Re: Linker plugins should be aware of --defsym during symbol resolution

["Teresa Johnson via binutils" <binutils at sourceware dot org>]

On Tue, Feb 13, 2018 at 10:24 AM, Cary Coutant <ccoutant@gmail.com> wrote:

> >> Can you explain why this new resolution is needed? Why would
> >> LDPR_PREEMPTED_REG not work?
> >
> > They mean different things. For example, LDPR_PREEMPTED_REG will mean
> that
> > there is a prevailing definition in a regular object file. But if the IR
> def
> > has weak ODR linkage, the LTO implementation knows that all copies must
> be
> > the same, and we can and currently do keep that def around long enough
> for
> > inlining. That would be incorrect in the defsym case, where this symbol
> is
> > actually redefined by the linker. That is what we are trying to
> distinguish.
>
> Do you have a real-world example? I'm having trouble imagining a case
> where --defsym would be used to override a symbol that's subject to
> the ODR and yet remain a valid program.
>

I just concocted one:

$ cat bar.h
#include <stdio.h>
extern inline void bar()
{
  fprintf(stderr, "in bar\n");
}

extern inline void baz()
{
  fprintf(stderr, "in baz\n");
}

$ cat hello1.cc
#include <stdio.h>
#include "bar.h"

extern void foo();

int main() {
  baz();
  bar();
  foo();
  return 0;
}

$ cat hello2.cc
#include <stdio.h>
#include "bar.h"

void foo()
{
  bar();
}

Without defsym:

$ ~/llvm/llvm_8_build/bin/clang++ hello[12].cc  -fuse-ld=gold
$ a.out
in baz
in bar
in bar

With defsym:

$ ~/llvm/llvm_8_build/bin/clang++ hello[12].cc  -fuse-ld=gold
-Wl,--defsym,_Z3barv=_Z3bazv
$ a.out
in baz
in baz
in baz

In this case, because bar is inline it gets weak ODR linkage in the two
translation units.

Teresa


> -cary
>



-- 
Teresa Johnson |  Software Engineer |  tejohnson@google.com |  408-460-2413
<(408)%20460-2413>