Abstract: Establishing a branch target instruction cache (BTIC) entry for subroutine returns to reduce pipeline bubbles, and related systems, methods, and computer-readable media are disclosed. In one embodiment, a method of establishing a BTIC entry includes detecting a subroutine call in an execution pipeline. In response, at least one instruction fetched sequential to the subroutine call is written as a branch target instruction in a BTIC entry for a subroutine return. A next instruction fetch address is calculated, and is written into a next instruction fetch address field in the BTIC entry. In this manner, the BTIC may provide correct branch target instruction and next instruction fetch address data for the subroutine return, even if the subroutine return is encountered for the first time or the subroutine is called from different calling locations.

Abstract: Systems and methods for dependency-prediction include executing instructions in an instruction pipeline of a processor and detecting a conditionality-imposing control instruction, such as an If-Then (IT) instruction, which imposes dependent behavior on a conditionality block size of one or more dependent instructions. Prior to executing a first instruction, a dependency-prediction is made to determine if the first instruction is a dependent instruction of the conditionality-imposing control instruction, based on the conditionality block size and one or more parameters of the instruction pipeline. The first instruction is executed based on the dependency-prediction. When the first instruction is dependency-mispredicted, an associated dependency-misprediction penalty is mitigated. If the first instruction is a branch instruction, the mitigation involves training a branch prediction tracking mechanism to correctly dependency-predict future occurrences of the first instruction.

Abstract: Branch prediction using Least-Recently-Used (LRU)-class linked list branch predictors, and related circuits, methods, and computer-readable media are disclosed. In one aspect, a branch predictor circuit comprises branch direction prediction logic and a linked list comprising a plurality of predictor entries, each comprising a link address register. The branch predictor circuit also comprises a LRU indicator indicative of a relative age of each of the predictor entries. The branch predictor circuit is configured to detect a first branch instruction in an instruction stream, and determine whether the first branch instruction is predicted to be taken.

Abstract: Fusing conditional write instructions having opposite conditions in instruction processing circuits and related processor systems, methods, and computer-readable media are disclosed. In one embodiment, a first conditional write instruction writing a first value to a target register based on evaluating a first condition is detected by an instruction processing circuit. The circuit also detects a second conditional write instruction writing a second value to the target register based on evaluating a second condition that is a logical opposite of the first condition. Either the first condition or the second condition is selected as a fused instruction condition, and corresponding values are selected as if-true and if-false values. A fused instruction is generated for selectively writing the if-true value to the target register if the fused instruction condition evaluates to true, and selectively writing the if-false value to the target register if the fused instruction condition evaluates to false.

Abstract: Eliminating redundant masking operations in instruction processing circuits and related processor systems, methods, and computer-readable media are disclosed. In one embodiment, a first instruction in an instruction stream indicating an operation writing a value to a first register is detected by an instruction processing circuit, the value having a value size less than a size of the first register. The circuit also detects a second instruction in the instruction stream indicating a masking operation on the first register. The masking operation is eliminated upon a determination that the masking operation indicates a read operation and a write operation on the first register and has an identity mask size equal to or greater than the value size. In this manner, the elimination of the masking operation avoids potential read-after-write hazards and improves performance of a CPU by removing redundant operations from an execution pipeline.

Abstract: A method and apparatus for allowing an out-of-order processor to reuse an in-use physical register is disclosed herein. The method and apparatus uses identifiers, such as tokens and/or other identifiers in a rename map table (RMT) and a physical register file (PRF), to indicate whether an instruction result is allowed or disallowed to be written into a physical register.

Abstract: Embodiments disclosed herein include eliminating redundant synchronization barriers from execution pipelines in instruction processing circuits. Related processor systems, methods, and computer-readable media are also disclosed. By tracking the occurrence of synchronization events, unnecessary software synchronization operations may be identified and eliminated, thus improving performance of a central processing unit (CPU). In one embodiment, a method for eliminating redundant synchronization barriers in an instruction stream is provided. The method comprises determining whether a next instruction comprises a synchronization barrier of a type corresponding to a first synchronization event. The method also comprises eliminating the next instruction from the instruction stream, responsive to determining that the next instruction comprises a synchronization barrier of a type corresponding to the first synchronization event.

Abstract: A processor includes a queue for storing instructions processed within the context of a current value of a register field, where for some embodiments the instruction is undefined or defined, depending upon the register field at time of processing. After a write instruction (an instruction that writes to the register field) executes, the queue is searched for any entries that contain instructions that depend upon the executed write instruction. Each such entry stores the value of the register field at the time the instruction in the entry was processed. If such an entry is found in the queue and its stored value of the register field does not match the value that the write instruction wrote to the register field, then the processor flushes the pipeline and restarts at a state so as to correctly execute the instruction.

Abstract: A processor to a store constant value (immediate or literal) in a cache upon decoding a move immediate instruction in which the immediate is to be moved (copied or written) to an architected register. The constant value is stored in an entry in the cache. Each entry in the cache includes a field to indicate whether its stored constant value is valid, and a field to associate the entry with an architected register. Once a constant value is stored in the cache, it is immediately available for forwarding to a processor pipeline where a decoded instruction may need the constant value as an operand.

Abstract: An apparatus and method for executing call branch and return branch instructions in a processor by utilizing a link register stack. The processor includes a branch counter that is initialized to zero, and is set to zero each time the processor decodes a link register manipulating instruction other than a call branch instruction. The branch counter is incremented by one each time a call branch instruction is decoded and an address is pushed onto the link register stack. In response to decoding a return branch instruction and provided the branch counter is not zero, a target address for the decoded return branch instruction is popped off the link register stack, the branch counter is decremented, and there is no need to check the target address for correctness.

Abstract: An instruction in an instruction cache line having a first portion that is cacheable, a second portion that is from a page that is non-cacheable, and crosses a cache line is prevented from executing from the instruction cache. An attribute associated with the non-cacheable second portion is tracked separately from the attributes of the rest of the instructions in the cache line. If the page crossing instruction is reached for execution, the page crossing instruction and instructions following are flushed and a non-cacheable request is made to memory for at least the second portion. Once the second portion is received, the whole page crossing instruction is reconstructed from the first portion saved in the previous fetch group. The page crossing instruction or portion thereof is returned with the proper attribute for a non-cached fetched instruction and the reconstructed instruction can be executed without being cached.

Abstract: Fusing immediate value, write-based instructions in instruction processing circuits, and related processor systems, methods, and computer-readable media are disclosed. In one embodiment, a first instruction indicating an operation writing an immediate value to a register is detected by an instruction processing circuit. The circuit also detects at least one subsequent instruction indicating an operation that overwrites at least one first portion of the register while maintaining a value of a second portion of the register. The at least one subsequent instruction is converted (or replaced) with a fused instruction(s), which indicates an operation writing the at least one first portion and the second portion of the register.

Abstract: Establishing a branch target instruction cache (BTIC) entry for subroutine returns to reduce pipeline bubbles, and related systems, methods, and computer-readable media are disclosed. In one embodiment, a method of establishing a BTIC entry includes detecting a subroutine call in an execution pipeline. In response, at least one instruction fetched sequential to the subroutine call is written as a branch target instruction in a BTIC entry for a subroutine return. A next instruction fetch address is calculated, and is written into a next instruction fetch address field in the BTIC entry. In this manner, the BTIC may provide correct branch target instruction and next instruction fetch address data for the subroutine return, even if the subroutine return is encountered for the first time or the subroutine is called from different calling locations.

Abstract: An instruction in an instruction cache line having a first portion that is cacheable, a second portion that is from a page that is non-cacheable, and crosses a cache line is prevented from executing from the instruction cache. An attribute associated with the non-cacheable second portion is tracked separately from the attributes of the rest of the instructions in the cache line. If the page crossing instruction is reached for execution, the page crossing instruction and instructions following are flushed and a non-cacheable request is made to memory for at least the second portion. Once the second portion is received, the whole page crossing instruction is reconstructed from the first portion saved in the previous fetch group. The page crossing instruction or portion thereof is returned with the proper attribute for a non-cached fetched instruction and the reconstructed instruction can be executed without being cached.

Abstract: A processor includes a conditional branch instruction prediction mechanism that generates weighted branch prediction values. For weakly weighted predictions, which tend to be less accurate than strongly weighted predictions, the power associating with speculatively filling and subsequently flushing the cache is saved by halting instruction prefetching. Instruction fetching continues when the branch condition is evaluated in the pipeline and the actual next address is known. Alternatively, prefetching may continue out of a cache. To avoid displacing good cache data with instructions prefetched based on a mispredicted branch, prefetching may be halted in response to a weakly weighted prediction in the event of a cache miss.

Abstract: Fusing flag-producing and flag-consuming instructions in instruction processing circuits and related processor systems, methods, and computer-readable media are disclosed. In one embodiment, a flag-producing instruction indicating a first operation generating a first flag result is detected in an instruction stream by an instruction processing circuit. The instruction processing circuit also detects a flag-consuming instruction in the instruction stream indicating a second operation consuming the first flag result as an input. The instruction processing circuit generates a fused instruction indicating the first operation generating the first flag result and indicating the second operation consuming the first flag result as the input. In this manner, as a non-limiting example, the fused instruction eliminates a potential for a read-after-write hazard between the flag-producing instruction and the flag-consuming instruction.

Abstract: A processor architecture to qualify software target-branch hints with hardware-based predictions, the processor including a branch target address cache having entries, where an entry includes a tag field to store an instruction address, a target field to store a target address, and a state field to store a state value. Upon decoding an indirect branch instruction, the processor determines whether an entry in the branch target address cache has an instruction address that matches the address of the decoded indirect branch instruction; and if there is a match, depending upon the state value stored in the entry, the processor will use the stored target address as the predicted target address for the decoded indirect branch instruction, or will use a software provided target address hint if available.

Abstract: Preventing execution of parity-error-induced unpredictable instructions, and related processor systems, methods, and computer-readable media are disclosed. In this regard, a method for processing instructions in a central processing unit (CPU) is provided. The method comprises decoding an instruction comprising a plurality of bits, and generating a parity error indicator indicating whether a parity error exists in the plurality of bits prior to execution of the instruction. If the parity error indicator indicates that the parity error exists in the plurality of bits, one or more of the plurality of bits are modified to indicate a no execution operation (NOP), without effecting a roll back of a program counter of the CPU and without re-decoding the instruction. In this manner, the possibility of the parity error causing an inadvertent execution of an unpredictable instruction is reduced.

Abstract: The disclosure relates to predicting simple and polymorphic branch instructions. An embodiment of the disclosure detects that a program instruction is a branch instruction, determines whether a program counter for the branch instruction is stored in a program counter filter, and, if the program counter is stored in the program counter filter, prevents the program counter from being stored in a first level predictor.

Abstract: Fusing conditional write instructions having opposite conditions in instruction processing circuits and related processor systems, methods, and computer-readable media are disclosed. In one embodiment, a first conditional write instruction writing a first value to a target register based on evaluating a first condition is detected by an instruction processing circuit. The circuit also detects a second conditional write instruction writing a second value to the target register based on evaluating a second condition that is a logical opposite of the first condition. Either the first condition or the second condition is selected as a fused instruction condition, and corresponding values are selected as if-true and if-false values. A fused instruction is generated for selectively writing the if-true value to the target register if the fused instruction condition evaluates to true, and selectively writing the if-false value to the target register if the fused instruction condition evaluates to false.