[lldb-dev] Parsing Line Table to determine function prologue?

[Zachary Turner via lldb-dev]

I see.  It's not the end of the world because I can just parse the whole
line table when requested.  It's just that in PDB-land the format is such
that a) I know the exact address of the prologue and epilogue at the time I
parse the function record, and b) When parsing the line table, I can
quickly scan to the address range for the function making the whole table
parsing less efficient than necessary.  But it's definitely sufficient.

On Mon, Oct 8, 2018 at 12:41 PM Jim Ingham <jingham at apple.com> wrote:

> A single sequence in the line table needs to be run from beginning to end
> to make sense of it.  It doesn't really have addresses, it generally has a
> start address, then a sequence of "increment line, increment address"
> instructions.  So you have to run the state machine to figure out what the
> addresses are.
>
> However, the line table does not have to be one continuous sequence.  The
> DWARF docs state this explicitly, and there is an "end_sequence"
> instruction to implement this.  I can't see any reason why you couldn't get
> the compiler to emit line tables in per-function sequences, and have the
> debugger optimize reading the line table by first scanning for sequence
> ends to get the map of chunks -> addresses, and then reading the line table
> in those chunks.  I don't think anybody does this, however.  clang emitted
> the whole CU as one sequence in the few examples I had sitting around.
>
> Jim
>
>
> > On Oct 8, 2018, at 12:28 PM, Leonard Mosescu via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > Even if we do need to parse the line table, could it be done just for
> the function in question?  The debug info tells us the function's address
> range, so is there some technical reason why it couldn't parse the line
> table only for the given address range?
> >
> > My understanding is that there's one DWARF .debug_line "program" per CU,
> and normally you'd need to "execute" the whole line number program.
> >
> > On Sat, Oct 6, 2018 at 8:05 PM, Zachary Turner via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> > While implementing native PDB support I noticed that LLDB is asking to
> parse an entire compile unit's line table in order to determine if 1
> address is a function prologue or epilogue.
> >
> > Is this necessary in DWARF-land?  It would be nice if I could just pass
> the prologue and epilogue byte size directly to the constructor of the
> lldb_private::Function object when I construct it.
> >
> > It seems unnecessary to parse the entire line table just to set a
> breakpoint by function name, but this is what ends up happening.
> >
> > Even if we do need to parse the line table, could it be done just for
> the function in question?  The debug info tells us the function's address
> range, so is there some technical reason why it couldn't parse the line
> table only for the given address range?
> >
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