[cfe-dev] Setting breakpoints before assignments or calls

[Vedant Kumar via cfe-dev]

> On Aug 23, 2018, at 1:43 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Thu, Aug 23, 2018 at 1:00 PM Vedant Kumar <vsk at apple.com <mailto:vsk at apple.com>> wrote:
> 
>> On Aug 23, 2018, at 12:22 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> +all the usual debugger folks
>> 
>> I reckon adding nops would be a pretty unfortunate way to go in terms of code size, etc, if we could help it. One alternative that might work and we've tossed it around a bit - is emitting more accurate "is_stmt" flags. In your first example, the is_stmt flag could be set on the bar() call instruction - and any breakpoint set on an instruction that isn't "is_stmt" could instead set a breakpoint on the statement that covers that instruction (the nearest previous is_stmt instruction*)
> 
> I hadn't considered using "is_stmt" at all, thanks for bringing that up!
> 
> Given the rule you've outlined, I'm not sure that the debugger could do a good job in the following scenarios:
> 
> Ah, one thing that might've been confusing is when I say "before" I mean "before in the instruction stream/line table" not "before in the source order".
>  
> 
>   1|  foo =
>   2|    bar();
>   3|  foo =    //< Given a breakpoint on line 3, would the debugger stop on line 2?
>   4|    bar();
> 
> 
> Given the line table (assuming a sort of simple pseudocode):
> 
>   %x1 = call bar(); # line 2
>   store %x1 -> @foo # line 1
>   %x2 = call bar(); # line 4
>   store %x2 -> @foo # line 3
> 
> We could group lines 1 and 2 as part of the same statement and lines 3 and 4 as part of the same statement - then when the line table is emitted, the lexically (in the assembly, not in the source code) first instruction that's part of that statement gets the "is_stmt" flag. So in this case line 2 and 4 would have "is_stmt", a debugger, when requested to set a breakpoint on line 3 would look at the line table and say "well, I have this location on line 3, but it's not the start of a statement/not a good place to break, so I'll back up in the instruction stream (within a basic block - so that might get tricky for conditional operators and the like) until I find an instruction marked with "is_stmt" - and it would find the second call instruction and break there. The debugger could make the UI nicer (rather than showing line 4 when the user requested line 3) by highlighting the whole statement (all lines attributed to instructions between this "is_stmt" instruction and the next (Or the end of the basic block)).
> 
> Does that make sense?

Got it, that makes sense.

vedant

>  
> or
> 
>   1|  if (...)
>   2|    foo();
>   3|  else
>   4|    bar();
>   5|  baz =   //< Given a breakpoint on line 5, would the debugger stop on line 2, 4, or possibly either?
>   6|    func();
> 
> Is there another way to apply and interpret "is_stmt" flags to resolve these sorts of ambiguities?
> 
> 
>> The inlining case seems to me like something that could be fixed without changes to the DWARF, purely in the consumer - the consumer has the info that the call occurs on a certain line and when I ask to break on that line it could break on that first instruction in the inlined subroutine (potentially giving me an artificial view that makes it look like I'm at the call site and leting me 'step' (though a no-op) into the inlined function).
> 
> Oh, that's a great point. Yes, there is a TAG_inlined_subroutine for "inline_me" which contains AT_call_file and AT_call_line. That's enough information to do the right thing on the debugger side.
> 
> thanks,
> vedant
> 
>> 
>> * This is a bit problematic when a statement gets interleaved/mixed up with other statements - DWARF has no equivalent of "ranges" for describing a statement. Likely not a problem at -O0 anyway, though (because little interleaving occurs there).
>> 
>> On Wed, Aug 22, 2018 at 2:01 PM Vedant Kumar via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> 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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