[llvm-dev] lld symbol choice for symbol present in both a shared and a static library, with and without LTO

[Rui Ueyama via llvm-dev]

On Sat, Jun 15, 2019 at 4:58 AM Eli Friedman via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I filed https://bugs.llvm.org/show_bug.cgi?id=42273 last night, about an
> inconsistency between LTO and non-LTO workflows.
>
>
>
> The basic scenario is that we have an object file which calls a function
> “foo”, a static library that provides an implementation of “foo”, and a
> shared library that also provides an implementation of “foo”.  Currently,
> whether lld chooses the symbol from the static library or the shared
> library depends on the order the files are specified on the command-line.
> For “obj.o static.a shared.so”, or “static.a obj.o shared.so”, lld chooses
> the symbol from the static library. For any other order, it chooses the
> symbol from the shared library.  Is this the expected behavior?  (As far as
> I can tell, this matches binutils ld except for the “static.a obj.o
> shared.so” case.)
>

This is what I expected. When lld visits an object file A and find an
undefined symbol, and there's a file B that appears before the object file
in the command line that defines the symbol, then B gets linked. If there's
more than one file that define the symbol, the leftmost one is chosen.

If “obj.o” is built with LTO enabled, and the function is specifically a
> runtime function, the behavior is different.  For example, suppose the IR
> contains a call to “llvm.memcpy”, and the generated code eventually calls
> “memcpy”.  Or suppose the IR contains a “resume” instruction, and the
> generated code eventually calls “_Unwind_Resume”.  In this case, the choice
> is different: lld always chooses the “memcpy” or “_Unwind_Resume” from the
> shared library, ignoring the order the files are specified on the
> command-line.  Is this the expected behavior?
>

That's not expected, but I suspect that that only occurs when you use a
builtin function like memcpy. Does this happen when you define some random
function like "foo"?
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