vector
Tells the compiler
that the loop should be vectorized according to the argument keywords.
Syntax
#pragma vector
{always[assert]|dynamic_align|nodynamic_align|temporal|nontemporal|[no]vecremainder|vectorlength(n1[,
n2]...)}
#pragma vector nontemporal
[
(
var1
[,
var2
,
...
])
]
Arguments
- always
- Instructs the compiler to override any efficiency heuristic during the decision to vectorize or not, and vectorize non-unit strides or very unaligned memory accesses; controls the vectorization of the subsequent loop in the program; optionally takes the keyword assert.
- dynamic_align
- Instructs the compiler to perform dynamic alignment optimization for the loop.
- nodynamic_align
- Disables dynamic alignment optimization for the loop.
- nontemporal
- Instructs the compiler to use non-temporal (that is, streaming) stores on systems based on all supported architectures, unless otherwise specified; optionally takes a comma-separated list of variables.When this pragma is specified, it is your responsibility to also insert any fences as required to ensure correct memory ordering within a thread or across threads. One typical way to do this is to insert a_mm_sfenceintrinsic call just after the loops (such as the initialization loop) where the compiler may insert streaming store instructions.
- temporal
- Instructs the compiler to use temporal (that is, non-streaming) stores on systems based on all supported architectures, unless otherwise specified.
- vecremainder
- Instructs the compiler to vectorize the remainder loop when the original loop is vectorized.
- novecremainder
- Instructs the compiler not to vectorize the remainder loop when the original loop is vectorized.
- vectorlength(n1[,n2]...)
- Instructs the vectorizer which vector length/factor to use when generating the main vector loop.
Description
The
vector
pragma indicates that the
loop should be vectorized, if it is legal to do so, ignoring normal heuristic
decisions about profitability. The
vector
pragma takes several
argument keywords to specify the kind of loop vectorization required. The
compiler does not apply the
vector
pragma to nested loops,
each nested loop needs a preceding pragma statement. Place the pragma before
the loop control statement.
Using the
always
keywordWhen the
always
argument keyword is used,
the pragma controls the vectorization of the subsequent loop in the program. If
assert
is added, the
compiler will generate an error-level assertion test to display a message
saying that the compiler efficiency heuristics indicate that the loop cannot be
vectorized.
Using the
dynamic_align
and
nodynamic_align
keywordsDynamic alignment
is an optimization the compiler can perform to improve alignment of memory
references inside the loop. It involves peeling iterations from the vector loop
into a scalar loop (which may, in turn, also be vectorized) before the vector
loop so that the vector loop aligns with a particular memory reference.
Specifying
dynamic_align
enables the optimization to be performed,
but the compiler will still use efficiency heuristics to determine whether the
optimization will be applied to the loop. Specifying
nodynamic_align
disables the optimization. By default, the
compiler does not perform optimization.
Using the
nontemporal
and
temporal
keywordsThe
nontemporal
and
temporal
argument keywords are
used to control how the "stores" of register contents to storage are performed
(streaming versus non-streaming) on systems based on IA-32 and Intel® 64
architectures.
By default, the compiler automatically determines whether
a streaming store should be used for each variable.
Streaming stores may cause significant performance
improvements over non-streaming stores for large numbers on certain processors.
However, the misuse of streaming stores can significantly degrade performance.
Using the
[no]vecremainder
keywordIf keyword
vecremainder
is specified, the compiler tries to vectorize
the remainder loop when the main loop is vectorized. Even if the
always
keyword is specified, the remainder loop vectorization is still a
subject of compiler efficiency heuristics.
If keyword
novecremainder
is specified, the compiler vectorizes the
main loop, but it does not vectorize the remainder loop.
Using the
vectorlength
keywordn
is an integer power of 2; the value must be 2, 4, 6, 8,
16, 32, or 64. If more than one value is specified, the vectorizer will choose
one of the specified vector lengths based on a cost model decision.
The pragma
vector
should be used with
care.
Overriding the
efficiency heuristics of the compiler should only be done if the programmer is
absolutely sure that vectorization will improve performance.