Method and apparatus for enforcing membar instruction semantics in an execute-ahead processor

A computer system for executing a membar instruction in an execute-ahead processor, wherein the membar instruction forces buffered loads and stores to complete before allowing any following loads or stores to complete, comprising:

• a processor;
• a memory;
• an execution mechanism within the processor configured to issue instructions for execution in a normal-execution mode, and further configured to enter an execute-ahead mode if, while issuing instructions for execution in the normal-execution mode, an instruction cannot be executed because of an unresolved data dependency;
• wherein while issuing instructions for execution in the execute-ahead mode, instructions that cannot be executed because of an unresolved data dependency are deferred by placing the instructions in a deferred queue, and non-deferred instructions are executed in program order;
• wherein, upon encountering a membar instruction while issuing instructions for execution in the normal-execution mode, the execution mechanism is configured to determine if either a load buffer or a store buffer is non-empty and, if so, the execution mechanism is configured to:
• defer the membar instruction by placing the membar instruction in the deferred queue;
• execute subsequent instructions in the execute-ahead mode;
• and
• defer all load and store instructions encountered by the execution mechanism in execute-ahead mode after deferring the membar instruction by placing these instructions in the deferred queue in program order following the membar instruction;
• and
• wherein the execution mechanism is configured to initialize a stores-after-launch counter upon each entry to execute-ahead mode that is caused by an unresolved data dependency other than the membar instruction, wherein the stores-after-launch counter is incremented for each store encountered while the processor is executing in execute-ahead mode, and wherein the stores-after-launch counter is not decremented after the stores-after-launch counter is initialized upon each entry to execute-ahead mode that is caused by an unresolved data dependency other than the membar instruction;
• when all stores and loads that precede the membar instruction have been committed to memory from the store buffer and the load buffer, the execution mechanism is configured to enter a deferred mode, wherein the execution mechanism executes the deferred instructions including the membar instruction;
• and
• if all deferred instructions are executed in the deferred mode, the mechanism is configured to return to the normal-execution mode to resume normal program execution from the point where the execute-ahead mode left off.

A method for executing a membar instruction in an execute-ahead processor, wherein the membar instruction forces buffered loads and stores to complete before allowing any following loads or stores to complete, comprising:

• issuing instructions for execution in a normal-execution mode;
• if, while issuing instructions for execution in the normal-execution mode, an instruction cannot be executed because of an unresolved data dependency, entering an execute-ahead mode, wherein instructions that cannot be executed because of an unresolved data dependency are deferred by placing the instructions in a deferred queue, and wherein non-deferred instructions are executed in program order;
• upon encountering a membar instruction while issuing instructions for execution in the normal-execution mode, determining if either a load buffer or a store buffer is non-empty and, if so, deferring the membar instruction by placing the membar instruction in the deferred queue and executing subsequent instructions in the execute-ahead mode;
• deferring all load and store instructions encountered by the execution mechanism in execute-ahead mode after deferring the membar instruction by placing these instructions in the deferred queue in program order following the membar instruction;
• initializing a stores-after-launch counter upon each entry to execute-ahead mode that is caused by an unresolved data dependency other than the membar instruction;
• incrementing the stores-after-launch counter for each store encountered while the processor is executing in execute-ahead mode;
• when all stores and loads that precede the membar instruction have been committed to memory from the store buffer and the load buffer during the execute-ahead mode, entering a deferred mode, wherein the processor executes the deferred instructions including the membar instruction;
• and
• if all deferred instructions are executed in deferred mode, returning to the normal-execution mode to resume normal program execution from the point where the execute-ahead mode left off;
• wherein the stores-after-launch counter is not decremented after the stores-after-launch counter is initialized upon each entry to execute-ahead mode that is caused by an unresolved data dependency other than the membar instruction.