genIType bitfieldReverse(genIType value);
genUType bitfieldReverse(genUType value);
genIType bitCount(genIType value);
genIType bitCount(genUType value);
genIType findLSB(genIType value);
genIType findLSB(genUType value);
genIType findMSB(genIType value);
genIType findMSB(genUType value);
The function bitfieldExtract() extracts bits through
+-1 from each component in , returning them in the
least significant bits of corresponding component of the result. For
unsigned data types, the most significant bits of the result will be set
to zero. For signed data types, the most significant bits will be set to
the value of bit +-1. If is zero, the result will be
zero. The result will be undefined if or is negative, or
if the sum of and is greater than the number of bits used
to store the operand. Note that for vector versions of bitfieldExtract(),
a single pair of and values is shared for all components.
The function bitfieldInsert() inserts the least significant bits of
each component of into the corresponding component of .
The result will have bits numbered through +-1
taken from bits 0 through -1 of , and all other bits taken
directly from the corresponding bits of . If is zero, the
result will simply be . The result will be undefined if or
is negative, or if the sum of and is greater than
the number of bits used to store the operand. Note that for vector
versions of bitfieldInsert(), a single pair of and values
is shared for all components.
The function bitfieldReverse() reverses the bits of . The bit
numbered of the result will be taken from bit (-1)- of
, where is the total number of bits used to represent
.
The function bitCount() returns the number of one bits in the binary
representation of .
The function findLSB() returns the bit number of the least significant one
bit in the binary representation of . If is zero, -1 will
be returned.
The function findMSB() returns the bit number of the most significant bit
in the binary representation of . For positive integers, the
result will be the bit number of the most significant one bit. For
negative integers, the result will be the bit number of the most
significant zero bit. For a of zero or negative one, -1 will be
returned.
(support for unsigned integer add/subtract with carry-out)
Syntax:
genUType uaddCarry(genUType x, genUType y, out genUType carry);
genUType usubBorrow(genUType x, genUType y, out genUType borrow);
The function uaddCarry() adds 32-bit unsigned integers or vectors and
, returning the sum modulo 2^32. The value is set to zero if
the sum was less than 2^32, or one otherwise.
The function usubBorrow() subtracts the 32-bit unsigned integer or vector
from , returning the difference if non-negative or 2^32 plus the
difference, otherwise. The value is set to zero if x >= y, or
one otherwise.
(support for signed and unsigned multiplies, with 32-bit inputs and a
64-bit result spanning two 32-bit outputs)
Syntax:
void umulExtended(genUType x, genUType y, out genUType msb,
out genUType lsb);
void imulExtended(genIType x, genIType y, out genIType msb,
out genIType lsb);
The functions umulExtended() and imulExtended() multiply 32-bit unsigned
or signed integers or vectors and , producing a 64-bit result. The
32 least significant bits are returned in ; the 32 most significant
bits are returned in .
GLX Protocol
None.
Dependencies on ARB_gpu_shader_fp64
This extension, ARB_gpu_shader_fp64, and NV_gpu_shader5 all modify the set
of implicit conversions supported in the OpenGL Shading Language. If more
than one of these extensions is supported, an expression of one type may
be converted to another type if that conversion is allowed by any of these
specifications.
If ARB_gpu_shader_fp64 or a similar extension introducing new data types
is not supported, the function overloading rule in the GLSL specification
preferring promotion an input parameters to smaller type to a larger type
is never applicable, as all data types are of the same size. That rule
and the example referring to "double" should be removed.
Dependencies on NV_gpu_shader5
This extension, ARB_gpu_shader_fp64, and NV_gpu_shader5 all modify the set
of implicit conversions supported in the OpenGL Shading Language. If more
than one of these extensions is supported, an expression of one type may
be converted to another type if that conversion is allowed by any of these
specifications.
If NV_gpu_shader5 is supported, integer data types are supported with four
different precisions (8-, 16, 32-, and 64-bit) and floating-point data
types are supported with three different precisions (16-, 32-, and
64-bit). The extension adds the following rule for output parameters,
which is similar to the one present in this extension for input
parameters:
5. If the formal parameters in both matches are output parameters, a
conversion from a type with a larger number of bits per component is
better than a conversion from a type with a smaller number of bits
per component. For example, a conversion from an "int16_t" formal
parameter type to "int" is better than one from an "int8_t" formal
parameter type to "int".
Such a rule is not provided in this extension because there is no
combination of types in this extension and ARB_gpu_shader_fp64 where this
rule has any effect.
Errors
None
New State
None
New Implementation Dependent State
None
Issues
(1) What should this extension be called?
UNRESOLVED. This extension borrows from GL_ARB_gpu_shader5, so creating
some sort of a play on that name would be viable. However, nothing in
this extension should require SM5 hardware, so such a name would be a
little misleading and weird.
Since the primary purpose is to add integer related functions from
GL_ARB_gpu_shader5, call this extension GL_MESA_shader_integer_functions
for now.
(2) Why is some of the formatting in this extension weird?
RESOLVED: This extension is formatted to minimize the differences (as
reported by 'diff --side-by-side -W180') with the GL_ARB_gpu_shader5
specification.
(3) Should ldexp and frexp be included?
RESOLVED: Yes. Few GPUs have native instructions to implement these
functions. These are generally implemented using existing GLSL built-in
functions and the other functions provided by this extension.
(4) Should umulExtended and imulExtended be included?
RESOLVED: Yes. These functions should be implementable on any GPU that
can support the rest of this extension, but the implementation may be
complex. The implementation on a GPU that only supports 32bit x 32bit =
32bit multiplication would be quite expensive. However, many GPUs
(including OpenGL 4.0 GPUs that already support this function) have a
32bit x 16bit = 48bit multiplier. The implementation there is only
trivially more expensive than regular 32bit multiplication.
(5) Should the pack and unpack functions be included?
RESOLVED: No. These functions are already available via
GL_ARB_shading_language_packing.
(6) Should the "BitsTo" functions be included?
RESOLVED: No. These functions are already available via
GL_ARB_shader_bit_encoding.
Revision History
Rev. Date Author Changes
---- ----------- -------- -----------------------------------------
2 7-Jul-2016 idr Fix typo in #extension line
1 20-Jun-2016 idr Initial version based on GL_ARB_gpu_shader5.