On 27/04/2024 21:40, Martin Frb via fpc-devel wrote:
Maybe a miss understanding: storing the difference between 2 local
labels would be the asm?
But if (e.g. on Linux) it is cross units then its the linker.
There are 2 consideration here.
1)
First, telling the asm to store ".l99 - .ldebuginfo" or telling the asm
to store 733 (having done the math already) will result in the same
dwarf info.
Of course this only works for local labels.
The big question here is, if it is ok to use labels (potentially many
labels) for this purpose.
Sure. The current DWARF writer also does this.
From the mails at the dwarf list, it seems Joost wanted to avoid this,
but it is not clear why.
Joost then went on, to count the bytes that are added by each bit of
dwarf info written (same as the asm would do), and therefore be able to
write the number directly (or in some cases iirc write it later into the
gap).
That leads to a very big changeset, as the entire dwarf generating code
needs to do counting.
(And IMHO, that is very easy to accidentally break / and if just one
very specific dwarf encoding breaks it, it may be months until found)
There's one case in which you would have to do it manually: if you want
to save those offsets in leb128 encoding. At least on macOS there's no
support for storing the difference between two labels in s/uleb128 in DWARF.
2)
At least on Linux, references for dwarf go across compilation units.
On Windows, if "unit1" has "TForm1 = class(TForm)", that leads to *all*
base classes being encoded into that unit. Almost every unit will
therefore have a copy of TObject (if it is oop). On Linux, only one unit
has this.
I must say, I haven't checked how Joost's code deals with that. (and
mine would probably still need work for that too, but before doing
anything, I am trying to find out what is ok).
Afaik that is why there is: tf_dwarf_only_local_labels in
target_info.flags
Correct.
Jonas
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