[PATCH] D38088: Fix out-of-order stepping behavior in programs with hoisted constants.

Wed Oct 25 14:59:10 PDT 2017

I talked to Matthew this morning about this debate.  He will update the patch to use the merge API and repost it.
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Tuesday, October 24, 2017 10:01 AM
To: Adrian Prantl
Cc: Robinson, Paul; reviews+D38088+public+4bc8f3de9212eeb7 at reviews.llvm.org; Voss, Matthew; craig.topper at gmail.com; echristo at gmail.com; llvm-commits at lists.llvm.org
Subject: Re: [PATCH] D38088: Fix out-of-order stepping behavior in programs with hoisted constants.

On Tue, Oct 24, 2017 at 9:52 AM Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>> wrote:

On Oct 24, 2017, at 9:23 AM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:

On Tue, Oct 24, 2017 at 9:20 AM Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>> wrote:

On Oct 24, 2017, at 9:12 AM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:

On Tue, Oct 24, 2017 at 9:08 AM Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>> wrote:

On Oct 24, 2017, at 9:05 AM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:

It merges as well as hoisting

Ok. But re-reading the motivation of the patch, it looks like merging the locations (let's assume they are identical) would not actually solve the issue the hoisted constant's location being unexpected, right?
Were you suggesting to merge the location of the constant with the location of the InsertPoint?

Yeah, I'm really not sure how to address it - if the constant is used in two different basic blocks but has the same location in both, but the constant is hoisted into a third basic block? We probably want to null out the location in that case (so I'm not sure merging it with the insertion point is ideal - letting the insertion point's location flow into this instruction if that's how it shakes out in the backend is fine, but actually specifying that location seems a bit off - if the instruction gets moved around some more, etc)

Yes. Based on what the patch is trying to do the current implementation seems right.

I guess the outstanding question I have is: is it worth trying to merge the location in the cases where it isn't hoisted into some other basic block? Any thoughts/strong feelings? Otherwise I'll probably just accept it as-is, even if the constant's used in two places in a single basic block and could have its location preserved witohut tainting profiling data.

Ah I see. The distinction between it being hoisted into some other basic block is relevant for PGO because it only confuses PGO when the constant is moved to a different basic block. Yes, it would be better to merge to locations in that case, since it doesn't affect the PGO workflow or stepping in the debugger negatively, so we shouldn't throw away the location.

Well it can/does still make for weird stepping behavior, I think - I think the constant can get hoisted to the start of the basic block, not just the first use (can anyone confirm this? If it only gets hoisted to the first use then, within a basic block, merging the locations would be fine - since you were already going to step there before this transformation) - so it could move far from any of the constant's uses.

- Dave

-- adrian

In the long term I don't think we should be erasing accurate (but confusing) source locations, and instead (for example) be more principled about what we mark with is_stmt in the linetable, so we could have multiple classes of line table entries and a debugger may choose to ignore the more accurate ones if it messes with stepping (but have them available for crashes/breakpoints/etc). I'm not sure if this also applies exactly the same way to profilers, but in the worst case we could add another flag to the line table to mark is_interesting_profiler_location.

-- adrian

-- adrian

On Tue, Oct 24, 2017 at 9:00 AM Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>> wrote:

> On Oct 24, 2017, at 8:48 AM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:
>
>
>
> On Tue, Sep 26, 2017 at 2:05 PM Robinson, Paul <paul.robinson at sony.com<mailto:paul.robinson at sony.com>> wrote:
> Doesn't need to be merged with the insertion point, I don't think? Does this merge across basic blocks? or just raise the uses of a constant to the beginning of a single basic block?
>
>
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> I read the pass as looking for a dominating insertion point across multiple basic blocks.  Doesn't mean the insertion point necessarily is in a different block, but I *think* it could be.
>
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> Yeah, still seems like it might be nice to keep the location if it doesn't cross a basic block boundary.
>
> Adrian - any ideas/thoughts on all this?

Looks like I missed the original review... Does Constant Hoisting just move a single constant to the top, or does it merge multiple identical constants (while hoisting them)? In the latter case getMergedLocation should definitely be used. In the former case I'm not so sure if erasing the location is the right thing to do in the first place (but it probably doesn't hurt much in practice). So I guess my question is: What are you suggesting should the location be merged with?

-- adrian
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