Method and mechanism for cache compaction and bandwidth reduction

A method in a hierarchical memory system comprising:

• conveying a request from a cache for a first sub-block within a first cache line comprising a plurality of sub-blocks of data;
• accessing a mask corresponding to the first cache line, in response to receiving said request;
• identifying one or more sub-blocks of the cache line which are predicted by the mask to be accessed;
• and
• transferring to the cache only the first sub-block and said one or more sub-blocks;
• wherein in response to detecting an access to a second cache line within the cache, the method further comprises:
• identifying a particular sub-block of the second cache line which is addressed by the access;
• and
• storing an indication in an entry in a first mask which corresponds to the particular sub-block, wherein the mask comprises a separate entry for each of the sub-blocks within the second cache line, said indication comprising a prediction that the sub-block is likely to be accessed while the second cache line is present in the cache;
• wherein in response to determining a selected cache line stored within the cache is to be evicted, the method further comprises:
• accessing a selected word mask corresponding to the selected cache line prior to evicting the selected cache line;
• and then
• transferring to a lower-level memory only those sub-blocks of the selected cache line which are identified by the selected word mask as being predicted likely to be accessed.

A computing system comprising:

• a main memory;
• a processing unit comprising a cache located between one or more processing cores and the main memory, wherein the cache is configured to convey a request for a first sub-block within a first cache line comprising a plurality of sub-blocks of data;
• a lower-level memory, coupled to the processing unit, configured to store a cache line which is evicted from the first cache;
• wherein, in response to receiving said request, the lower-level memory is further configured to:
• access a mask corresponding to the first cache line;
• identify one or more sub-blocks within the first cache line which are predicted by the mask to be accessed;
• and
• transfer to the cache only the first sub-block and said one or more sub-blocks;
• wherein the cache, in response to detecting an access to a second cache line within the cache, is further configured to:
• identify a particular sub-block of the second cache line which is addressed by the access;
• and
• store an indication in an entry in a first mask which corresponds to the particular sub-block, wherein the first mask comprises a separate entry for each of the sub-blocks within the second cache line, said indication comprising a prediction that the sub-block is likely to be accessed while the second cache line is present in the cache;
• wherein in response to determining a selected cache line stored within the cache is to be evicted, the cache is further configured to access a selected word mask corresponding to the selected cache line prior to evicting the selected cache line, and then transfer to the lower-level memory only those sub-blocks of the selected cache line which are identified by the selected word mask as being predicted likely to be accessed.