public abstract class ValueProfile extends Profile
Specialized value profile to capture certain properties of Object runtime values.
Value profiles require a runtime check in their initialized state to verify their profiled
assumption. Value profiles are limited to capture monomorphic profiles only. This means that if
two or more identities or classes are profiles within a single profile then the profile has no
effect and no overhead after compilation. There are specialized versions of this profile with
less interpreter footprint for boolean, byte,
int, long, float and
double values.
Usage example:
class SampleNode extends Node {
final ValueProfile profile = ValueProfile.create{Identity,Class}Profile();
Object execute(Object input) {
Object profiledValue = profile.profile(input);
// compiler may know now more about profiledValue
return profieldValue;
}
}
A profile is a Truffle utility class that uses the Truffle compiler
directives to guard for and/or forward runtime information to the compiler.
Usage: Profiles should be stored in final or compilation final fields of node classes to ensure that they can get optimized properly.
Profiles must not be shared between ASTs. Using the same profile multiple times in a single
node or in multiple nodes which link to the
same root is allowed. Never store profiles inside runtime values that
leave the scope of the originating AST. This limitation exists because the used mechanism to
invalidate compiled code performs local invalidations only. For global speculations use
assumptions instead.
Compilation: Some profiles like branch profiles do not induce
additional overhead in compiled code. Others like value profiles might
require a runtime check to verify their assumptions which are forwarded to the compiler. Even if
profiles do not induce direct overhead in compiled code it still might get invalidated as a
result of using profiles. Invalidating profiles will result in the invalidation of compiled code.
It is therefore essential to place these profiles in way that is neither too aggressive nor too
conservative.
Footprint: Whether profiling information can be forwarded to the compiler depends on the
capabilities of the runtime system. If the runtime returns
true in TruffleRuntime.isProfilingEnabled() then runtime information will
get collected. This comes at at the cost of additional overhead and footprint in interpreted
mode. Thats why the factory methods of profiles can return implementations where profiling is
disabled. Using disabled profiles makes sense for runtimes that are unable to use the collected
profiling information. Even runtime implementations that are able to use this information might
decide to turn off profiling for benchmarking purposes.
Profile subclasses:
BranchProfile to profile on unlikely branches like errors.ConditionProfile to profile on conditionals or boolean values.LoopConditionProfile to profile on conditionals of loops with special support for
counted loops.ValueProfile to profile on properties like type and identity of values.ByteValueProfile to profile on byte values.IntValueProfile to profile on int values.LongValueProfile to profile on long values.FloatValueProfile to profile on float values.DoubleValueProfile to profile on double values.PrimitiveValueProfile to profile on objects by identity and on primitives by value.
ValueProfile.createIdentityProfile(),
ValueProfile.createClassProfile()| Modifier and Type | Method and Description |
|---|---|
static ValueProfile |
createClassProfile()
Returns a value profile that profiles the exact class of a value.
|
static ValueProfile |
createIdentityProfile()
Returns a value profile that profiles the object identity of a value.
|
static ValueProfile |
getUncached()
Returns the uncached version of the profile.
|
abstract <T> T |
profile(T value) |
clonepublic abstract <T> T profile(T value)
public static ValueProfile createClassProfile()
Returns a value profile that profiles the exact class of a value. It will check the class of
the profiled value and provide additional information to the compiler if only non-null values
of exactly one concrete Java class are passed as a parameter to the
ValueProfile.profile(T) method. This can be beneficial if subsequent code can take
advantage of knowing the concrete class of the value. The profile will degrade to the generic
case if a null value or if at least two instances of two different Java classes are
registered.
Compilation notes: Value profiles require a runtime check in their initialized state to verify their profiled class. If two classes have been seen on a single profile instance then this profile will transition to a generic state with no overhead.
usage examplepublic static ValueProfile createIdentityProfile()
Returns a value profile that profiles the object identity of a value. A single instance can only profile one particular instance.
Compilation notes: Identity profiles require a runtime check to verify their profiled object identity. If two identities have been seen on a single profile instance then this profile will transition to a generic state with no overhead.
public static ValueProfile getUncached()