Debugging system

A non-transitory data carrier carrying processor control code configured to implement a method of returning to a state in the history of execution of a computer program, said state comprising a set of values of one or more of registers of a processor on which the computer program is running and of working memory space to which the computer program has access, the method comprising:

• identifying, in machine code representing said computer program, instances of machine code instructions associated with substantially non-deterministic events;
• generating a first instrumented version of said program machine code instructions to handle said substantially non-deterministic events, the first instrumented version comprising first instrumented machine code;
• executing said first instrumented machine code;
• storing a time series of said states, including a log of said non-deterministic events during a reference execution such that the program machine code can be rerun in a deterministic manner to reproduce states of the processor and the memory during the rerunning, wherein the log of non-deterministic events is captured during the reference execution comprises capturing a succession of snapshots of program state information at successive times during running of the reference execution of the computer program or during deterministic rerunning of the reference execution, the non-deterministic events include system calls, RDTSC (Read Time-Stamp counter) instructions, thread-switches and reads from a shared memory;
• restoring said stored state;
• executing said first instrumented machine code forward in time starting at said restored state to return to said state in said program history of execution;
• inputting data defining program analysis data to be performed on said program machine code;
• performing a reverse search on the program analysis data, backwards from a debug point in the computer program, to identify a defined condition in the program analysis data, wherein the reverse search comprises running the first instrumented machine code forward one or more times to reproduce the reference execution and to identify a most recent time at which the defined condition is met prior to the debug point;
• generate a second instrumented version of said program machine code comprising second instrumented machine code to replay execution of said machine code representing the computer program and to capture and store program state information during said replayed execution, wherein said program state information comprises one or both of one or more values of registers of said processor and one or more values of memory locations used by said computer program, wherein the first instrumented machine code and the second instrumented machine code are different;
• run said second instrumented machine code whilst reproducing said non-deterministic events during said running to reproduce said reference execution of said computer program;
• store program analysis data generated by said second instrumented machine code when said second instrumented machine code is run.