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Implementation of breakpoints and single step in JIT mode

Implementation of breakpoints and single step in JIT mode



DRLVM uses code instrumentation to set breakpoints in a method in the JIT mode. When JVMTI is turned on by a command line switch, only Jitrino.JET is used to compile methods. JET maintains one-to-one mapping between method bytecodes and native code instructions, so that for each bytecode it is possible to find the instruction starting address, and for each address in compiled code it is possible to determine the corresponding bytecode. Using this mapping you can to set a breakpoint in any point of a method and notify the agent when execution reaches this point.

DRLVM has an abstraction layer, class VMBreakPoints, that allows the following interfaces to register breakpoints in the registry: thread-local single-stepping, breakpoints in the interpreter mode and in native Java code for NCAI. This way, each breakpoints interface uses its specific processing callback that it gives to the breakpoints registry. Each interface maintains a list of references to the global list of currently enabled breakpoints. This list has a global synchronization lock to maintain its integrity.

Each interface has a priority, and breakpoints are processed in the order of interfaces' priority. Prioritizing is necessary because the same location may have multiple breakpoints registered by different interfaces. The JVMTI specification states that for the same location in the code, single-step events have to be sent before breakpoints event, so the single-step interface has a higher priority than the breakpoints interface.

Setting a breakpoint

To implement the JVMTI API function SetBreakpoint, DRLVM uses the global interface registered in the breakpoints registry and the JVMTI breakpoints interface. First, the code locks the list, checks if a reference with the same method and bytecode location has been already registered, and if not, adds a new one.

When a new reference to a breakpoint is added, the breakpoint registry has to instrument the code. In the interpreter mode, it uses the interpreter internal function for this, and in the JIT mode, the registry uses JET to find out the native address for the requested bytecode. This address is instrumented with a single byte instruction, such as INT3 (used on Linux* OS) or CLI (used on Windows* OS). Instrumentation is done by remembering the byte that was previously in memory at the target address and then writing the single byte opcode into memory. Because instrumentation is done when the thread owns the global breakpoints lock, atomic operations are unnecessary.

After a new reference to a breakpoint is added, the global breakpoints list lock is released.

There is a possible situation that a method where user wants to set a breakpoint is not compiled yet. The method may be loaded into memory by the class loader, but not executed a single time, so there is no native code generated for it. To set a breakpoint it such a method, breakpoints registry adds a breakpoint to a list of pending breakpoints. Each method has a pointer to such a list, which is initialized with NULL. For each method after VM calls the compilation, and if it is successful, VM checks this list, and if it is not empty it performs all the instrumentation.

Clearing a breakpoint

To implement JVMTI API function ClearBreakpoint, DRLVM uses the same global JVMTI breakpoints interface. For that, the code locks the global breakpoints list and tries to find a reference to a breakpoint for the requested method and bytecode location. If no reference is found, an error is returned. If the reference is found, the code calls the breakpoints registry to remove it.

When a reference to a breakpoint is removed, the breakpoints registry checks all registered interfaces and their references to find any other existing references to the same breakpoint. If no references are found, the location in the code is de-instrumented: in the interpreter mode breakpoints registry uses interpreter function, and in the JIT mode - by writing the original saved byte back to the memory to restore the original instruction. If the target method where breakpoint is removed has not been compiled yet, the breakpoint is removed from its list of pending breakpoints.

After a reference to a breakpoint is removed, the global breakpoints list lock is released.

Reacting to a breakpoint

In the JIT mode when execution reaches instrumented location, the Java process receives a signal (exception on Windows). Signals in DRLVM are handled by the crash handler. VM registers a signal callback for the breakpoint type signal so that it can react to it. This callback calls the breakpoints registry to process the breakpoint.

The registry processes all registered interfaces in the order of their priority. If an interface has a registered reference to the breakpoint at the address where the signal has been received, the breakpoints registry calls this interface's callback. In this callback, normal event JVMTI processing is done. For each JVMTI environment that has enabled receiving interface-specific event type (e.g. JVMTI_BREAKPOINT or JVMIT_SINGLE_STEP), this environment's callback is called.

Before calling the callback, the registry unlocks the global list of breakpoints. Unlocking is necessary because the callback executes arbitrary JVMTI agent code that may work for a long time (e.g. wait on a JVMTI raw monitor). At the same time, another thread may need to modify the breakpoints list, and if at the same time this thread owns the same raw monitor, the VM process would dead lock.

After all of the interface's callbacks are called, it is necessary to return to executing the code of the method starting with the instruction that is instrumented. However, it is not possible to remove instrumentation from this instruction because another thread may be executing the same code at the same time, and with removed instrumentation the other thread would not receive the breakpoint signal.

To execute the original instruction, JVMTI has a thread-local memory buffer where this instruction is copied to. To copy this instruction, the breakpoints registry code uses a simple disassembler, which parses a single instruction and determines its length. The registry copies the original instrumented byte and the rest of the instruction into this thread-local buffer and after it inserts a jump instruction to the address next to the instrumented instruction.

NOTE: On x86_64 platforms, it is impossible to create a jump to a 64-bit address in the instruction's immediate operand. So instead of a jump, a sequence of push and ret instructions is used. It is necessary because no registers can be modified while executing the code emulating the original instruction.

Certain instructions depend on their location in the memory, such as relative jumps and calls. These instructions are handled in a specific way: for relative jumps the generated code contains a jump to an absolute address, for calls the generated sequence consists of pushing the "return address", which is the original address of the instrumented call instruction, and jumping to the calculated absolute target address.

After creating the buffer with the code emulating the behavior of the instrumented instruction, the breakpoints processing code modifies the registers' context to make the IP pointer to point to this buffer. With such modified registers context, the breakpoint processing code returns to the crash handler, which transfers execution control to the thread-local buffer.

Single step


The single-step mode implements Step In and Step Over commands in the debugger. In this mode, for each executed bytecode VM must send an event to the JVMTI agent with method and location in the method parameters. The agent may request to receive this event only on a specific set of threads or globally for all of executed threads.

Single stepping in DRLVM is done with breakpoints. Breakpoints are set on bytecodes and after sending the event to the agent, breakpoints are moved to the next bytecodes. Each thread in single-step mode uses its own breakpoints interface to add and remove single-step breakpoints. These interfaces have priority of single-step, that is higher than priority of ordinary breakpoints.

Enabling single step

When the JVMTI agent enables the single-step mode, DRLVM suspends all target threads except current. If the agent enables the single step globally, all currently running threads are considered target threads. In all target threads, VM creates a thread-local single step state, which is a breakpoint interface registered in the breakpoints registry with type "single step". Presence of this single-step state means that single step is enabled in the thread and is often used as a flag in the code.

For each thread, JVMTI code determines the execution position. If the thread executes Java code, the currently executed bytecode is the starting bytecode. If the thread executes native code, no action is taken. For every starting bytecode, JVMTI code determines the bytecode next to it and sets a breakpoint of single step type on it. These breakpoints are kept in the single-step state.

After setting breakpoints in all threads that execute Java code, DRLVM resumes all threads.

Disabling single step

When the agent disables the single-step mode, threads are suspended as with enabling this mode. In each thread, VM uses the single step state to remove references to the single step breakpoints, de-allocates the single-step state and resumes target threads.

Processing single step

The single step breakpoints interface has its own breakpoint processing callback different from the callback for ordinary breakpoints. In this callback, VM sends single-step events to the agent's environments that have requested to receive such events. Environment callbacks are called in the same way as for other JVMTI events.

If after all callbacks are called, the agent hasn't turned off the single-step mode in the thread, JVMTI has to continue executing to the next bytecode. It does the following:

  1. Removes all breakpoints predicted for the current thread - they were used on the previous step.
  2. Depending on the currently executed bytecode, predicts which bytecode(s) may be executed next to it.
    1. For ordinary bytecodes that don't transfer execution control the next bytecode is the one next to the current one in the method's bytecode array.
    2. For bytecodes that transfer control, all of the possible target locations are counted as predicted locations.

JVMTI code doesn't try to interpret the Java state and determine which target location is going to take place when the bytecode is executed. Instead, JVMTI does the following:

If the target method is not compiled yet, single-step breakpoints are added to its list of pending breakpoints in the same way as described in the Breakpoints section.

After predicting all possible target locations JVMTI code sets single step breakpoints on them. When this is done, execution is returned back to the code breakpoints registry which in turn returns execution back to the java method in the way described in breakpoints section.

Transitions from Java to native and back

Native code may be a user-program native method or VM code, such as class loader, that calls the Java user class loader to load classes. When native code calls a Java method, and if a thread has the single-step mode enabled, the first bytecode of the Java method is counted as predicted, and VM code sets the single-step breakpoint there.

If the single-step state is present when the execution returns to VM from the Java method, VM tries to find a Java method up the stack which has invoked the native code. It may happen implicitly with invoke or explicitly when compiled code calls VM to resolve a method. The bytecode next to the location which has called native code is counted predicted and VM code sets the single step breakpoint there.


When an exception is raised, all single-step predicted breakpoints are removed because execution control may not reach them. VM tries to find the exception handler code. If a Java exception handler is found, its starting IP location is counted as predicted and VM sets a single-step breakpoint on it. If the exception handler is not found, or a native method is reached in stack unwinding, no predicted breakpoint is set. Single-stepping in native code is not possible, and the predicted breakpoint would be set automatically when native code returns to Java as described in the previous section.

Invokevirtual and invokeinterface

The two bytecodes that transfer control have no fixed target address that could be found by analyzing the code statically. Instead, the target address can be found at run time. When VM sends a single-step event and execution is stopped on invokevirtual or invokeinterface bytecode, VM inserts a special single-step type breakpoint (it has a flag to make it different from ordinary single-step breakpoints) that doesn't trigger a single-step event.

The location of such a breakpoint is determined in a special way. Compiled code for invokevirtual and invokeinterface bytecodes may be long and may contain several call instructions before the actual call of the target method. But there is a special contract with JIT that the last call inside of the code range that corresponds to the invoke bytecode is the actual call of the method. So VM tries to find this last call instruction in the code range and instrument it with the special type of single step breakpoint.

To find this last call instruction, VM uses the same disassembler facility that is used for processing breakpoints. It iterates through all instructions of the code generated for the invoke type bytecode and checks whether the instruction is a call. Iteration continues until the whole range of code for the invoke bytecode is scanned. The last call encountered in this range is the one that calls the method.

When a special type of single step breakpoint is hit, the register context contains all runtime information to find the target address. VM uses the call instruction argument to find the target address of the call. It looks up the method in the compiled methods table and if such method is found, counts its first bytecode as predicted. If no method is found, this means that the method is not compiled yet and no action is taken.

Single-step breakpoints for newly compiled methods are set after they are compiled, at the same time when pending breakpoints are inserted, as described in breakpoints section. For every method that is compiled, its first bytecode is considered as predicted, and VM sets a single-step type breakpoint on it.