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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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package jdk.internal;
import java.lang.annotation.*;
/**
* The {@code @HotSpotIntrinsicCandidate} annotation is specific to the
* HotSpot Virtual Machine. It indicates that an annotated method
* may be (but is not guaranteed to be) intrinsified by the HotSpot VM. A method
* is intrinsified if the HotSpot VM replaces the annotated method with hand-written
* assembly and/or hand-written compiler IR -- a compiler intrinsic -- to improve
* performance. The {@code @HotSpotIntrinsicCandidate} annotation is internal to the
* Java libraries and is therefore not supposed to have any relevance for application
* code.
*
* Maintainers of the Java libraries must consider the following when
* modifying methods annotated with {@code @HotSpotIntrinsicCandidate}.
*
* <ul>
* <li>When modifying a method annotated with {@code @HotSpotIntrinsicCandidate},
* the corresponding intrinsic code in the HotSpot VM implementation must be
* updated to match the semantics of the annotated method.</li>
* <li>For some annotated methods, the corresponding intrinsic may omit some low-level
* checks that would be performed as a matter of course if the intrinsic is implemented
* using Java bytecodes. This is because individual Java bytecodes implicitly check
* for exceptions like {@code NullPointerException} and {@code ArrayStoreException}.
* If such a method is replaced by an intrinsic coded in assembly language, any
* checks performed as a matter of normal bytecode operation must be performed
* before entry into the assembly code. These checks must be performed, as
* appropriate, on all arguments to the intrinsic, and on other values (if any) obtained
* by the intrinsic through those arguments. The checks may be deduced by inspecting
* the non-intrinsic Java code for the method, and determining exactly which exceptions
* may be thrown by the code, including undeclared implicit {@code RuntimeException}s.
* Therefore, depending on the data accesses performed by the intrinsic,
* the checks may include:
*
* <ul>
* <li>null checks on references</li>
* <li>range checks on primitive values used as array indexes</li>
* <li>other validity checks on primitive values (e.g., for divide-by-zero conditions)</li>
* <li>store checks on reference values stored into arrays</li>
* <li>array length checks on arrays indexed from within the intrinsic</li>
* <li>reference casts (when formal parameters are {@code Object} or some other weak type)</li>
* </ul>
*
* </li>
*
* <li>Note that the receiver value ({@code this}) is passed as a extra argument
* to all non-static methods. If a non-static method is an intrinsic, the receiver
* value does not need a null check, but (as stated above) any values loaded by the
* intrinsic from object fields must also be checked. As a matter of clarity, it is
* better to make intrinisics be static methods, to make the dependency on {@code this}
* clear. Also, it is better to explicitly load all required values from object
* fields before entering the intrinsic code, and pass those values as explicit arguments.
* First, this may be necessary for null checks (or other checks). Second, if the
* intrinsic reloads the values from fields and operates on those without checks,
* race conditions may be able to introduce unchecked invalid values into the intrinsic.
* If the intrinsic needs to store a value back to an object field, that value should be
* returned explicitly from the intrinsic; if there are multiple return values, coders
* should consider buffering them in an array. Removing field access from intrinsics
* not only clarifies the interface with between the JVM and JDK; it also helps decouple
* the HotSpot and JDK implementations, since if JDK code before and after the intrinsic
* manages all field accesses, then intrinsics can be coded to be agnostic of object
* layouts.</li>
*
* Maintainers of the HotSpot VM must consider the following when modifying
* intrinsics.
*
* <ul>
* <li>When adding a new intrinsic, make sure that the corresponding method
* in the Java libraries is annotated with {@code @HotSpotIntrinsicCandidate}
* and that all possible call sequences that result in calling the intrinsic contain
* the checks omitted by the intrinsic (if any).</li>
* <li>When modifying an existing intrinsic, the Java libraries must be updated
* to match the semantics of the intrinsic and to execute all checks omitted
* by the intrinsic (if any).</li>
* </ul>
*
* Persons not directly involved with maintaining the Java libraries or the
* HotSpot VM can safely ignore the fact that a method is annotated with
* {@code @HotSpotIntrinsicCandidate}.
*
* The HotSpot VM defines (internally) a list of intrinsics. Not all intrinsic
* are available on all platforms supported by the HotSpot VM. Furthermore,
* the availability of an intrinsic on a given platform depends on the
* configuration of the HotSpot VM (e.g., the set of VM flags enabled).
* Therefore, annotating a method with {@code @HotSpotIntrinsicCandidate} does
* not guarantee that the marked method is intrinsified by the HotSpot VM.
*
* If the {@code CheckIntrinsics} VM flag is enabled, the HotSpot VM checks
* (when loading a class) that (1) all methods of that class that are also on
* the VM's list of intrinsics are annotated with {@code @HotSpotIntrinsicCandidate}
* and that (2) for all methods of that class annotated with
* {@code @HotSpotIntrinsicCandidate} there is an intrinsic in the list.
*
* @since 9
*/
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR})
@Retention(RetentionPolicy.RUNTIME)
public @interface HotSpotIntrinsicCandidate {
}