[LLVMdev] debug stoppoints and control flow

[Chris Lattner]

On Wed, 13 Oct 2004, Michael McCracken wrote:
> Hi, I'm just getting back to working on the cfe debug info after a brief hiatus.

Great!

> It appears that the appropriate place to be inserting stoppoints is
> starting in llvm_expand_stmt, using STMT_LINENO(t) . If that's not the
> best place, comments would be appreciated.

I think that makes sense, along with EXPR_WITH_FILE_LOCATION.  You might
also want to only emit a line number if the previously emitted line number
is not the same as the current one.  This would allow us to only emit one
for:

   --x; ++y;

> Using the debug_hooks seems to be a non-starter, because they're
> called during rtl generation, which apparently isn't done when
> EMIT_LLVM is set.

Correct.

> A more pressing question is how to chain the debug intrinsics together
> around control flow.
> The current examples of debug info have no control flow except for
> calls. I started with a simplistic version that chained them in file
> order:
>
> %1 = stoppoint(%0)
> ...
> %2 = stoppoint(%1)
> etc..
>
> However, doing this obviously won't work with control flow, since an
> instruction generated for one side of a branch may get linked to one
> on the other side, resulting in instructions that don't dominate their
> uses.

Yes.  This is equivalent to fully constructing SSA, which you don't want
to do. :)

> I wanted to ask if there was already a good explanation of what to do
> somewhere (or off the top of someone's head) before I came up with one
> on my own.

Basically you need to use the 'alloca trick'.  Basically, memory in LLVM
is not in SSA form, so do something like this:

   %dbg = alloca {}*    ;; In the entry block.

   %tmp = call {}* %llvm.db.func.start(...)
   store {}* %tmp, {}** %dbg

   ...
   %tmp.1 = load {}** %dbg
   %tmp.2 = call {}* %llvm.dbg.stoppoint({}* %tmp.1, ...
   store {}* %tmp.2, {}** %dbg

When the -mem2reg pass is run, it will promote this to SSA form.

-Chris

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