[llvm-dev] is_stmt and column numbers

[Roger Pack via llvm-dev]

On Tue, Dec 17, 2019 at 8:35 AM Robinson, Paul <paul.robinson at sony.com> wrote:
>
>
>
> > -----Original Message-----
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Roger Pack
> > via llvm-dev
> > Sent: Tuesday, December 17, 2019 8:31 AM
> > To: llvm-dev <llvm-dev at lists.llvm.org>
> > Subject: [llvm-dev] is_stmt and column numbers
> >
> > It appears to me that from this:
> >
> > https://iis-git.ee.ethz.ch/H2020-
> > Compiler/llvm/commit/1d68fc5021ca8c704e21f171d98bb18eb396a7fa
> >
> > That LLVM basically emits DWARF expressions where is_stmt is true only
> > "once per line".  Is that still true?  Is there an option to emit more
> > than one per line?
> >
> >
> > Thanks all.
> >
> > -Roger Pack-
> >
> > refs:
> > https://github.com/crystal-lang/crystal/pull/8499
> > https://github.com/crystal-lang/crystal/issues/8319
>
> is_stmt is based fairly strictly on "the line number changed" (while
> ignoring line 0).  This works reasonably well at -O0 and gets clunkier as
> optimization level increases.  I've never been happy about it, but have
> never found the time to come up with something better.  The optimizer has
> been getting better at not introducing gratuitous line-number changes, if
> that helps.
>
> There is no option to make it work differently.  We can (usually do) emit
> column numbers, which gets you more line-table entries, although they are
> not flagged with is_stmt.  I know at least one debugger that gave up on
> is_stmt "because it's not reliable."
> --paulr

OK that makes sense.  I've noticed that there'll be some DWARF
statements that set the line number to "0".

  [0x0000048c]  Special opcode 72: advance Address by 5 to 0x1000015b0
and Line by -3 to 0
  [0x0000048d]  Special opcode 78: advance Address by 5 to 0x1000015b5
and Line by 3 to 3

I assume this means some kind of internal processing?  Anyway the
current work around crystal uses is to basically only accept DWARF
statements that have a positive line number [not sure if that's right
or not...], or if they're is_stmt then it accepts them :)
Thanks!