Apple Inc.

Mips Tech Limited

Oracle International Corporation

Imagination Technologies Limited

Mips Technologies, Inc.

Debasish Chandra

Yuan C Chou

Deepankar Duggal

Richard F Russo

Mridul Agarwal

Niket K Choudhary

Haoyan Jia

Shailender Chaudhry

Robert Cypher

Ranjit Joseph Rozario

Ranganathan Sudhakar

Era Kasturia Nangia

Yanran Yang

Chang Xu

Richard Thuy Van

Disclosed techniques relate to re-use of speculative results from an incorrect execution path. In some embodiments, when a first control transfer instruction is mispredicted, a second control transfer instruction may have been executed on the wrong path because of the misprediction. Result storage circuitry may record information indicating a determined direction for the second control transfer instruction. Control flow tracker circuitry may store, for the first control transfer instruction, information indicating a reconvergence point. Re-use control circuitry may track registers written by instructions prior to the reconvergence point, determine, based on the tracked registers, that the second control transfer instruction does not depend on data from any instruction between the first control transfer instruction and the reconvergence point, and use the recorded determined direction for the second control transfer instruction, notwithstanding the misprediction of the first control transfer instruction.

Re-use of speculative control transfer instruction results from wrong path

A livelock resolution circuit may be used to detect and remove a livelock from a processor. The livelock resolution circuit may use a counter, whose value may be reset or incremented depending on a status of a retire pointer in a reorder buffer circuit (ROB). When the retire pointer fails to move from one operation to another over a period of time, the value in the counter may continuously increment to exceed a threshold, indicating that the period of time elapses without a successful retirement of an operation from the ROB. Thus, the livelock resolution circuit may determine that a livelock occurs. To remove the livelock, the livelock resolution circuit may involve a single-step operational mode, where the livelock resolution circuit may demand only one operation (or microoperation) to be executed at each step. As a result, the operation may be retired and the livelock may be removed.

Livelock detection and resolution using oldest operation tracking

Disclosed techniques relate to re-use of speculative results from an incorrect execution path. In some embodiments, when a control transfer instruction is mispredicted, a load instruction may have been executed on the wrong path. In disclosed embodiments, result storage circuitry records information that indicates destination registers of speculatively-executed load instructions including a first load instruction. Control flow tracker circuitry may store information indicating a reconvergence point for the control transfer instruction. Re-use control circuitry may track registers written by instructions prior to the reconvergence point, determine that the first load instruction does not depend on data from any instruction between the control transfer instruction and the reconvergence point, and use, as a result of the first load instruction, a value from a recorded destination register that was written based on speculative execution of the first load, notwithstanding the misprediction of the control transfer instruction.

Re-use of speculative load instruction results from wrong path

Techniques are disclosed relating to predicting values for load operations. In some embodiments, front-end circuitry is configured to predict values of load operations based on multiple value tagged geometric length predictor (VTAGE) prediction tables (based on program counter information and branch history information). Training circuitry may adjust multiple VTAGE learning tables based on completed load operations. Control circuitry may pre-compute access information (e.g., an index) for a VTAGE learning table for a load based on branch history information that is available to the front-end circuitry but that is unavailable to the training circuitry, store the pre-computed access information, and provide the pre-computed access information from the first storage circuitry to the training circuitry to access the VTAGE learning table based on completion of the load. This may facilitate VTAGE training without pipelining the branch history information.

Managing table accesses for tagged geometric length (TAGE) load value prediction

Techniques are disclosed relating to load value prediction. In some embodiments, a processor includes learning table circuitry that is shared for both address and value prediction. Loads may be trained for value prediction when they are eligible for both value and address prediction. Entries in the learning table may be promoted to an address prediction table or a load value prediction table for prediction, e.g., when they reach a threshold confidence level in the training table. In some embodiments, the learning table stores a hash of a predicted load value and control circuitry uses a probing load to retrieve the actual predicted load value for the value prediction table.

Shared learning table for load value prediction and load address prediction

In one aspect, a processor has a register file, a private Level 1 (L1) cache, and an interface to a shared memory hierarchy (e.g., an Level 2 (L2) cache and so on). The processor has a Load Store Unit (LSU) that handles decoded load and store instructions. The processor may support out of order and multi-threaded execution. As store instructions arrive at the LSU for processing, the LSU determines whether a counter, from a set of counters, is allocated to a cache line affected by each store. If not, the LSU allocates a counter. If so, then the LSU updates the counter. Also, in response to a store instruction, affecting a cache line neighboring a cache line that has a counter that meets a criteria, the LSU characterizes that store instruction as one to be effected without obtaining ownership of the effected cache line, and provides that store to be serviced by an element of the shared memory hierarchy.

Processors and methods for cache sparing stores

Methods and systems that reduce the number of instance of a shared resource needed for a processor to perform an operation and/or execute a process without impacting function are provided. a method of processing in a processor is provided. Aspects include determining that an operation to be performed by the processor will require the use of a shared resource. A command can be issued to cause a second operation to not use the shared resources N cycles later. The shared resource can then be used for a first aspect of the operation at cycle X and then used for a second aspect of the operation at cycle X+N. The second operation may be rescheduled according to embodiments.

Resource sharing using process delay

The described embodiments include a processor that determines instructions that can be issued based on unresolved data dependencies. In an issue unit in the processor, the processor keeps a record of each instruction that is directly or indirectly dependent on a base instruction. Upon determining that the base instruction has been deferred, the processor monitors instructions that are being issued from an issue queue to an execution unit for execution. Upon determining that an instruction from the record has reached a head of the issue queue, the processor immediately issues the instruction from the issue queue.

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Fremont, CA, United States of America

Debasish Chandra