[PATCH] D33680: [ELF] - Resolve references properly when using .symver directive

[George Rimar via llvm-commits]

>>    // It is an error if the specified version is not defined.
>> -  error(toString(File) + ": symbol " + S + " has undefined version " + Verstr);
>> +  if (Config->Shared)
>> +    error(toString(File) + ": symbol " + S + " has undefined version " +
>> +          Verstr);
>
>Why do you needed to add "if (Config->Shared)"? The patch looks good
>otherwise, but I would like to understand this part.
>
>Cheers,
>Rafael

Testcase does not specify --version-script and would fail otherwise with "has undefined version error",
after doing this change we are consistent with gnu linkers which do not require version script file for such case when
building executable.

And reason for that I believe is next. Spec (https://sourceware.org/binutils/docs/as/Symver.html) says:
"Use the .symver directive to bind symbols to specific version nodes within a source file. This is only supported on ELF platforms,
 and is typically used when assembling files to be linked into a shared library. There are cases where it may make sense to use this 
in objects to be bound into an application itself so as to override a versioned symbol from a shared library."
(last sentence).

Example. Imagine dso.s:
.globl foo1
foo1:
  ret
.globl foo2
foo2:
  ret

And version script ver.txt: VERSION_1.0 { global : foo1; foo2; local : *; };

Build DSO:
llvm-mc -filetype=obj -triple=x86_64-pc-linux dso.s -o dso.o 
ld.bfd dso.o -shared -o dso.so --version-script ver.txt

Now we want to override one of foo() in executable. Usually version script is not provided when linking executable,
so we probably should be able just to do:

(app.s)
.global zed
.symver zed, foo1@@VERSION_1.0
zed:
  .quad 0

.global _start
_start:
 call foo1
 call foo2

llvm-mc -filetype=obj -triple=x86_64-pc-linux app.s -o app.o 
ld.bfd app.o dso.so -o out

And end up with overriden foo1:
    17: 00000000006003b0     0 NOTYPE  GLOBAL DEFAULT   10 foo2@@VERSION_1.0
    18: 00000000006003b0     0 NOTYPE  GLOBAL DEFAULT   10 _edata
    19: 00000000006003b0     0 NOTYPE  GLOBAL DEFAULT   10 _end
    20: 0000000000400278     0 NOTYPE  GLOBAL DEFAULT    8 foo1@@VERSION_1.0

Without "if (Config->Shared)" we would error out here.

George.