Condition code flag emulation for program code conversion
View Patent ↗An emulator ( 30 ) allows subject code ( 10 ) written for a subject processor ( 12 ) having subject processor registers ( 14 ) and condition code flags ( 16 ) to run in a non-compatible computing environment ( 2 ). The emulator ( 30 ) identifies and records parameters of instructions in the subject code ( 10 ) that affect status of the subject condition code flags ( 16 ). Then, when an instruction in the subject code ( 10 ) is encountered, such as a branch or jump, that uses the flag status to make a decision, the flag status is resolved from the recorded instruction parameters. Advantageously, emulation overhead is substantially reduced.
1. A method for emulating condition code flags during program code conversion, comprising the steps of:
(a) identifying a flag-affecting instruction in a sequence of subject code instructions, wherein the flag-affecting instruction selectively modifies a flag;
(b) recording parameters of the identified flag-affecting instruction;
(c) detecting a flag-usage instruction in the sequence of subject code instructions, wherein the flag-usage instruction uses the flag; and
(d) resolving, in response to detecting the flag-usage instruction, a status of the flag with respect to the recorded parameters.
2. The method of claim 1 , wherein the step (d) comprises explicitly calculating a flag status by emulating effects of the identified flag-affecting instruction on one or more subject condition code flags.
3. The method of claim 2 , wherein the step (b) comprises setting a flag status indicator to a pending state to indicate that parameters have been recorded, and the step (d) comprises setting the flag status indicator to a normalised state to indicate that a flag status has been explicitly calculated.
4. The software of claim 2 further operable to perform the step of recording the first flag affecting instruction as the most recent flag affecting instruction.
5. The method of claim 1 , wherein the step (d) comprises implicitly determining flag status with reference to the recorded parameters.
6. The method of claim 1 , wherein the step (c) comprises determining a flag usage type with reference to a type of the detected flag-usage instruction, and the step (d) comprises resolving flag status either by explicitly calculating a flag status or by implicitly determining a flag status, according to the determined flag usage type.
7. The method of claim 1 , wherein the step (a) comprises identifying a second flag-identifying instruction in the sequence of subject code instructions, and the step (b) comprises replacing the recorded parameters with parameters of the second flag-affecting instruction.
8. The method of claim 1 , wherein the step (a) comprises identifying a type of flag-affecting instruction as one of a plurality of predetermined types, and the step (b) comprises recording parameters for each type of flag-affecting instruction.
9. The method of claim 8 , wherein the predetermined types include a first type instruction affecting a full set of condition code flags, and a second type instruction affecting a subset of the full set of condition code flags.
10. The method of claim 9 , wherein the step (d) comprises resolving flag status for a full set of condition code flags with respect to the recorded first-type parameters, and resolving flag status with respect to the subset of the condition code flags with respect to the recorded second-type parameters.
11. The method of claim 10 , wherein the step (d) comprises explicitly determining flag status from the recorded first-type parameters, and modifying the explicitly determined flag status from the recorded second-type parameters.
12. The method of claim 8 , wherein the predetermined types include instruction types each affecting a condition code flag set amongst a plurality of condition code flag sets.
13. The method of claim 1 , wherein the recorded parameters include an instruction type parameter and one or more instruction operand parameters.
14. The method of claim 13 , wherein recording an instruction operand parameter comprises storing a reference to a location containing an operand.
15. The method of claim 13 , wherein recording an instruction operand parameter comprises determining that an operand remains available unmodified at an original location and storing a reference to the original location as the instruction operand parameter, or else copying the operand from the original location to a dedicated operand parameter storage location.
16. The method of claim 13 , comprising providing a plurality of abstract registers representing registers of a subject processor, and supplementing the abstract registers with one or more additional abstract registers each for storing an operand as an instruction operand parameter.
17. A computer-readable recording medium containing program code constructions for performing the method of claim 1 .
18. A method for emulating condition code flags during program code conversion, comprising:
selecting a sequence of subject code instructions;
identifying a flag-affecting instruction in the sequence of subject code instructions, the flag-affecting instruction specifying an operation that selectively affects subject condition code flags of a subject processor;
recording parameters of the identified flag-affecting instruction including an instruction type parameter and one or more operand parameters;
detecting a flag-usage instruction in the sequence of subject code instructions, wherein the flag-usage instruction uses at least one of the subject condition code flags, the flag-usage instruction specifying an operation with reference to a flag status of one or more of the subject condition code flags; and
in response to detecting a flag-usage instruction, resolving a flag status of one or more of the subject condition code flags with respect to the recorded parameters.
19. The method of claim 18 , comprising updating the recorded parameters when a new flag-affecting instruction is encountered in the sequence of subject code instructions.
20. The method of claim 18 , comprising recording parameters for a plurality of flag-affecting instructions.
21. The method of claim 20 , wherein the plurality of flag-affecting instructions are each associated with differing sets of subject condition code flags.
22. The method of claim 18 , comprising resolving flag status by at least one of (a) explicitly calculating a status for one or more subject condition code flags by emulating an effect of the recorded flag-affecting instruction, or (b) implicitly representing the flag-usage instruction using the recorded parameters, or (c) selecting between options (a) and (b) according to a type of the flag-usage instruction.
23. The method of claim 18 , wherein the recorded parameters include at least one instruction operand parameter, and wherein recording the instruction operand parameter comprises at least one of (a) copying an operand to a predetermined storage location, or (b) storing a reference to an original location containing the operand, or (c) selectively performing (a) or (b).
24. A computer-readable recording medium containing program code constructions for performing the method of claim 18 .
25. An emulator apparatus for use in a target computing environment for emulating a subject processor of a subject computing environment when translating subject code appropriate to the subject computing environment to produce target code appropriate to the target computing environment, the emulator apparatus comprising:
means for identifying a flag-affecting instruction in a sequence of subject code instructions, wherein the flag-affecting instruction selectively modifies a flag;
an instruction parameter store for recording parameters of the identified flag-affecting instruction;
means for detecting a flag-usage instruction in the sequence of subject code instructions, wherein the flag-usage instruction uses the flag; and
means for resolving, in response to detecting the flag-usage instruction, a status of the flag with respect to the instruction parameters recorded in the instruction parameter store.
26. A computing platform comprising the emulator apparatus of claim 25 .