[cfe-dev] RFC: Remove uninteresting debug locations at -O0

Reid Kleckner via cfe-dev cfe-dev at lists.llvm.org
Wed Apr 29 14:52:17 PDT 2020

Sure, I can try to elaborate, but I am assuming a bit about what the users
desired stepping behavior is. I am assuming in this case that the user is
doing the equivalent of `s` or `n` in gdb, and they want to get from one
semicolon to the next. If that is not the case, if the user wants the
debugger to stop at the location of both getFoo() calls in your example, my
suggestion isn't helpful.

However, since the last dev meeting, I have been thinking about having IR
intrinsics that mark the IR position where a statement begins. The
intrinsic would carry a source location. The next instruction emitted with
that source location would carry the DWARF is_stmt line table flag.

Making this idea work with optimizations is harder, since the instructions
that make up a statement may all be removed or hoisted. To make this work,
we would have to establish which instructions properly belong to the
statement. This could be done by adding a level to the DIScope hierarchy.
The statement markers would remain in place, and code motion would happen
around them. Late in the codegen pipeline, the first instruction belonging
to the most recently activated statement will be emitted with the is_stmt

On Wed, Apr 29, 2020 at 9:36 AM Adrian Prantl <aprantl at apple.com> wrote:

> Thanks to all of you for sharing your perspective! You all brought up
> important aspects that I hadn't considered. The argument that really
> clicked for me was Pavel's that you might want to change the value before
> the load happens.
> One of my worries here is that what I called "less interesting" locations
> might delete more interesting ones when instructions and their locations
> are merged. But if we indeed consider the stack slot loads to be
> interesting then that is really the best we can do. After all, storing more
> than source location per instruction would be a UI design nightmare for a
> debugger.
> Reid, I would like to learn more about what you mean by statement
> tracking. I'm thinking of a statements as the expressions separated by
> semicolons, but since my whole example was a single statement I'm assuming
> you have something more fine-grained in mind. Perhaps you can post an
> example to illustrate what you have in mind?
> thanks!
> -- adrian
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