Built-In Functions - Praise for OpenGL ES 30 Programming Guide - page 464

Applications should be encouraged to use the built-in functions rather
than do the equivalent computations in their own shader code because
the built-in functions are assumed to be optimal (e.g., perhaps supported
directly in hardware).
When the built-in functions are specified below, where the input
arguments (and corresponding output) can be float, , vec2 , vec3 , or
vec4 , genType is used as the argument. Where the input arguments (and
corresponding output) can be int , ivec2 , ivec3 , or ivec4 , genIType is
used as the argument. Where the input arguments (and corresponding
output) can be uint , uvec2 , uvec3 , or uvec4 , genUType is used as the
argument. Where the input arguments (or corresponding output) can be
bool , bvec2 , bvec3 , or bvec4 , genBType is used as the argument. For
any specific use of a function, the actual types substituted for genType ,
genIType , genUType , or genBType have to have the same number of
components for all arguments and for the return type. Similarly for mat ,
which can be any matrix basic type.
The precision of built-in functions is dependent on the function and
arguments. There are three categories:
• Some functions have predefined precisions. The precision is specified;
for example,
highp ivec2 textureSize (gsampler2D sampler , int lod )
• For the texture sampling functions, the precision of the return type
matches the precision of the sampler type:
uniform lowp sampler2D sampler;
highp vec2 coord;
. . .
// texture() returns lowp
lowp vec4 col = texture(sampler, coord);
• For other built-in functions, a call will return a precision qualification
matching the highest precision qualification of the call's input
arguments.
The built-in functions are assumed to be implemented according to the
equations specified in the following sections.