Abstract: A method and load and store buffer for issuing a load instruction to a data cache. The method includes determining whether there are any unresolved store instructions in the store buffer that are older than the load instruction. If there is at least one unresolved store instruction in the store buffer older than the load instruction, it is determined whether the oldest unresolved store instruction in the store buffer is within a speculation window for the load instruction. If the oldest unresolved store instruction is within the speculation window for the load instruction, the load instruction is speculatively issued to the data cache. Otherwise, the load instruction is stalled until any unresolved store instructions outside the speculation window are resolved. The speculation window is a short window that defines a number of instructions or store instructions that immediately precede the load instruction.

Abstract: A technique for restoring a register renaming map is described. In one example, a restore table having a number of storage locations saves a copy of the register renaming map whenever a flow-risk instruction is passed to a re-order buffer. When all storage locations are full, further instructions still pass to the re-order buffer, but a copy of the map is not saved. A storage location subsequently becomes available when its associated flow-risk instruction is executed. A register renaming map state for an unrecorded flow-risk instruction passed to the re-order buffer whilst the storage locations were full is generated and stored in the available location. This is generated using the restore table entry for a previous flow-risk instruction and re-order buffer values for intervening instructions between the previous and unrecorded flow-risk instructions. The restore table can be used to restore the map if an unexpected change in instruction flow occurs.

Abstract: Register files for use in an out-of-order processor that have been divided into a plurality of sub-register files. The register files also have a plurality of buffers which are each associated with one of the sub-register files. Each buffer receives and stores write operations destined for the associated sub-register file which can be later issued to the sub-register file. Specifically, each clock cycle it is determined whether there is at least one write operation in the buffer that has not been issued to the associated sub-register file. If there is at least one write operation in the buffer that has not been issued to the associated sub-register file, one of the non-issued write operations is issued to the associated sub-register file.

Abstract: A return stack buffers (RSB) is modified to store index values instead of addresses. When a function is called, the address following the function call is stored in a look-up table and the index at which the address is stored is pushed to the RSB. When a function returns, an index is popped from the RSB and used to identify an address in the look-up table. In another embodiment, the RSB is modified such that each entry comprises two or more address slots. When a function is called, the address following the function call is pushed to the RSB and stored in a selected one of the address slots in a top entry in the RSB. One or more pointer bits within the entry are set to indicate which slot the address was stored in.

Abstract: A method and load and store buffer for issuing a load instruction to a data cache. The method includes determining whether there are any unresolved store instructions in the store buffer that are older than the load instruction. If there is at least one unresolved store instruction in the store buffer older than the load instruction, it is determined whether the oldest unresolved store instruction in the store buffer is within a speculation window for the load instruction. If the oldest unresolved store instruction is within the speculation window for the load instruction, the load instruction is speculatively issued to the data cache. Otherwise, the load instruction is stalled until any unresolved store instructions outside the speculation window are resolved. The speculation window is a short window that defines a number of instructions or store instructions that immediately precede the load instruction.

Abstract: A return stack buffer (RSB) is modified such that each entry comprises two or more address slots. When a function is called, the address following the function call is pushed to the RSB and stored in a selected one of the address slots in a top entry in the RSB. One or more pointer bits within the entry are set to indicate which slot the address was stored in.

Abstract: Methods and migration units for use in out-of-order processors for migrating data to register file caches associated with functional units of the processor to satisfy register read operations. The migration unit receives register read operations to be executed for a particular functional unit. The migration unit reviews entries in a register renaming table to determine if the particular functional unit has recently accessed the source register and thus is likely to comprise an entry for the source register in its register file cache. In particular, the register renaming table comprises entries for physical registers that indicate what functional units have accessed the physical register. If the particular functional unit has not accessed the particular physical register the migration unit migrates data to the register file cache associated with the particular functional unit.

Abstract: Register files for use in an out-of-order processor that have been divided into a plurality of sub-register files. The register files also have a plurality of buffers which are each associated with one of the sub-register files. Each buffer receives and stores write operations destined for the associated sub-register file which can be later issued to the sub-register file. Specifically, each clock cycle it is determined whether there is at least one write operation in the buffer that has not been issued to the associated sub-register file. If there is at least one write operation in the buffer that has not been issued to the associated sub-register file, one of the non-issued write operations is issued to the associated sub-register file.

Abstract: Methods and branch predictors for predicting a target location of a jump table switch statement in a program. The method includes continuously monitoring instructions at the branch predictor to determine if they write to registers used to store an input variable to a jump table switch statement. Any update to a monitored register is stored in a register table maintained by the branch predictor. Then when it comes time to make a prediction for a jump table switch statement instruction the branch predictor uses the register value stored in the table is used to predict where the jump table switch statement will branch to.

Abstract: Methods and migration units for use in out-of-order processors for migrating data to register file caches associated with functional units of the processor to satisfy register read operations. The migration unit receives register read operations to be executed for a particular functional unit. The migration unit reviews entries in a register renaming table to determine if the particular functional unit has recently accessed the source register and thus is likely to comprise an entry for the source register in its register file cache. In particular, the register renaming table comprises entries for physical registers that indicate what functional units have accessed the physical register. If the particular functional unit has not accessed the particular physical register the migration unit migrates data to the register file cache associated with the particular functional unit.

Abstract: A processor and method of efficiently running a 32-bit operating system on a 64-bit processor includes 64-bit hardware and when running a 64-bit operating system operates as a single-threaded processor. However, when running a 32-bit operating system (which may be a guest operating system running on a virtual machine), the processor operates as a two-threaded core. The register file is logically divided into two portions, one for each thread, and logic within an execution unit may be split between threads, shared between threads or duplicated to provide an instance of the logic for each thread. Configuration bits may be set to indicate whether the processor should operate as a single-threaded or multi-threaded device.

Abstract: A datapath pipeline which uses replicated logic blocks to increase the throughput of the pipeline is described. In an embodiment, the pipeline, or a part thereof, comprises a number of parallel logic paths each comprising the same logic. Input register stages at the start of each logic path are enabled in turn on successive clock cycles such that data is read into each logic path in turn and the logic in the different paths operates out of phase. The output of the logic paths is read into one or more output register stages and the logic paths are combined using a multiplexer which selects an output from one of the logic paths on any clock cycle. Various optimization techniques are described and in various examples, register retiming may also be used. In various examples, the datapath pipeline is within a processor.

Abstract: A technique for restoring a register renaming map is described. In one example, a restore table having a number of storage locations saves a copy of the register renaming map whenever a flow-risk instruction is passed to a re-order buffer. When all storage locations are full, further instructions still pass to the re-order buffer, but a copy of the map is not saved. A storage location subsequently becomes available when its associated flow-risk instruction is executed. A register renaming map state for an unrecorded flow-risk instruction passed to the re-order buffer whilst the storage locations were full is generated and stored in the available location. This is generated using the restore table entry for a previous flow-risk instruction and re-order buffer values for intervening instructions between the previous and unrecorded flow-risk instructions. The restore table can be used to restore the map if an unexpected change in instruction flow occurs.

Abstract: Methods, reservation stations and processors for allocating resources to a plurality of threads based on the extent to which the instructions associated with each of the threads are speculative. The method comprises receiving a speculation metric for each thread at a reservation station. Each speculation metric represents the extent to which the instructions associated with a particular thread are speculative. The more speculative an instruction, the more likely the instruction has been incorrectly predicted by a branch predictor. The reservation station then allocates functional unit resources (e.g. pipelines) to the threads based on the speculation metrics and selects a number of instructions from one or more of the threads based on the allocation. The selected instructions are then issued to the functional unit resources.

Abstract: A technique for restoring a register renaming map is described. In one example, a restore table having a number of storage locations saves a copy of the register renaming map whenever a flow-risk instruction is passed to a re-order buffer. When all storage locations are full, further instructions still pass to the re-order buffer, but a copy of the map is not saved. A storage location subsequently becomes available when its associated flow-risk instruction is executed. A register renaming map state for an unrecorded flow-risk instruction passed to the re-order buffer while the storage locations were full is generated and stored in the available location. This is generated using the restore table entry for a previous flow-risk instruction and re-order buffer values for intervening instructions between the previous and unrecorded flow-risk instructions. The restore table can be used to restore the map if an unexpected change in instruction flow occurs.

Abstract: A return stack buffer (RSB) is modified such that each entry comprises two or more address slots. When a function is called, the address following the function call is pushed to the RSB and stored in a selected one of the address slots in a top entry in the RSB. One or more pointer bits within the entry are set to indicate which slot the address was stored in.

Abstract: A return stack buffers (RSB) is modified to store index values instead of addresses. When a function is called, the address following the function call is stored in a look-up table and the index at which the address is stored is pushed to the RSB. When a function returns, an index is popped from the RSB and used to identify an address in the look-up table. In another embodiment, the RSB is modified such that each entry comprises two or more address slots. When a function is called, the address following the function call is pushed to the RSB and stored in a selected one of the address slots in a top entry in the RSB. One or more pointer bits within the entry are set to indicate which slot the address was stored in.

Abstract: Methods and apparatus for predicting the value of a stack pointer which store data when an instruction is seen which grows the stack. The information which is stored includes a size parameter which indicates by how much the stack is grown and one or both of: the register ID currently holding the stack pointer value or the current stack pointer value. When a subsequent instruction shrinking the stack is seen, the stored data is searched for one or more entries which has a corresponding size parameter. If such an entry is identified, the other information stored in that entry is used to predict the value of the stack pointer instead of using the instruction to calculate the new stack pointer value. Where register renaming is used, the information in the entry is used to remap the stack pointer to a different physical register.

Abstract: Methods of predicting stack pointer values of variables stored in a stack are described. When an instruction is seen which stores a variable in the stack in a position offset from the stack pointer, an entry is added to a data structure which identifies the physical register which currently stores the stack pointer, the physical register which stores the value of the variable and the offset value. Subsequently when an instruction to load a variable from the stack from a position which is identified by reference to the stack pointer is seen, the data structure is searched to see if there is a corresponding entry which includes the same offset and the same physical register storing the stack pointer as the load instruction. If a corresponding entry is found the architectural register in the load instruction is mapped to the physical register storing the value of the variable from the entry.

Abstract: Methods, reservation stations and processors for allocating resources to a plurality of threads based on the extent to which the instructions associated with each of the threads are speculative. The method comprises receiving a speculation metric for each thread at a reservation station. Each speculation metric represents the extent to which the instructions associated with a particular thread are speculative. The more speculative an instruction, the more likely the instruction has been incorrectly predicted by a branch predictor. The reservation station then allocates functional unit resources (e.g. pipelines) to the threads based on the speculation metrics and selects a number of instructions from one or more of the threads based on the allocation. The selected instructions are then issued to the functional unit resources.