Methods and apparatus for storing prefetch metadata

A method comprising:

• making predictions of data in advance of said data being processed by processing circuitry;
• and
• determining metadata items, each being associated with predictions of data for a given stream of instructions, for storage in metadata storage comprising a plurality of entries;
• responsive to a given entry of the plurality of entries being associated with the given stream associated with a given metadata item, updating the given entry based on the given metadata item;
• responsive to no entry of the plurality of entries being associated with the given stream associated with a given metadata item:
• selecting an entry according to a default replacement policy;
• allocating said given stream to the selected entry;
• and
• updating the selected entry based on the given metadata item, and
• responsive to a switch condition being met, identifying one or more of said entries to lock, and switching said default selection policy to an alternative selection policy comprising locking each identified entry by preventing allocation of streams to the locked entries,
• wherein the switch condition is indicative of a number of active streams of instructions exceeding a capacity of the prefetch metadata storage.

A non-transitory computer-readable storage medium to store computer-readable code for fabrication of an apparatus comprising:

• processing circuitry to process streams of instructions;
• prefetch circuitry to prefetch data in advance of said data being processed by said streams of instructions;
• and
• prefetch metadata storage to store metadata associated with said prefetching, the prefetch metadata storage comprising a plurality of entries and the prefetch circuitry being configured to:
• determine metadata items, each being associated with a given stream of instructions, for storage in the prefetch metadata storage;
• responsive to a given entry of the plurality of entries being associated with the given stream associated with a given metadata item, updating the given entry based on the given metadata item;
• responsive to no entry of the plurality of entries being associated with the given stream associated with a given metadata item:
• selecting an entry according to a default replacement policy;
• allocating said given stream to the selected entry;
• and
• updating the selected entry based on the given metadata item, and
• responsive to a switch condition being met, identifying one or more of said entries to lock, and switching said default selection policy to an alternative selection policy comprising locking each identified entry by preventing allocation of streams to the locked entries,
• wherein the switch condition is indicative of a number of active streams of instructions exceeding a capacity of the prefetch metadata storage.