578157 - Single-stepping should work efficiently under JaegerMonkey

Status: RESOLVED DUPLICATE of bug 610793

(Core :: JavaScript Engine, defect)

Description

[Jim Blandy :jimb]

It should be possible for a debugger to mix single-stepping through JaegerMonkey-compiled code and continuing while still retaining good performance.

adrake suggests introducing a new JSTRAP_ value, JSTRAP_STEP, indicating that the program should continue a single step.  JM can recompile the code placing traps at points of interest:

- any jump to a different statement (regardless of source line)
- *before* any function call
- *before* any return
- *before* any throw
- *before* any eval

It should also be possible to single-step across calls, returns, and throws.

If the statement contains a function call, we should stop immediately before the function call; and the next step should bring us into the function call, with arguments in place.

If the statement contains (is) a return, we should stop immediately before returning, at a point where we can still see local variables and such. The next step should bring us into the caller, with the return value available.

Eval should behave like any other function call.

Comment 1

[Andrew Drake [:adrake]]

Proposed implementation is to recompile adding traplikes on JSTRAP_STEP, and recompile removing them on JSTRAP_CONTINUE or function scope exit.

Comment 2

[Andrew Drake [:adrake]]

It looks like the way to do this currently is preserve SetInterrupt semantics, since it's used for single stepping. We may want to bump this along with associated semantic differences to debug API v2.

Comment 3

[David Mandelin [:dmandelin]]

I'm gonna merge this into the new single-step bug.