[llvm-dev] [LLVMdev] Improving the quality of debug locations / DbgValueHistoryCalculator

Vikram TV via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 12 08:11:42 PDT 2015

Hi all,

An early implementation of extending debug ranges and providing multiple
location support is done here: http://reviews.llvm.org/D11933

Design document:

On Jun 24, 2015, at 12:12 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:

Hi Adrian,

You might want to take a look at abandoned http://reviews.llvm.org/D2658
where I tried to implement something similar.
Looks like we're now at the point where we *do* require complicated
solution and analysis in DbgValueHistoryCalculator...

On Tue, Jun 23, 2015 at 4:41 PM, Adrian Prantl <aprantl at apple.com> wrote:

> Here is a proposal for improving DbgValueHistoryCalculator and the
> overall quality of debug locations.
> Focus: This is about lowering the DBG_VALUE machine instructions to
> DWARF location lists.
> Non-focus: This is not about (typical -O0) variables that permanently
> reside at a frame index and are described with dbg.declare intrinsics
> in the IR. These variables are stored in the MMI side-table and
> processed separately.
> The semantics of DBG_VALUE
> ==========================
> Examples:
>         DBG_VALUE %EAX, %noreg, !"a", <<0x7fd4bbc17470>>; line no:3
>         DBG_VALUE %RBX, 56, !"a", <<0x7ffd5ac3b3c0>>; line no:4 indirect
>         DBG_VALUE 0, 0, !"a", <<0x7fd4bbc17470>>; line no:3
> The DBG_VALUE machine instruction informs us that a variable (or a
> piece of it) is henceforth stored in a specific location or has a
> constant value. This is valid until the next DBG_VALUE instruction
> describing the same variable or the location is being clobbered.
> Lowering today
> ==============
> 1. DbgValueHistoryCalculator takes the MachineFunction graph and
>    produces a list of live ranges for each variable (the
>    DbgValueHistoryMap).
>    Note: Variable live ranges are to be consumed by a debugger. They
>    refer to the physical addresses and are agnostic of control flow.
>    The live ranges produced by DbgValueHistoryCalculator are pairs of
>    MachineInstructions, the first of which is always the DBG_VALUE
>    describing the variable and location.
>    The ranges are calculated as follows:
>    - If the location is a register the range extends until the
>      register is clobbered or until the end of the basic block.
>    - If the location is a constant the range extends until the next
>      DBG_VALUE instruction describing the same variable.
>    - If the location is indirect and the register is not clobbered
>      outside the function prologue and epilogue the the range is the
>      entire function. This is a heuristic to make stack
>      frame-allocated variables work better. Otherwise the range
>      extends until the next DBG_VALUE or the end of the basic block.
> 2. The buildLocationList() function takes the list of ranges of one
>    variable and builds a location list. A variable may consist of many
>    pieces which have their own live ranges. A live ranges for a piece
>    is split up so that no piece's live range starts or ends in the
>    middle of another piece's live range.  Any two consecutive ranges
>    with identical location contents are merged.  Labels are requested
>    for start and end of each range.
> 3. The location list is finalized by lowering it into DWARF and
>    emitted into a buffer.
> Shortcomings with the current apporach
> ======================================
> Problems with the current approach include inaccurate live ranges for
> constant values, ranges ending too early (usually at basic block
> boundaries), poor handling of frame-register-indirect variables that
> were introduced by spill code, poor handling of variables that are in
> more than one location at once.
> A better DbgValueHistoryCalculator
> ==================================
> Currently DbgValueHistoryCalculator does a very simple, linear pass
> through all MachineInstructions, with a couple of heuristics to make
> common cases such as the frame index variables work.
> It would be possible to address many of the current problems by having
> earlier passes emit many more DBG_VALUE instructions. There are
> several problem with this approach though: It distributes the
> complexity across many more passes and thus imposes a maintenance
> burden on authors of prior passes, increases machine code size, and
> stores a lot of redundant information.
> What I'm proposing instead is to make DbgValueHistoryCalculator
> smarter at creating ranges. The goal is to make hack^H^H^Heuristics such as
> the frame index handling unnecessary by correctly propagating DBG_VALUE
> liveness across basic block boundaries.
> To illustrate what I mean I'll use a notation where
> DbgValueHistoryCalculator is defined in terms of a data-flow analysis.
> First a couple of data types used in the following pseudo code:
> - A range is a pair of MachineInstructions (start, end) that both belong
>   to the same basic block.
> - range[var] is the final list of live ranges for a variable.

How come you never remove ranges from range[var]? What if you have smth.

  DBG_VALUE %RAX, %noreg, !"a"
  jmp BB2
  // more minsns
  clobber %RAX
  clobber %RAX
  DBG_VALUE %RBX, %noreg, !"a"
  jmp BB2

Looks like you would handle BB1, then BB2, and create a range for "a" from
the beginning of BB2 to clobber instruction.
But this might be incorrect - if we have jumped to BB2 from BB3, then the
location of "a" is anything but %RAX.

To guarantee termination new ranges are only committed to ranges[var] when
we know they are valid on all paths.
BB1 is visited first and we record a range for a from the DBG_VALUE to the
end of BB1:
  ranges[a].push_back( (DBG_VALUE, BB2.first_insn) )
(There’s a bug in the pseudo-code doesn’t set the end of the range
correctly, but the comment conveys the intention.)
When BB2 is visited the data-flow analysis magic kicks in via the join()
function, which I (as Paul pointed out) left out from the pseudo-code.
Sorry for the extra confusion. Next time I write up such a proposal I
should just implement it so I can make sure that all the pieces are
actually there and I don’t just imagine them :-)
The first time BB2 is visited, we only have the information from BB1, so
join(BB2, /*pred1*/ BB1, /*pred2*/  BB3) returns an empty set. After BB3 is
visited join() will still come back with an empty set because the
intersection of the location for “a” coming from BB1 and BB3 is different.

I think you should respect the control flow somehow, and consider locations
of "a" at the beginning of basic block BB only
if you are certain that locations of "a" at the end of all BB's
predecessors don't contradict.

It does take the control flow into account, but I left out the important
part from the pseudo-code and replaced it with a hand-wavy statement about
it being a data-flow analysis. Sorry about that!

- open_ranges[BB][var] is the list of not-yet-terminated ranges for
  var inside the current basic block BB.
> - outgoing_loc[BB][var] is the list of locations for var valid at
>   the end of a basic block BB.
> // This code is probably buggy because I didn't run it through a
> // compiler yet, but I hope it serves to illustrate my point.
> // Visit a machine instruction.

I'm kind of concerned about the runtime cost of this. Looks like this can
be at least
O(number_of_minsn * number_of_local_variables), which is already
significant. And it seems
that you can iterate over the same basic block several times.

Yes, we’ll have to benchmark it very carefully before deciding that the
extra complexity is worth it.

thanks for all the feedback
-- adrian

> transfer(MInsn) {
>   // A DBG_VALUE marks the beginning of a new range.
>   if (MInsn is a DBG_VALUE(var, loc)) {
>     for ((rvar, start) : open_ranges[BB])
>       // A DBG_VALUE terminates a range started by a previous
>       // DBG_VALUE for the same variable, if the described pieces
>       // overlap.
>       if (var == rvar && piece_overlaps(MInsn, start)) {
>         ranges[var].push_back((start, MInsn));
>         open_ranges[BB][rvar].remove(start));
>       }
>     open_ranges[BB][var].push_back(MInsn)
>   }
>   // A def of a register may mark the end of a range.
>   if (MInsn is a def(reg)) {
>     for ((var, start) : open_ranges[BB])
>       if (start.loc == reg) {
>         ranges[var].push_back((start, MInsn));
>         open_ranges[BB][var].remove(start));
>       }
>   // End all ranges in the current basic block.
>   if (MInsn.isTerminator())
>     for (range : open_ranges[BB]) {
>       ranges[var].push_back(range);
>       open_ranges[BB].remove(range);
>       outgoing_loc[BB][range.var].push_back(range.loc);
>       changed = true;
>    }
> }
> // Visit the beginning of a basic block BB with two predecessors.
> join(BB, BB1, BB2) {
>   for ((var1, loc1) : outgoing_loc[BB1])
>     for (loc2 : outgoing_loc[BB2][var1])
>       // It's only safe to propagate a range if all predecessors end
>       // with the same location.
>       if (loc1.kind == loc2.kind &&
>           loc1.val  == loc2.val)
>         // Not sure how to best implement this.
>         open_ranges[BB][var].push_back(new DBG_VALUE(loc1));

Yep, that's kind of problem, we can't modify MachineFunction at this point.

> }
> analyze(MF) {
>   for (BB : MF)
>     workset.push_back(BB);
>   while (!workset.empty()) {
>     changed = false;
>     for (MI : workset.pop_front())
>       transfer(MI)
>     if (changed)
>       workset.append(BB.successors())
>    }
> }
> A couple of observations about the pseudo-code above: The analysis
> terminates, because ranges is write-only, making this effectively a
> bit-vector problem. It is also safe, because we are only propagating
> ranges into the next basic block if all predecessors have the same
> outgoing location for a variable piece.
> One problem that is not addressed by the approach above is how to
> become better at handling variables that are in more than one location
> at once: As Keno noted on llvm-commits recently, the fundamental
> problem is that DbgValueHistoryCalculator cannot safely distinguish
> between a DBG_VALUE that provides an additional valid location and one
> describing an updated location for a variable. IMO this is best
> addressed on a case-by-case basis in the pass that introduces the second
> DBG_VALUE, either by marking it as alternative location or not emitting
> it at all, but I’m open for suggestions.
> Comments?
> -- adrian
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Alexey Samsonov
vonosmas at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150812/7de7d3b2/attachment-0001.html>

More information about the llvm-dev mailing list