Load/store unit employing last-in-buffer indication for rapid load-hit-store

Abstract

A load/store unit includes a buffer configured to retain store memory operations which have probed the data cache. Each entry in the buffer includes a last-in-buffer (LIB) indication which identifies whether or not the store in that entry is the youngest store in the buffer to update the memory locations specified by the corresponding store address. Load addresses are compared to the store addresses, and the comparison result is qualified with the corresponding LIB indication such that only the youngest store is identified as a hit. At most one load hit store is detected. In one embodiment, the buffer also stores loads which have probed the data cache. To associate the load with the youngest store which is older than the load, the buffer records the store instruction tag of a store which is hit by the load during the initial probe (according to the LIB indications during the initial probe). During reprobes, the LIB indications are ignored and instead the store instruction tags are compared to the recorded store instruction tag. As stores are inserted into the buffer, the store address is compared to the addresses in the buffer. If a hit is detected, the LIB indication for the hit store is set to a state indicating that the hit store in not the last in buffer to update the corresponding store address. The LIB indication for the newly inserted store is set to a last-in-buffer state.