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

[Francois Pichet via llvm-dev]

Good point.

Currently yes a DEBUG_VALUE "x", vreg0 will be added in BB2. Now I realize
this might be wrong in some (corner?) cases where vreg0 no longer refer to
"x"

My fix would be to propagate the DEBUG_VALUE only if "x" is associated with
only a single virtual register.
BTW, my goal is to generally improve DEBUG_VALUE for optimized code, not
make it 100% correct.


On Wed, May 11, 2016 at 4:43 PM, Adrian Prantl <aprantl at apple.com> wrote:

>
> > On May 11, 2016, at 1:12 PM, Francois Pichet <pichet2000 at gmail.com>
> wrote:
> >
> > Hello,
> >
> > Regarding the problem of debug range for optimized code.
> > Currently a DEBUG_VALUE will be inserted after the <def>vregX
> > DEBUG_VALUE are only valid until the end of the current
> MachineBasicBlock. That's the main problem.
> > Why not simply iterate over all uses of vregX and insert an DEBUG_VALUE
> in all the MachineBasicBlocks where vregX is used. (pre regalloc)
> >
> > I prototyped a small pass to do that and at first it seems to improve
> .debug_loc range validity and enhance the debugging experience for
> optimized code.
>
> The problem that I see with this approach is that DEBUG_VALUEs are only
> valid until the next DEBUG_VALUE that describes the same variable. How does
> your pass handle:
>
>   BB0:
>     DEBUG_VALUE “x”, vreg0
>
>   | |
>   | BB1:
>   |  DEBUG_VALUE “x”, vreg1
>   | |
>   \_____BB2:
>     |    |
>     |____/
>   BB2:
>     vreg0 // still not clobbered here.
>
> Does it insert a DEBUG_VALUE “x”, vreg0 into BB2?
>
> -- adrian
>
>
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