How debuggers deal with out-of-order execution and branch prediction-CodePudding

I know that modern CPUs do OoO execution and got advanced branch predictors that may fail, how does the debugger deal with that? So, if the cpu fails in predicting a branch how does the debugger know that? I don't know if debuggers execute instructions in a simulated environment or something.

CodePudding user response：

Debuggers don't have to deal with it; those effects aren't architecturally visible, so everything (including debug breakpoints triggering) happens as-if instructions had executed one at a time, in program order. Anything else would break single-threaded code; they don't just arbitrary shuffle your program!

CPUs support precise exceptions so they can always recover the correct consistent state whenever they hit a breakpoint or unintended fault.

If you want to know how often the CPU mispredicted branches, you need to use its own hardware performance counters that can see and record the internals of execution. (Software programs these counters, and can later read back the count, or have it record an event or fire an interrupt when the counter overflows.) e.g. Linux perf stat counts branches and branch-misses by default.

(On Skylake for example, that generic event probably maps to br_misp_retired.all_branches which counts how many branch instructions eventually retired that were at one point mispredicted. So it doesn't count when the CPU detected mis-prediction of a branch that was itself only reached in the shadow of some other misprediction, either of a branch or a fault. Because such a branch wouldn't make it to retirement. Events like int_misc.clear_resteer_cycles or int_misc.recovery_cycles can count lost cycles for the front-end due to such things.)

For more about OoO exec, see

Out-of-order execution vs. speculative execution (including in the context of the Meltdown vulnerability, which suddenly made a lot more people care about the details of OoO exec). A modern OoO exec CPU treats everything as speculative until it reached retirement (which happens in program order to support precise exceptions.)
Difference between In-oder and Out-of-order execution in ARM architecture
Why memory reordering is not a problem on single core/processor machines? OoO exec preserves the illusion (for the local core) of instructions running in program order.