Apple Inc.
Handling non-cacheable loads in a non-coherent processor

Abstract:

Systems, apparatuses, and methods for processing load instructions are disclosed. A processor includes at least a data cache and a load queue for storing load instructions. The load queue includes poison indicators for load instructions waiting to reach non-speculative status. When a non-cacheable load instruction is speculatively executed, then the poison bit is automatically set for the load instruction. If a cacheable load instruction is speculatively executed, then the processor waits until detecting a first condition before setting the poison bit for the load instruction. The first condition may be detecting a cache line with data for the load instruction being evicted from the cache. If an ordering event occurs for a load instruction with a set poison bit, then the load instruction may be flushed and replayed. An ordering event may be a data barrier or a hazard on an older load targeting the same address as the load.