llvm dev - May 2016 - [LLVMdev] Improving the quality of debug locations / DbgValueHistoryCalculator

> 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

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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> On May 11, 2016, at 2:09 PM, Francois Pichet <pichet2000 at
gmail.com> wrote:
> 
> 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.
I hold the (perhaps somewhat extreme) position that having debug info the *may*
be correct is worse than having no debug info at all, because it means that — in
the end — you then cannot trust *anything* reported by the debugger. (There are
some debatable corner cases, for example, in contrast to the debugger, the user
may know over which path the control flow arrived at the current break point,
but these are far in between.)

That said, I’m very open to improvements to the quality and correctness, so
please don’t feel discouraged :-)

-- adrian
> 
> On Wed, May 11, 2016 at 4:43 PM, Adrian Prantl <aprantl at apple.com
<mailto:aprantl at apple.com>> wrote:
> 
> > On May 11, 2016, at 1:12 PM, Francois Pichet <pichet2000 at
gmail.com <mailto: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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