[PATCH] D88405: [LiveDebugValues][InstrRef][1/2] Try harder to recover clobbered variable locations

[Jeremy Morse via Phabricator via llvm-commits]

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Hi,

Here's a patch that improves variable location PC-range coverage when using the instruction-referencing LiveDebugValues implementation. I've mostly been focusing on ensuring there are no regressions in variable availability (i.e., the number of variables that have at least one location), and wasn't really looking at the 'PC Ranges covered' output of llvm-locstats. This patch rectifies some of that to give a modest improvement in covered PC ranges -- however the real benefits come with entry values (see next patch, an additional 5% PC ranges).

In various circumstances, when we clobber a register there may be alternative locations that the value is live in. The classic example would be a value loaded from the stack, and then clobbered: the value is still available on the stack. InstrRefBasedLDV was coping with this at block starts where it's forced to pick a location; however it wasn't searching for alternative locations when values were clobbered. This patch notifies the "TransferTracker" object when clobbers occur, and it's able to find alternatives and issue DBG_VALUEs for the that location. See: the added test.

More generally, it's pretty clear that this should be done by something like DbgEntityHistoryCalculator instead, which would also be able to avoid redundant location transfers. But that's an improvement for another day.

Here's the output of llvm-locstats when buildling clang-3.4 before and after this patch, when forcing the use of InstrRefBasedLV, for which I've added an -mllvm switch.

Before:

  =================================================
      cov%           samples         percentage(~)
  -------------------------------------------------
    0%               872495               26%
    (0%,10%)          47486                1%
    [10%,20%)         50330                1%
    [20%,30%)         57127                1%
    [30%,40%)         48881                1%
    [40%,50%)         46630                1%
    [50%,60%)         59643                1%
    [60%,70%)         57581                1%
    [70%,80%)         67858                2%
    [80%,90%)         77362                2%
    [90%,100%)       100012                2%
    100%            1866414               55%
  =================================================
  -the number of debug variables processed: 3351819
  -PC ranges covered: 56%
  -------------------------------------------------
  -total availability: 60%
  =================================================

After:

  =================================================
      cov%           samples         percentage(~)
  -------------------------------------------------
    0%               872508               26%
    (0%,10%)          44994                1%
    [10%,20%)         49960                1%
    [20%,30%)         56512                1%
    [30%,40%)         48070                1%
    [40%,50%)         46135                1%
    [50%,60%)         58738                1%
    [60%,70%)         55552                1%
    [70%,80%)         65360                1%
    [80%,90%)         74530                2%
    [90%,100%)        97362                2%
    100%            1882113               56%
  =================================================
  -the number of debug variables processed: 3351834
  -PC ranges covered: 56%
  -------------------------------------------------
  -total availability: 60%
  =================================================

So that's roughly 16,000 more variable locations in the 100% bucket, and other variable locations have shifted up a few buckets too.

This patch isn't based on any of the other instruction referencing work, only the LiveDebugValues implementation in tree. After a few months of prodding it, it could definitely do with a refactor, but I'd like to do that from a position where it's working well. Given that it's "officially" in an experimental state, it's not clear if it needs "full" review, I'll probably take my lead from the child patch that I'll upload in a moment.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D88405

Files:
  llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
  llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.cpp
  llvm/test/DebugInfo/MIR/InstrRef/livedebugvalues_recover_clobbers.mir

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