56 results about "Branch misprediction" patented technology

A system and method for performance monitoring may use data collected from a hardware event agent comprising a hardware sampling mechanism and / or one or more hardware counters to increment one or more synthesized performance counters by an amount dependent on an expression involving the collected data. Each synthesized performance counter may be configured to count events of a different type and may comprise a machine addressable storage location. The event types may include various memory references or misses, branches, branch mispredictions, or any other event of interest in performance monitoring. The hardware event agent may comprise one or more instruction counters, cycle counters, timers, or other hardware performance counters. One hardware performance counter may be used in a time-multiplexed or data-multiplexed manner to monitor events of multiple event types. The hardware sampling mechanism may return a statistical packet for sampled instructions, which may be examined to determine the event type.

A branch target buffer 10 is provided which maintains its entries across context switches within a virtually addressed system. Branch mispredictions are detected for individual entries 12 within the branch target buffer 10 and those individual entries are invalidated.

A method and system for changing the executable status of an operation following a branch misprediction. In one embodiment, a method may include predicting an execution path of a first conditional branch operation stored in an entry of a trace cache, and in response to predicting the execution path, if a first operation stored in the entry of the trace cache is not in the execution path according to the prediction, assigning to the first operation a non-executable status indicative that the first operation is not in the execution path. The method may further include detecting that the prediction is incorrect subsequent to assigning the non-executable status to the first operation and assigning an executable status to the first operation in response to detecting the incorrect prediction, where the executable status is indicative that the first operation is in the execution path.

An apparatus and method for cycle accounting for a microprocessor are disclosed, in which a performance monitor includes a plurality of silos, a prioritizer, and a combiner. The silos receive delay reason signals from the main processor pipeline, and output staged signals. The prioritizer receives the staged signals, and outputs a plurality of prioritized signals. The combiner selectively combines various of the prioritize signals, and provides signals indicative of microprocessor performance. Each silo includes, in series, a plurality of stages, with each stage containing a single latch. The stages of the silo are synchronized with the stages of the main processor pipeline. The performance monitor operates in real-time, at the same frequency as the microprocessor, and in parallel to the main processor pipeline, and correctly accounts for buffering effects of decoupling buffers. Outputted signals include various signals indicative of microprocessor performance, for example, cache misses, branch mispredictions, and so forth, but only for those miss-events that contribute to a program's visible delay, thereby providing an accurate picture of where cycles are being wasted.

An apparatus and method for cycle accounting for a microprocessor are disclosed, in which a performance monitor includes a plurality of silos, a prioritizer, and a combiner. The silos receive delay reason signals from the main processor pipeline, and output staged signals. The prioritizer receives the staged signals, and outputs a plurality of prioritized signals. The combiner selectively combines various of the prioritize signals, and provides signals indicative of microprocessor performance. Each silo includes, in series, a plurality of stages, with each stage containing a single latch. The stages of the silo are synchronized with the stages of the main processor pipeline. The performance monitor operates in real-time, at the same frequency as the microprocessor, and in parallel to the main processor pipeline. Outputted signals include various signals indicative of microprocessor performance, for example, cache misses, branch mispredictions, and so forth, but only for those miss-events that contribute to a program's visible delay, thereby providing an accurate picture of where cycles are being wasted.

In one embodiment, a processor comprises a branch resolution unit and a redirect recovery cache. The branch resolution unit is configured to detect a mispredicted branch operation, and to transmit a redirect address for fetching instructions from a correct target of the branch operation responsive to detecting the mispredicted branch operation. The redirect recovery cache comprises a plurality of cache entries, each cache entry configured to store operations corresponding to instructions fetched in response to respective mispredicted branch operations. The redirect recovery cache is coupled to receive the redirect address and, if the redirect address is a hit in the redirect recovery cache, the redirect recovery cache is configured to supply operations from the hit cache entry to a pipeline of the processor, bypassing at least one initial pipeline stage.

A processor core and method for managing branch misprediction in an out-of-order processor pipeline. In one embodiment, the pipeline of the processor core includes a front-end instruction fetch portion, a back-end instruction execution portion, and pipeline control logic. Operation of the instruction fetch portion is decoupled from operation of the instruction execution portion. Following detection of a control transfer misprediction, operation of the instruction fetch portion is halted and instructions residing in the instruction fetch portion are invalidated. When the instruction associated with the misprediction reaches a selected pipeline stage, instructions residing in the instruction execution portion of the pipeline are invalidated and the flow of instructions from the instruction fetch portion to the instruction execution portion of the processor pipeline is restarted. A mispredict instruction identification checker and instruction identification tags are used to determine if a control transfer instruction is permitted to redirect instruction fetching.

A computer system may recognize a busy-wait loop in program instructions at compile time and / or may recognize busy-wait looping behavior during execution of program instructions. The system may recognize that an exit condition for a busy-wait loop is specified by a conditional branch type instruction in the program instructions. In response to identifying the loop and the conditional branch type instruction that specifies its exit condition, the system may influence or override a prediction made by a dynamic branch predictor, resulting in a prediction that the exit condition will be met and that the loop will be exited regardless of any observed branch behavior for the conditional branch type instruction. The looping instructions may implement waiting for an inter-thread communication event to occur or for a lock to become available. When the exit condition is met, the loop may be exited without incurring a misprediction delay.

A system and method for reducing branch misprediction penalty. In response to detecting a mispredicted branch instruction, circuitry within a microprocessor identifies a predetermined condition prior to retirement of the branch instruction. Upon identifying this condition, the entire corresponding pipeline is flushed prior to retirement of the branch instruction, and instruction fetch is started at a corresponding address of an oldest instruction in the pipeline immediately prior to the flushing of the pipeline. The correct outcome is stored prior to the pipeline flush. In order to distinguish the mispredicted branch from other instructions, identification information may be stored alongside the correct outcome. One example of the predetermined condition being satisfied is in response to a timer reaching a predetermined threshold value, wherein the timer begins incrementing in response to the mispredicted branch detection and resets at retirement of the mispredicted branch.

A bytecode interpreter in a computing system is provided. The interpreter assists in branch prediction by a host processor that processes a virtual machine such as JAVA TM and DALVIK TM , thereby reducing branch misprediction and achieving high performance.

An internal call / return stack (CRS) correction apparatus in a pipelined microprocessor is disclosed. Each time the microprocessor updates the CRS in response to a call or return instruction (call / ret), the microprocessor also stores correction information into a first correction stack. The microprocessor includes two distinct stages that detect invalidating events, such as a branch misprediction or exception. Once a call / ret passes the first detecting stage, the correction information associated with that call / ret is moved from the first correction stack to a second correction stack. If an invalidating event is detected at the upper detecting stage, then only the correction information in the first stack is used to correct the CRS. However, if an invalidating event is detected at the lower detecting stage, then the correction information in both the first and second stack is used to correct the CRS.

A method for trace prediction includes using trace prediction to predict a trace specifying branch decisions. When a branch misprediction is detected, trace prediction is terminated and prediction is continued using branch prediction.

Checkpoints may be used to recover from branch mispredictions using scalable rename map table recovery.

A system and method for efficient branch prediction. A processor includes two branch predictors. A first branch predictor generates branch prediction data, such as a branch direction and a branch target address. The second branch predictor generates branch prediction data at a later time and with higher prediction accuracy. Control logic may determine whether the branch prediction data from each of the first and the second branch predictors match. If a mismatch occurs, the first predictor may be trained with the branch prediction data generated by the second branch predictor. A stored indication of hysteresis may indicate a given branch instruction exhibits a frequently alternating pattern regarding its branch direction. Such behavior may lead to consistent branch mispredictions due to the training is unable to keep up with the changing branch direction. When such a condition is determined to occur, the control logic may prevent training of the first predictor.