[PATCH] D67500: [DebugInfo] LiveDebugValues: don't create transfer records for potentially invalid locations

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added a comment.

Thanks for the writeup, this is very helpful. I am indeed thinking of LiveDebugValues as the first kind of analysis.

One thing that we must define, too, is what `true` and `false` mean in the context of LiveDebugValues. My understanding is that for each DBG_VALUE d in the analyzed MIR function and each instruction i

- true means: "it is correct to insert d at position i"
- false means: "it would be wrong to insert d at position i"

In other words at each instruction i the set of DBG_VALUEs that are "true" are the set of "live" DBG_VALUEs.

As for your loop example: *My* mental model actually used a three-element lattice that is suspiciously close to your second one, but using the opposite direction:

  \top (wrong to place DBG_VALUE = false in the above sense / DBG_VALUE is not in the set)
     |
  true (DBG_VALUE may be placed / DBG_VALUE is in the set)
     |
  \bot (not fully analyzed yet)

We'd initialize everything with \bot.
join is a commutative function that is
join(x, x) = x
join(x, \bot) = x
join(x, \top) = \top

After processing the first basic block once, we get:

  entry: ; {\bot}
    dbg.value(%0, ...) ; {true}
    br label %loop
  loop: ; {\bot}
    call void @nop() ; {\bot}
    br i1 %cond, label %loop, label %exit ; {\bot}
  exit: ; {\bot}
    ret void ; {\bot}

processing the second block once, we get

  entry: ; {\bot}
    dbg.value(%0, ...) ; {true}
    br label %loop
  loop: ; {join(true (entry), \bot (backedge)) = true}
    call void @nop() ; {true}
    br i1 %cond, label %loop, label %exit ; {true}
  exit: ; {\bot}
    ret void ; {\bot}

processing the second block twice, we get

  loop: ; {join(true (entry), true (backedge)) = true}

and then true is propagated to the final block. Every time a value changes all successor blocks are added to the worklist. The join function ensures that values can only move in one direction, from \bot -> true -> \top.

I *think* I just described something isomorphic to your type (2), just using other names.


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  rL LLVM

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