Mechanism for handling unfused multiply-accumulate accrued exception bits in a processor

1. A processor, comprising:

a floating-point unit configured to execute an unfused multiply-accumulate instruction defined with an instruction set architecture, wherein the unfused multiply-accumulate instruction includes a multiply sub-operation and an accumulate sub-operation;

a storage configured to maintain floating-point exception state information; and

exception logic including a storage array and configured to capture uncommitted floating-point exception state within the storage array after completion of the multiply sub-operation and prior to completion of the accumulate sub-operation, and to update the storage, after the unfused multiply-accumulate instruction commits, with the floating-point exception state from the storage array;

wherein the storage array includes a first port and a second port; and

wherein the exception logic is further configured to:

read exception information for a first unfused multiply-accumulate instruction defined in the instruction set architecture from the first port in response to a determination that the first unfused multiply-accumulate instruction has committed;

read exception information for a second unfused multiply-accumulate instruction defined the in the instruction set architecture from the second port in response to a determination that the second unfused multiply-accumulate instruction has committed;

select one of the exception information for the first unfused multiply-accumulate instruction and the exception information for the second unfused multiply-accumulate instruction dependent upon which of the first unfused multiply-accumulate instruction and the second unfused multiply-accumulate instruction commits; and

determine an architectural state dependent upon the selected exception information;

wherein the first unfused multiply-accumulate instruction and the second unfused multiply-accumulate instruction are concurrently executed.

2. The processor as recited in claim 1 , wherein the floating-point unit is configured to provide one or more exception signals corresponding to one or more respective exceptions generated in response to execution of each of the multiply sub-operation and the accumulate sub-operation.

3. The processor as recited in claim 1 , wherein the floating-point exception state information includes accrued exception bits and current exception bits.

4. The processor as recited in claim 3 , wherein the floating-point unit is further configured to generate one or more particular accrued exception bits and one or more current exception bits that are indicative of the floating-point exception state after the multiply sub-operation and the accumulate sub-operation.

5. The processor as recited in claim 4 , wherein, to generate a given accrued exception bit, the exception logic is configured to perform a logical OR operation between a first exception signal generated after execution of the multiply sub-operation and a second exception signal generated after execution of the accumulate sub-operation.

6. The processor as recited in claim 3 , wherein the exception logic is further configured to discard floating-point exception bits corresponding to the floating-point exception state prior to updating the storage in response to one or more exception traps being enabled.

7. The processor as recited in claim 1 , wherein the floating-point unit is configured to execute a first instruction before a second instruction, wherein the second instruction occurs before the first instruction in program order.

8. The processor as recited in claim 1 , wherein the floating-point exceptions are defined in the IEEE Standard 754-1985.

9. The processor as recited in claim 1 , wherein during execution of an unfused multiply-accumulate instruction, the floating-point unit is further configured to perform a first rounding operation subsequent to the multiply sub-operation and a second rounding operation subsequent to the accumulate sub-operation.

10. A system comprising:

a system memory; and

a processor coupled to the system memory;

wherein the processor includes:

a floating-point unit configured to execute an unfused multiply-accumulate instruction defined with an instruction set architecture, wherein the unfused multiply-accumulate instruction includes a multiply sub-operation and an accumulate sub-operation;

a storage configured to maintain floating-point exception state information; and

exception logic including a storage array and configured to capture uncommitted floating-point exception state within the storage array after completion of the multiply sub-operation and prior to completion of the accumulate sub-operation, and to update the storage, after the unfused multiply-accumulate instruction commits, with the floating-point exception state from the storage array;

wherein the storage array includes a first port and a second port; and

wherein the exception logic is further configured to:

read exception information for a first unfused multiply-accumulate instruction defined in the instruction set architecture from the first port in response to a determination that the first unfused multiply-accumulate instruction has committed;

read exception information for a second unfused multiply-accumulate instruction defined in the instruction set architecture from the second port in response to a determination that the second unfused multiply-accumulate instruction has committed;

select one of the exception information for the first unfused multiply-accumulate instruction and the exception information for the second unfused multiply-accumulate instruction dependent upon which of the first unfused multiply-accumulate and the second unfused multiply-accumulate instruction commits; and

determine an architectural state dependent upon the selected exception information;

wherein the first unfused multiply-accumulate instruction and the second unfused multiply-accumulate instruction are concurrently executed.

11. A method comprising:

a floating-point unit of a processor executing an unfused multiply-accumulate instruction defined with an instruction set architecture, wherein the unfused multiply-accumulate instruction includes a multiply sub-operation and an accumulate sub-operation;

maintaining floating-point exception state information within a storage; and

exception logic capturing uncommitted floating-point exception state within a storage array of the exception logic after completion of the multiply sub-operation and prior to completion of the accumulate sub-operation, and updating the storage, after the unfused multiply-accumulate instruction commits, with the floating-point exception state from the storage array;

wherein the storage array includes a first port and a second port; and

wherein the exception logic is further configured to:

read exception information for a first unfused multiply-accumulate instruction defined in the instruction set architecture from the first port in response to a determination that the first unfused multiply-accumulate instruction has committed;

read exception information for a second unfused multiply-accumulate instruction defined in the instruction set architecture from the second port in response to a determination that the second unfused multiply-accumulate instruction as committed;

select one of the exception information for the first unfused multiply-accumulate instruction and the exception information for the second unfused multiply-accumulate instruction dependent upon which of the first unfused multiply-accumulate instruction and the second unfused multiply-accumulate instruction commits; and

determine an architectural state dependent upon the selected exception information;

wherein the first unfused multiply-accumulate instruction and the second unfused multiply-accumulate instruction are concurrently executed.

12. The method as recited in claim 11 , further comprising the floating-point unit providing one or more exception signals corresponding to one or more respective exceptions generated in response to execution of each of the multiply sub-operation and the accumulate sub-operation.

13. The method as recited in claim 11 , wherein the floating-point exception state information includes accrued exception bits and current exception bits.

14. The method as recited in claim 13 , further comprising the exception logic discarding floating-point exception bits corresponding to the floating-point exception state prior to updating the storage in response to one or more exception traps being enabled.

15. The method as recited in claim 11 , further comprising the floating-point unit generating one or more particular accrued exception bits and one or more current exception bits that are indicative of the floating-point exception state after the multiply sub-operation and the accumulate sub-operation.

16. The method as recited in claim 15 , wherein, generating a given accrued exception bit includes the exception logic performing a logical OR operation between a first exception signal generated after execution of the multiply sub-operation and a second exception signal generated after execution of the accumulate sub-operation.

17. The method as recited in claim 11 , wherein during execution of an unfused multiply-accumulate instruction, the floating-point unit performs a first rounding operation subsequent to the multiply sub-operation and a second rounding operation subsequent to the accumulate sub-operation.

18. The method as recited in claim 11 , further comprising the floating-point unit executing a first instruction before a second instruction, wherein the second instruction occurs before the first instruction in program order.