llvm dev - Dec 2019 - 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

> -----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

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!