llvm dev - Aug 2013 - [LLVMdev] [lld] -emit-yaml doesnot contain linker added symbols specified with command line options

Hi Nick,
>> The YAML writer does not have anything to add here.
>>
>> The problem can be solved by having the YAML writer append a list of
undefined atoms specified by the -u option, but the problem I have is each
flavor has extra command line options
>> for which it wants to create a DefinedAtom/UndefinedAtom. The flavor
also may want to add extra linker internal files in the future.
>>
>> I prefer addImplicitFiles calling the reader to add more files, which
means addFiles API moves to the reader.
> But won’t that fail too if you were using a YAML Reader and ELF Writer?
Yeah :(
> Since we are talking about files/atoms that are created because of command
line options, perhaps the Driver should be creating the files/atoms.
Currently the Driver has a notion of all the files that it processes are 
all Linker Inputs. InputGraph could have an additional function, to just 
add to it a list of files created implicitly by the linker. The 
UniversalDriver can then look at this list and append the list of files 
to the vector of files and then process the InputFiles.
>   In the case of -u, it would be nice to group them all in one file named
“command line option -u” so that any error messages about undefined symbols says
the references was from “command line option -u”  (as opposed to the -u
UndefinedAtoms come from a generic internal file).
Agree.
>   Also for the case of -u, the Driver can make generic UndefinedAtoms.
ELF would want to treat the undefined symbols as a weak undefined 
symbols. So the driver cant create undefined atoms.

Below is an example :-

$cat 1.c
int _start() { return 0; }
$gcc -c 1.c
$ld -u myundef 1.o
==> Does not throw any error, the resolver was hinted that /myundef /was 
a undefined weak symbol.
>   Other options may need platform specific atoms which may be created with
the help of static Writer methods.
We wouldnt be able to override the functionality with each flavor, if we 
have a static method, or you had thought of something else ?

Thanks

Shankar Easwaran

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation

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On Aug 28, 2013, at 4:24 PM, Shankar Easwaran <shankare at codeaurora.org>
wrote:
> Hi Nick,
> 
>>  Also for the case of -u, the Driver can make generic UndefinedAtoms. 
> ELF would want to treat the undefined symbols as a weak undefined symbols.
So the driver cant create undefined atoms.
> 
> Below is an example :-
> 
> $cat 1.c
> int _start() { return 0; }
> $gcc -c 1.c
> $ld -u myundef 1.o
> ==> Does not throw any error, the resolver was hinted that myundef was a
undefined weak symbol.
Wow.  Reading the gnu ld man page, that is not obvious.  How can you make a
non-weak undefined on the command line?  That is, how can you force and error if
something is not defined?

>>  Other options may need platform specific atoms which may be created
with the help of static Writer methods.
> We wouldnt be able to override the functionality with each flavor, if we
have a static method, or you had thought of something else ?
I’m not sure when we would want polymorphism.  If you wanted it in this case,
there could be a method on LinkingContext to create a file of UndefinedAtoms,
and ELFLinkingContext could override that method to make weak undefines.

-Nick

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Hi Nick,

On 8/28/2013 6:37 PM, Nick Kledzik wrote
> $cat 1.c
> int _start() { return 0; }
> $gcc -c 1.c
> $ld -u myundef 1.o
> ==> Does not throw any error, the resolver was hinted that myundef was a
undefined weak symbol.
> Wow.  Reading the gnu ld man page, that is not obvious.  How can you make a
non-weak undefined on the command line?  That is, how can you force and error if
something is not defined?
I think the linker will barf only if the input file contained an 
undefined symbol and not by any other means.
>>>   Other options may need platform specific atoms which may be
created with the help of static Writer methods.
>> We wouldnt be able to override the functionality with each flavor, if
we have a static method, or you had thought of something else ?
> I’m not sure when we would want polymorphism.  If you wanted it in this
case, there could be a method on LinkingContext to create a file of
UndefinedAtoms, and ELFLinkingContext could override that method to make weak
undefines.
Yes this will work for undefined atoms that need to be supported by all 
flavors. All the implicit files that are needed would be added to a 
vector of files in the inputGraph,
addImplicitFiles would splice/prepend to the list of input files which 
it already has.

Do you think that we still want the Writer::addFiles API, as this 
already covers the functionalities that is needed.

Thanks

Shankar Easwaran
-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the
Linux Foundation