Abstract: Techniques are disclosed relating to cache debug using control registers based on debug commands. In some embodiments, an apparatus includes a processor core, debug circuitry, and control circuitry. In some embodiments, the debug circuitry is configured to receive external debug inputs and send abstract commands to the processor core based on the external debug inputs. In some embodiments, the control circuitry is configured to, in response to an abstract command to read data from the cache: write cache address information to a first control register, assert a trigger signal to cause a read of the data from the cache to a second control register, based on the cache address information in the first control register, and send data from the second control register to the debug circuitry. In various embodiments, this may facilitate hardware cache debug using debug circuitry that also controls software debugging.

Abstract: An instruction buffer for a processor configured to execute multiple threads is disclosed. The instruction buffer is configured to receive instructions from a fetch unit and provide instructions to a selection unit. The instruction buffer includes one or more memory arrays comprising a plurality of entries configured to store instructions and/or other information (e.g., program counter addresses). One or more indicators are maintained by the processor and correspond to the plurality of threads. The one or more indicators are usable such that for instructions received by the instruction buffer, one or more of the plurality entries of a memory array can be determined as a write destination for the received instructions, and for instructions to be read from the instruction buffer (and sent to a selection unit), one or more entries can be determined as the correct source location from which to read.

Abstract: Techniques are disclosed relating to cache debug using control registers based on debug commands. In some embodiments, an apparatus includes a processor core, debug circuitry, and control circuitry. In some embodiments, the debug circuitry is configured to receive external debug inputs and send abstract commands to the processor core based on the external debug inputs. In some embodiments, the control circuitry is configured to, in response to an abstract command to read data from the cache: write cache address information to a first control register, assert a trigger signal to cause a read of the data from the cache to a second control register, based on the cache address information in the first control register, and send data from the second control register to the debug circuitry. In various embodiments, this may facilitate hardware cache debug using debug circuitry that also controls software debugging.

Abstract: An instruction buffer for a processor configured to execute multiple threads is disclosed. The instruction buffer is configured to receive instructions from a fetch unit and provide instructions to a selection unit. The instruction buffer includes one or more memory arrays comprising a plurality of entries configured to store instructions and/or other information (e.g., program counter addresses). One or more indicators are maintained by the processor and correspond to the plurality of threads. The one or more indicators are usable such that for instructions received by the instruction buffer, one or more of the plurality entries of a memory array can be determined as a write destination for the received instructions, and for instructions to be read from the instruction buffer (and sent to a selection unit), one or more entries can be determined as the correct source location from which to read.

Abstract: An instruction buffer for a processor configured to execute multiple threads is disclosed. The instruction buffer is configured to receive instructions from a fetch unit and provide instructions to a selection unit. The instruction buffer includes one or more memory arrays comprising a plurality of entries configured to store instructions and/or other information (e.g., program counter addresses). One or more indicators are maintained by the processor and correspond to the plurality of threads. The one or more indicators are usable such that for instructions received by the instruction buffer, one or more of the plurality entries of a memory array can be determined as a write destination for the received instructions, and for instructions to be read from the instruction buffer (and sent to a selection unit), one or more entries can be determined as the correct source location from which to read.

Abstract: A multi-threaded processor configured to allocate entries in a buffer for instruction cache misses is disclosed. Entries in the buffer may store thread state information for a corresponding instruction cache miss for one of a plurality of threads executable by the processor. The buffer may include dedicated entries and dynamically allocable entries, where the dedicated entries are reserved for a subset of the plurality of threads and the dynamically allocable entries are allocable to a group of two or more of the plurality of threads. In one embodiment, the dedicated entries are dedicated for use by a single thread and the dynamically allocable entries are allocable to any of the plurality of threads. The buffer may store two or more entries for a given thread at a given time. In some embodiments, the buffer may help ensure none of the plurality of threads experiences starvation with respect to instruction fetches.

Abstract: A multithreaded microprocessor includes an instruction fetch unit including a perceptron-based conditional branch prediction unit configured to provide, for each of one or more concurrently executing threads, a direction branch prediction. The conditional branch prediction unit includes a plurality of storages each including a plurality of entries. Each entry may be configured to store one or more prediction values. Each prediction value of a given storage may correspond to at least one conditional branch instruction in a cache line. The conditional branch prediction unit may generate a separate index value for accessing each storage by generating a first index value for accessing a first storage by combining one or more portions of a received instruction fetch address, and generating each other index value for accessing the other storages by combining the first index value with a different portion of direction branch history information.

Abstract: A processor including instruction support for large-operand instructions that use multiple register windows may issue, for execution, programmer-selectable instructions from a defined instruction set architecture (ISA). The processor may also include an instruction execution unit that, during operation, receives instructions for execution from the instruction fetch unit and executes a large-operand instruction defined within the ISA, where execution of the large-operand instruction is dependent upon a plurality of registers arranged within a plurality of register windows. The processor may further include control circuitry (which may be included within the fetch unit, the execution unit, or elsewhere within the processor) that determines whether one or more of the register windows depended upon by the large-operand instruction are not present. In response to determining that one or more of these register windows are not present, the control circuitry causes them to be restored.

Abstract: Various techniques for mitigating dependencies between groups of instructions are disclosed. In one embodiment, such dependencies include “evil twin” conditions, in which a first floating-point instruction has as a destination a first portion of a logical floating-point register (e.g., a single-precision write), and in which a second, subsequent floating-point instruction has as a source the first portion and a second portion of the same logical floating-point register (e.g., a double-precision read). The disclosed techniques may be applicable in a multithreaded processor implementing register renaming. In one embodiment, a processor may enter an operating mode in which detection of evil twin “producers” (e.g., single-precision writes) causes the instruction sequence to be modified to break potential dependencies. Modification of the instruction sequence may continue until one or more exit criteria are reached (e.g., committing a predetermined number of single-precision writes).

Abstract: A control transfer instruction (CTI), such as a branch, jump, etc., may have an offset value for a control transfer that is to be performed. The offset value may be usable to compute a target address for the CTI (e.g., the address of a next instruction to be executed for a thread or instruction stream). The offset may be specified relative to a program counter. In response to detecting a specified offset value, the CTI may be modified to include at least a portion of a computed target address. Information indicating this modification has been performed may be stored, for example, in a pre-decode bit. In some cases, CTI modification may be performed only when a target address is a “near” target, rather than a “far” target. Modifying CTIs as described herein may eliminate redundant address calculations and produce a savings of power and/or time in some embodiments.

Abstract: Various techniques for dynamically allocating instruction tags and using those tags are disclosed. These techniques may apply to processors supporting out-of-order execution and to architectures that supports multiple threads. A group of instructions may be assigned a tag value from a pool of available tag values. A tag value may be usable to determine the program order of a group of instructions relative to other instructions in a thread. After the group of instructions has been (or is about to be) committed, the tag value may be freed so that it can be re-used on a second group of instructions. Tag values are dynamically allocated between threads; accordingly, a particular tag value or range of tag values is not dedicated to a particular thread.

Abstract: Techniques and structures are described which allow the detection of certain dependency conditions, including evil twin conditions, during the execution of computer instructions. Information used to detect dependencies may be stored in a logical map table, which may include a content-addressable memory. The logical map table may maintain a logical register to physical register mapping, including entries dedicated to physical registers available as rename registers. In one embodiment, each entry in the logical map table includes a first value usable to indicate whether only a portion of the physical register is valid and whether the physical register includes the most recent update to the logical register being renamed. Use of this first value may allow precise detection of dependency conditions, including evil twin conditions, upon an instruction reading from at least two portions of a logical register having an entry in the logical map table whose first value is set.

Abstract: An instruction buffer for a processor configured to execute multiple threads is disclosed. The instruction buffer is configured to receive instructions from a fetch unit and provide instructions to a selection unit. The instruction buffer includes one or more memory arrays comprising a plurality of entries configured to store instructions and/or other information (e.g., program counter addresses). One or more indicators are maintained by the processor and correspond to the plurality of threads. The one or more indicators are usable such that for instructions received by the instruction buffer, one or more of the plurality entries of a memory array can be determined as a write destination for the received instructions, and for instructions to be read from the instruction buffer (and sent to a selection unit), one or more entries can be determined as the correct source location from which to read.

Abstract: In one embodiment, a storage buffer includes a plurality of storage locations configured to store a plurality of incoming instructions. The storage buffer also includes a shift FIFO that is coupled to the plurality of storage locations. The shift FIFO includes an entry configured to store an instruction that is next in a program order. In response to receiving a shift signal, control functionality that is coupled to the plurality of storage locations and to the shift FIFO may cause the instruction that is next in the program order to be moved from a given location of the plurality of storage locations to the entry of the shift FIFO.

Abstract: A multi-threaded processor configured to allocate entries in a buffer for instruction cache misses is disclosed. Entries in the buffer may store thread state information for a corresponding instruction cache miss for one of a plurality of threads executable by the processor. The buffer may include dedicated entries and dynamically allocable entries, where the dedicated entries are reserved for a subset of the plurality of threads and the dynamically allocable entries are allocable to a group of two or more of the plurality of threads. In one embodiment, the dedicated entries are dedicated for use by a single thread and the dynamically allocable entries are allocable to any of the plurality of threads. The buffer may store two or more entries for a given thread at a given time. In some embodiments, the buffer may help ensure none of the plurality of threads experiences starvation with respect to instruction fetches.

Abstract: A processor including instruction support for large-operand instructions that use multiple register windows may issue, for execution, programmer-selectable instructions from a defined instruction set architecture (ISA). The processor may also include an instruction execution unit that, during operation, receives instructions for execution from the instruction fetch unit and executes a large-operand instruction defined within the ISA, where execution of the large-operand instruction is dependent upon a plurality of registers arranged within a plurality of register windows. The processor may further include control circuitry (which may be included within the fetch unit, the execution unit, or elsewhere within the processor) that determines whether one or more of the register windows depended upon by the large-operand instruction are not present. In response to determining that one or more of these register windows are not present, the control circuitry causes them to be restored.

Abstract: In one embodiment, a processor comprises a cache and a cache miss unit coupled to the cache. The cache is coupled to be accessed by cache accesses corresponding to a plurality of threads active in the processor. The cache miss unit is configured to record a plurality of cache misses detected in the cache and to associate each cache miss of the plurality of cache misses with a corresponding thread of the plurality of threads for which that cache miss is detected. Additionally, the cache miss unit is configured to initiate a cache fill for a selected cache miss of the plurality of cache misses. The cache miss unit is configured to select the selected cache miss based on a prioritization of the corresponding threads associated with the plurality of cache misses. In one implementation, the cache is an instruction cache and the cache misses are due to fetches corresponding to the plurality of threads.

Abstract: A multithreaded microprocessor includes an instruction fetch unit including a perceptron-based conditional branch prediction unit configured to provide, for each of one or more concurrently executing threads, a direction branch prediction. The conditional branch prediction unit includes a plurality of storages each including a plurality of entries. Each entry may be configured to store one or more prediction values. Each prediction value of a given storage may correspond to at least one conditional branch instruction in a cache line. The conditional branch prediction unit may generate a separate index value for accessing each storage by generating a first index value for accessing a first storage by combining one or more portions of a received instruction fetch address, and generating each other index value for accessing the other storages by combining the first index value with a different portion of direction branch history information.

Abstract: Various techniques for dynamically allocating instruction tags and using those tags are disclosed. These techniques may apply to processors supporting out-of-order execution and to architectures that supports multiple threads. A group of instructions may be assigned a tag value from a pool of available tag values. A tag value may be usable to determine the program order of a group of instructions relative to other instructions in a thread. After the group of instructions has been (or is about to be) committed, the tag value may be freed so that it can be re-used on a second group of instructions. Tag values are dynamically allocated between threads; accordingly, a particular tag value or range of tag values is not dedicated to a particular thread.

Abstract: In one embodiment, a processor comprises a fetch unit and a pick unit. The fetch unit is configured to fetch instructions for execution by the processor. The pick unit is configured to schedule instructions fetched by the fetch unit for execution in the processor. The pick unit is configured to inhibit scheduling a delayed control transfer instruction (DCTI) until a delay slot instruction of the DCTI is available for scheduling. For example, in some embodiments, the pick unit may inhibit scheduling until the delay slot instruction is written to an instruction buffer, until the delay slot instruction is fetched, etc.