[LLVMdev] Status of YAML IO?

Shankar Easwaran
Tue, 30 Oct 2012 22:48:51 -0000

Hi Nick,

Thanks for your reply.

How about if the atom flags could be overridden ? The Atom flag could have a
MIN/MAX and anything above the MAX or lower than the MIN are platform specific,
like how its dealt with section indexes ?

> I know ELF file format has some ranges for various values that are
specifically reserved for processors or "user" defined functionality.
It serves the needs of ELF well.  It allows processor and software tools teams
to use ELF but work independently (and/or in secret) on new functionality
without needed to coordinate with a central ELF owner.
I am not sure, if we will run out of possible enumerations for the
individual atom flags across the many platforms :)
>
> But lld is different. It is not a file format.  It is an API. If a
particular processor needs to express something not captured in the Atom model,
we should discuss what that functionality is and see if we can grow the Atom
model.  There may well be another processor that needs some similar
functionality.    If we added a generic uint32_t DefinedAtom::flags() method, I
would be concerned that lld porters would be quick to just use the bits for
whatever they need and not see if the Atom model needs expanding.
>
> An example of something I added (but am not happy with) is
DefinedAtom::isThumb().  This is something only applicable to ARM (and only if
you care about interop of thumb and arm code).
>
> Given that the Reference::Kind field is already platform specific, I'm
leaning towards saying that the way to add platform specific atom attributes is
to add a Reference with no target to the Atom with a Kind field that for that
platform means whatever attribute you need.
To get the Atom flags, I felt the interface is

a) more cumbersome
b) leads to slow performance
>>>> 5) are you planning to support dwarf information too ?
>>> Debugging information is another big open question.  The dwarf
format is very much tied to the section model.  Not only is the debug
information put is sections with special names, but the dwarf debug into
references code by its address in the .o files (the Atom model does not model
addresses).    I'm sure the lldb guys have some ideas on direction of where
they would like debug information to go.  It may be that the Atom model has a
different representation for debug info.  And when generating a final linked
image you can choose the debug format you want.  A Writer could convert the
debug info to dwarf if requested.
>> Wouldnt it be hard to get the source / line information right if the
linker tries to write the debug information ?
> Just as hard as reading and writing dwarf debug information in general ;-)
>
> Let me also mention why the debug information is not an issue for
MacOS/iOS.  Dwarf is designed to work with "dumb" linkers or
"smart" linkers.  A dumb linker just copies all the dwarf sections
from all input files to the output file, and applies any relocations.  This is
simple, but the resulting dwarf is huge with tons of "dead" dwarf in
it (because of coalescing by the linker).  A smart linker knows how to parse up
dwarf and optimize the combining of sections.  The resulting dwarf is much
smaller, but it takes a lot of computation to do the merge.
>
> When we (Apple/darwin) switched from stabs to dwarf years ago, we decided
to take a different approach. We realized a dumb linker would be slow because of
all the I/O copying dwarf.  A smart linker would be slow because of all the
computation needed.  So, instead the darwin linker just ignores all dwarf in .o
files!  Instead it writes "debug notes" to the final linked image that
lists the paths to all the .o files used to create the image.  This approach
makes linking fast.  Next, if you happen to run the program in the debugger, the
debugger would see the debug notes and go read the .o files' dwarf
information.  Lastly, if you are making a release build, you run a tool called
dsymutil on the final linked image.  dsymutil finds the debug notes, parses the
.o files' dwarf information then does all the computation to produce an
optimal dwarf output file (we use a .dSYM extension).  Later, if you need to
debug a release build, you point the debugger at the .dSYM file.
This functionality was prevalent in other linkers like solaris ld, and
hpux too.
>
> Perhaps the initial approach you should take for ELF is to go the dumb
linker route.  Have the ELF reader produce on Atom for each dwarf section with
all the fixups/References needed.  Then the ELF Writer will just concatenate
those sections into the output file, and apply the fixups.
Not sure, if the debuggers are going to understand the merged dwarf
sections. The debugger may still need the optimized version. What do you
think ?

Thanks

Shankar Easwaran

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