Store queue having restricted and unrestricted entries

1. A method for processing a load instruction, comprising:

receiving the load instruction;

determining if an entry in a store queue contains data that satisfies the load instruction;

if no entry in the store queue contains data that satisfies the load instruction, retrieving data for the load instruction from a cache; and

if an entry in the store queue contains data that satisfies the load instruction, conditionally forwarding data from entry by:

forwarding data from an unrestricted entry that contains the youngest store that satisfies the load instruction when any number of unrestricted or restricted entries contain data that satisfies the load instruction;

forwarding data from an unrestricted entry when only one restricted entry and no unrestricted entries contain data that satisfies the load instruction; and

deferring the load instruction by placing the load instruction in a deferred queue when two or more restricted entries and no unrestricted entries contain data that satisfies the load instruction.

2. The method of claim 1 , wherein determining if an unrestricted entry contains data that satisfies the load instruction involves:

determining if any unrestricted entry in a set of unrestricted entries in the store queue contains a buffered store to a same cache line as the load;

for each such unrestricted entry, determining if an age of the unrestricted entry matches a global age counter; and

if the age of an unrestricted entry matches the global age counter, using a bitmask for the unrestricted entry to determine if each byte needed to satisfy the load instruction is contained in the unrestricted entry;

wherein an unrestricted entry satisfies the load instruction when the buffered store is directed to the same cache line, the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry, and the age of the entry and the global age counter match.

3. The method of claim 1 , wherein determining if a restricted entry contains data that satisfies the load instruction involves:

determining if any one restricted entry in the store queue contains a store directed to the same cache line as the load; and

for the restricted entry, using a bitmask for the restricted entry to determine if each byte for the load instruction is contained in the restricted entry;

wherein a restricted entry satisfies the load instruction when the buffered store is directed to the same cache line and the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry.

4. The method of claim 1 , wherein the method further comprises re-executing the deferred load when no more than one of the restricted entries contain data that satisfies the load instruction.

5. The method of claim 1 , further comprising:

receiving a new store to a cache line; and

determining if one or more unrestricted entries in the store queue contain buffered stores that are directed to a same cache line;

if not, buffering the new store in a next entry in the store queue;

if so, determining an age of the one or more unrestricted entries;

if an age of an unrestricted entry matches a global age counter, merging the new store into the unrestricted entry,

otherwise, buffering the new store in the next entry in the store queue.

6. The method of claim 1 , wherein forwarding data involves copying the data from an entry in the store queue to a processor register so that the processor register can be used in subsequent computational operations.

7. An apparatus that processes a load instruction, comprising:

a processor;

a cache coupled to the processor, wherein the cache stores data for the processor;

a store queue in the processor, wherein the processor is configured to buffer stores into the store queue until the stores have been committed to the cache;

wherein the processor is configured to receive a load instruction;

in response to receiving the load instruction, the processor is configured to determine if an entry in the store queue contains data that satisfies the load instruction;

if no entry in the store queue contains data that satisfies the load instruction, the processor is configured to retrieve data for the load instruction from the cache; and

if an entry in the store queue contains data that satisfies the load instruction, the processor is configured to conditionally forward data from the entry by:

forwarding data from an unrestricted entry that contains the youngest store that satisfies the load instruction when any number of unrestricted or restricted entries contain data that satisfies the load instruction;

forwarding data from an unrestricted entry when only one restricted entry and no unrestricted entries contain data that satisfies the load instruction; and

deferring the load instruction by placing the load instruction in a deferred queue when two or more restricted entries and no unrestricted entries contain data that satisfies the load instruction.

8. The apparatus of claim 7 , wherein when determining if an unrestricted entry contains data that satisfies the load instruction, the processor is configured to:

determine if any unrestricted entry in a set of unrestricted entries in the store queue contains a buffered store to a same cache line as the load;

for each such unrestricted entry, use a bitmask for the unrestricted entry to determine if each byte needed to satisfy the load instruction is contained in the unrestricted entry; and

if each byte needed to satisfy the load instruction is contained in the unrestricted entry, determine if an age of the unrestricted entry matches a global age counter;

wherein an unrestricted entry satisfies the load instruction when the buffered store is directed to the same cache line, the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry, and the age of the entry and the global age counter match.

9. The apparatus of claim 7 , wherein when determining if a restricted entry contains data that satisfies the load instruction, the processor is configured to:

determine if any one restricted entry in the store queue contains a store directed to the same cache line as the load; and

for the restricted entry, use a bitmask for the restricted entry to determine if each byte for the load instruction is contained in the restricted entry;

wherein a restricted entry satisfies the load instruction when the buffered store is directed to the same cache line and the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry.

10. The apparatus of claim 7 , wherein the processor is configured to re-execute the deferred load when no more than one of the restricted entries contain data that satisfies the load instruction.

11. The apparatus of claim 7 , wherein the processor is further configured to:

receive a new store to a cache line; and

determine if one or more unrestricted entries in the store queue contain buffered stores that are directed to a same cache line;

if not, the processor is configured to buffer the new store in a next entry in the store queue;

if so, the processor is configured to determine an age of the one or more unrestricted entries;

if an age of an unrestricted entry matches a global age counter, the processor is configured to merge the new store into the unrestricted entry,

otherwise, the processor is configured to buffer the new store in the next entry in the store queue.

12. The apparatus of claim 7 , wherein when forwarding data, the processor is configured to copy the data from an entry in the store queue to a processor register so that the processor can use the register in subsequent computational operations.

13. A computer system, comprising:

a processor;

a cache coupled to the processor, wherein the cache is a fast-access memory that stores recently-used data for the processor;

a mass-storage device coupled to the cache, wherein the mass-storage device is a high-capacity, slow-access storage memory that stores data for the processor;

a store queue in the processor, wherein the processor is configured to buffer stores into the store queue until the stores have been committed to the cache;

wherein the processor is configured to receive a load instruction;

in response to receiving the load instruction, the processor is configured to determine if an entry in the store queue contains data that satisfies the load instruction;

if no entry in the store queue contains data that satisfies the load instruction, the processor is configured to retrieve data for the load instruction from the cache; and

if an entry in the store queue contains data that satisfies the load instruction, the processor is configured to conditionally forward data from the entry by:

forwarding data from an unrestricted entry that contains the youngest store that satisfies the load instruction when any number of unrestricted or restricted entries contain data that satisfies the load instruction;

forwarding data from an unrestricted entry when only one restricted entry and no unrestricted entries contain data that satisfies the load instruction; and

deferring the load instruction by placing the load instruction in a deferred queue when two or more restricted entries and no unrestricted entries contain data that satisfies the load instruction.

14. The computer system of claim 13 , wherein when determining if an unrestricted entry contains data that satisfies the load instruction, the processor is configured to:

determine if any unrestricted entry in a set of unrestricted entries in the store queue contains a buffered store to a same cache line as the load;

for each such unrestricted entry, use a bitmask for the unrestricted entry to determine if each byte needed to satisfy the load instruction is contained in the unrestricted entry; and

if each byte needed to satisfy the load instruction is contained in the unrestricted entry, determine if an age of the unrestricted entry matches a global age counter;

wherein an unrestricted entry satisfies the load instruction when the buffered store is directed to the same cache line, the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry, and the age of the entry and the global age counter match.

15. The computer system of claim 13 , wherein when determining if a restricted entry contains data that satisfies the load instruction, the processor is configured to:

determine if any one restricted entry in the store queue contains a store directed to the same cache line as the load; and

for the restricted entry, use a bitmask for the restricted entry to determine if each byte for the load instruction is contained in the restricted entry;

wherein a restricted entry satisfies the load instruction when the buffered store is directed to the same cache line and the bitmask indicates that each byte needed to satisfy the load instruction is contained in the entry.

16. The computer system of claim 13 , wherein the processor is configured to re-execute the deferred load when no more than one of the restricted entries contain data that satisfies the load instruction.

17. The computer system of claim 13 , wherein the processor is further configured to:

receive a new store to a cache line; and

determine if one or more unrestricted entries in the store queue contain buffered stores that are directed to a same cache line;

if not, the processor is configured to buffer the new store in a next entry in the store queue;

if so, the processor is configured to determine an age of the one or more unrestricted entries;

if an age of an unrestricted entry matches a global age counter, the processor is configured to merge the new store into the unrestricted entry,

otherwise, the processor is configured to buffer the new store in the next entry in the store queue.

18. The computer system of claim 13 , wherein when forwarding data, the processor is configured to copy the data from an entry in the store queue to a processor register so that the processor can use the register in subsequent computational operations.