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/**
* The top-level package in C1X containing options, metrics, timers and the main compiler class
* {@link com.sun.c1x.C1XCompiler}.
*
* <H2>{@code C1XCompiler} Overview</H2>
*
* C1X is intended to be used with multiple JVM's so makes no use of or reference to classes for a specific JVM, for
* example Maxine.
*
* The compiler is represented by the class {@code C1XCompiler}. {@code C1XCompiler} binds a specific target
* architecture and JVM interface to produce a usable compiler object. There are
* two variants of {@code compileMethod}, one of which is used when doing <i>on stack replacement</i> (OSR), discussed
* later. The main variant takes {@link com.sun.cri.ri.RiMethod} and {@link com.sun.cri.xir.RiXirGenerator} arguments.
* {@code RiMethod} is C1X's representation of a Java method and {@code RiXirGenerator} represents the interface through
* which the compiler requests the XIR for a given bytecode from the runtime system.
*
* <H3>The C1X Compilation Process</H3>
*
* {@link com.sun.c1x.C1XCompiler#compileMethod} creates a {@link C1XCompilation} instance and then returns the result of calling its
* {@link com.sun.c1x.C1XCompilation#compile} method. The {@code C1XCompilation} instance records whether {@code compileMethod} was invoked with
* the OSR variant, which is used later in the IR generation.
* <p>
* While there is only one {@code C1XCompiler} instance, there may be several compilations proceeding concurrently, each of
* which is represented by a unique {@code C1XCompilation} instance. The static method {@link com.sun.c1x.C1XCompilation#current}} returns the
* {@code C1XCompilation} instance associated with the current thread, and is managed using a {@link java.lang.ThreadLocal} variable. It
* is used when assigning the unique id that is used for tracing output to an HIR node. Each {@code C1XCompilation} instance
* has an associated {@link com.sun.cri.ci.CiStatistics} object that accumulates information about the compilation process, but is also
* used as a generator of, for example, basic block identifiers.
* <p>
* The compilation begins by calling {@link com.sun.c1x.C1XCompilation#emitHIR}, which creates the high-level intermediate representation (HIR) from the
* bytecodes of the method. The HIR is managed by the {@link com.sun.c1x.graph.IR} class, an instance of which is created by
* {@code emitHR}, which then calls the {{@link com.sun.c1x.graph.IR#build}} method and returns the result. The {@code C1XCompilation} and {@code IR}
* instances are are bi-directionally linked.
*
* <H3>Supported backends</H3>
*
* <ul>
* <li>AMD64/x64 with SSE2</li>
* </ul>
*
* <H2>Notes and Todos</H2> This is a collection of notes about the C1X compiler, including future directions,
* refactorings, missing features, broken features, etc.
*
*
* <h3>Anticipated Refactorings</h3>
*
* <ul>
* <li>
* The HIR nodes {@link com.sun.c1x.ir.UnsafePrefetch}, {@link com.sun.c1x.ir.UnsafePutObject}, etc should be replaced by uses of the newer
* {@link com.sun.c1x.ir.LoadPointer} and {@link com.sun.c1x.ir.StorePointer} nodes. Currently, the unsafe nodes are only generated by
* the creation of an OSR entry. Benefit: reduce the number of different IR nodes.</li>
*
* <li>
* Add a field to optionally store an {@link com.sun.c1x.ir.Info} object for each HIR node, and remove the
* {@link com.sun.c1x.ir.Instruction#exceptionHandlers} field, the {@link com.sun.c1x.ir.Instruction#bci} field, and any fields to store the Java
* frame state in subclasses. Benefit: saves space if most HIR nodes do not have exception handlers, a bci or Java frame
* state. Removes virtual dispatch on accessing debug information for nodes. Allows any node, regardless of its type, to
* have info attached.</li>
*
* <li>
* Migrate all HIR nodes to use the immutable {@link com.sun.c1x.value.FrameStateInfo} for debugging information. The {@link com.sun.c1x.value.FrameState}
* class is mutable and used throughout graph building. Benefit: {@code FrameStateInfo} would save both total space in
* the IR graph prevent many bugs due to the mutability of {@code FrameState}.</li>
*
* <li>
* Move the {@code FrameState} class to an inner class, or combine entirely, with the {@link com.sun.c1x.graph.GraphBuilder} class. After
* the introduction of the {@code FrameStateInfo} into HIR nodes, the mutable value stack should only need to be
* accessed from the graph builder.</li>
*
* </ul>
*
* <h3>Missing or incomplete features</h3>
*
* There are some features of C1 that were not ported forward or finished given the time constraints for the C1X port. A
* list appears below.
*
* <ul>
* <li>
* Deoptimization metadata. The locations of all local variables and stack values are not communicated back to the
* runtime system through the {@link com.sun.cri.ci.CiDebugInfo} class yet. Such values are known to the register allocator, and there
* vestigial logic to compute them still there in the
* {@link com.sun.c1x.alloc.LinearScan#computeDebugInfo} method. To complete this metadata, the
* {@link com.sun.c1x.alloc.LinearScan} class must implement the {@link ValueLocator} interface and pass it to the
* {@link com.sun.c1x.lir.LIRDebugInfo#createFrame} method after register allocation. The
* resulting debug info will be fed back to the runtime system by the existing logic that calls
* {@link com.sun.cri.ci.CiTargetMethod#recordCall(int, Object, CiDebugInfo, boolean)} and other methods. Obviously the runtime
* system will need to encode this metadata in a dense format, because it is huge.</li>
*
*
* <li>
* Tiered compilation support. C1 supported the ability to add instrumentation to branches, invocations, and checkcasts
* in order to feed profile information to the C2 compiler in a tiered compilation setup. It relied on adding some
* information to the HIR nodes that represent these operations ({@link Invoke}, {@link CheckCast}, etc). All of this
* logic was removed to simplify both the front end and back end in anticipation of designing a future instrumentation
* API. XIR should be general enough to allow instrumentation code to be added to invocation and checkcast sites, but
* currently has no support for adding code at branches.
*
* </li>
*
* <li>
* SPARC and other architecture support. There pretty well-delineated separation between the architecture-independent
* part of LIR backend and the architecture-dependent, but the only implementation that current exists is the X86
* backend ({@link com.sun.c1x.target.amd64.AMD64Backend}, {@link com.sun.c1x.target.amd64.AMD64LIRGenerator}, {@link com.sun.c1x.target.amd64.AMD64LIRAssembler}, etc).</li>
*
* <li>
* XIR for safepoints. The C1X backend should use XIR to get the code for safepoints, but currently it still uses the
* handwritten logic (currently only compatible with Maxine).</li>
*
* </ul>
*
* <h3>Untested features</h3>
*
* <ul>
*
* <li>
* Reference map for outgoing overflow arguments. If a C1X method calls another method that has overflow arguments, it
* is not clear if the outgoing overflow argument area, which may contain references, has the appropriate bits set in
* the reference map for the C1X method's frame. Such arguments may be live in the called method.</li>
*
* <li>
* Although it should work, inlining synchronized methods or methods with exception handlers hasn't been tested.</li>
* <li>
* On-stack replacement. C1X retains all of the special logic for performing an OSR compilation. This is basically a
* compilation with a second entrypoint for entry from the interpreter. However, the generation of a runtime-specific
* entry sequence was never tested.</li>
*
* <li>
* {@link com.sun.c1x.C1XIntrinsic Intrinsification} is the mechanism by which the compiler recognizes calls to special JDK or
* runtime methods and replaces them with custom code. It is enabled by the {@link com.sun.c1x.C1XOptions#OptIntrinsify} compiler
* option. The C1X backend has never been tested with intrinsified arithmetic or floating point operations. For best
* performance, it should generate specialized machine code for arithmetic and floating point, perhaps using global
* stubs for complex floating point operations. <br>
* <i>Note</i>: Folding of special intrinsified methods is supported, tested, and working. The runtime system may
* register methods to be folded by using the
* {@link com.sun.c1x.C1XIntrinsic#registerFoldableMethod(RiMethod, java.lang.reflect.Method)} call. When the compiler encounters a
* call to such a registered method where the parameters are all constants, it invokes the supplied method with
* reflection. If the reflective call produces a value and does not throw an exception, C1X replaces the call to the
* method with the result.</li>
* </ul>
*
* <h3>Broken features</h3>
*
* <ul>
* <li>
* {@link com.sun.c1x.opt.LoopPeeler Loop peeling} was written by Marcelo Cintra near the end of his internship. It was never completed
* and should be considered broken. It only remains as a sketch of how loop peeling would be implemented in C1X, or in
* case he would finish the implementation and test it.</li>
*
* <li>
* Calls to compiler stubs should allocate space on the caller's stack. On AMD64 currently, calls to compiler stubs poke the
* arguments onto the stack below the RSP (i.e. in the callee's stack). While normally this code sequence is
* uninterruptible and works fine in the VM, signal handlers triggered when debugging or inspecting this code sequence
* may destroy these values when the OS calls the signal handler. This requires knowing which compiler stubs are called
* before finalizing the frame size; currently only the calls to
* {@link com.oracle.max.asm.target.amd64.AMD64MacroAssembler#callRuntimeCalleeSaved}
* do not fit this pattern. This needs to be fixed so that all compiler stubs that are called by the assembled code are
* known before beginning assembling. The {@link com.sun.c1x.target.amd64.AMD64CompilerStubEmitter} controls how the compiler stubs accept their
* parameters. See {@link com.sun.c1x.target.amd64.AMD64CompilerStubEmitter#callerFrameContainsArguments} and its usages.
*
* </li>
* </ul>
*/
package com.sun.c1x;