[cfe-dev] Setting breakpoints before assignments or calls

[Vedant Kumar via cfe-dev]

Hello,

I'd like to improve the situation with setting breakpoints on lines with assignments or inlinable calls. This email outlines problem areas, possible solutions, and why I think emitting extra nops at -O0 might be the best solution.

# Problem 1: Assignments

Counter to user expectation, a breakpoint on a line containing an assignment is reached when the assignment happens, not before the r.h.s is evaluated.

## Example: Can't step into bar()

  1| foo = // Set a breakpoint here. Note that it's not possible to step into bar().
  2|   bar();

One solution is to set the location of the assignment to the location of the r.h.s (line 2). The problem with this approach is that it becomes impossible to set a breakpoint on line 1.

Another solution is to emit a nop (on line 1) prior to emitting the r.h.s, and to emit an artificial location on the assignment's store instruction. This makes it possible to step to line 1 before line 2, and prevents users from stepping back to line 1 after line 2.

# Problem 2: Inlinable calls

Instructions from an inlined function don't have debug locations within the caller. This can make it impossible to set a breakpoint on a line that contains a call to an inlined function.

## Example: Can't set a breakpoint on a call

It's easier to see the bug via Godbolt: https://godbolt.org/z/scwF20 <https://godbolt.org/z/scwF20>. Note that it's not possible to set a breakpoint on line 9 (on "inline_me"). At the IR-level, we do emit an unconditional branch with a location that's on line 9, but we have to drop that branch during ISel.

The only solution to this problem (afaik) is to insert a nop before inlinable calls. In this example the nop would be on line 9.

One alternative I've heard is to make the first inlined instruction look like it's located within the caller, but that actually introduces a bug. You wouldn't be able to set a breakpoint on the relevant location in the inlined callee.

# Proposal

As outlined above, I think the best way to address issues with setting breakpoints on assignments and calls is to insert nops with suitable locations at -O0. These nops would lower to a target-specific nop at -O0, and lower to nothing at -O1 or greater (like @llvm.donothing).

The tentative plan is to introduce an intrinsic (say, @llvm.dbg.nop) to accomplish this.

I don't anticipate there being a substantial compile-time impact, but haven't actually measured this yet. I'd like to get some feedback before going forward. Let me know what you think!

thanks,
vedant


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