[llvm-dev] Linker Option support for ELF

Cary Coutant via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 5 16:35:49 PST 2018

>> In general I'm in favor of the proposal. Defining a generic way to convey
>> some information from the compiler to the linker is useful, and it looks
>> like it is just a historical reason that the ELF lacks the feature at the
>> moment.
>> This is a scenario in which the feature is useful: when you include
>> math.h, a compiler (which is driven by some pragma) could added `-lm` to the
>> note section so that a linker automatically links libm.

I agree that this would be a very useful addition to ELF. I've always
wanted to reach the point where you could just type "ld main.o" and
have all the dependencies automatically linked in. (Go kind of
achieves this, I think.)

I'm not in favor of using yet another note section, however. SHT_NOTE
sections are intended for the use of "off-axis" tools, not for
something the linker would need to look for. I don't want to have the
linker parsing individual note entries looking for notes of interest
to itself, and then having to decide whether to edit those entries out
of the larger section, or merge them together. And I also don't want
to key off of individual section names -- the linker is not supposed
to have to care about the section name. There should be a new section
type for this feature. This is  the kind of extension that ELF was
designed for.

>> I think I'm also in favor of the format, which is essentially runs of
>> null-terminated strings (*1) that are basically opaque to compilers.
> Yes.  However, I think I want to clarify that we want this to be completely
> opaque to the backend.  The front end could possibly have some enhancements
> to make this better.  But, that will be a separate change, and that
> discussion should take place then.  We shouldn’t paint ourselves into a
> corner.  Basically, I think that there is some legitimate concerns here, but
> they would not be handled at this layer, but above.
>> However, you should define as a spec what options are allowed and what
>> their semantics are. We should not accept arbitrary linker options because
>> semantics of some linker options cannot be clearly defined when they appear
>> as embedded options. Just saying "this feature allows you to embed linker
>> options to object files" is too weak as a specification. You need to clearly
>> define a list of options that will be supported by linkers with their clear
>> semantics.
> Personally, I would like to see the ability to add support for additional
> options without having to modify the compiler.  That said, I think that
> there are options which can be scary (e.g. -nopie).  I think that the linker
> should make the decision of what it supports and error out on others.  This
> allows for us to enhance the support over time without a huge overhead.  As
> a starting point, I think that -l and -L are two that would be interesting.
> I can see -u being useful as well, but the point is that we can slowly grow
> the support after consideration by delaying the validation of the options.
>> (*1) One of the big annoyances that I noticed when I was implementing the
>> same feature for COFF is that the COFF's .drctve section that contains
>> linker options have to be tokenized in the same way as the Windows command
>> line does. So it needs to interpret double quotes and backslashes correctly
>> especially when handling space-containing pathnames. This is a design
>> failure that a COFF file contains just a single string instead of runs of
>> strings that have already been tokenized.

I too would like to keep the linker from having to tokenize the
strings. I kind of agree with Rafael that there should be defined tags
and values, much like a .dynamic section, but I wouldn't want to have
values pointing to strings in yet another section, so I'd prefer
something in between that and a free-form null-terminated string. I
also wouldn't want to open up the complete list of linker options, so
I'd prefer a defined list of tags in string form that could easily be
augmented without additional backend support. We could start with,
perhaps, "lib" to inject a library (a la "-l"), "file" to inject an
object file by full name, and "path" to provide a search path (a la
"-L"). I don't think an equivalent for "-u" would be needed, since the
compiler can simply generate an undef symbol for that case. For the
section format, I'd suggest a series of null-terminated strings,
alternating between tags and values, so that no quote or escape
parsing is necessary.

For the header files, a simple syntax like

   #pragma linker_directive "lib" "m"

would provide the extensibility needed to add new tags with no
additional support in the front end or back end.


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