Re: "-flto -O2" shouln't opt out "undefined reference" error

[Xuefer]

volatile: i tried already with clang/llvm it worked.

i'm using gentoo linux, trying to emerge everything with -flto. i'm not
sure if i understand linker/compiler bug or not a bug, i wonder how many
script is affected by this issue.

On Wed, Jan 18, 2017 at 7:53 AM Jim Wilson <jim.wil...@linaro.org> wrote:

On 01/16/2017 10:32 AM, Xuefer wrote:
> without -flto or without -O2 produce good (expected) result:
> 
> configure:5332: checking for dlsym
> ...

It isn't the linker that is the problem here.  It is the compiler.  But
it isn't a compiler bug.  An optimizing compiler is supposed to optimize
code like this.

I'd say the main problem is trying to use -flto at configure time.  This
is likely to break lots of configure scripts.

how could -lfto be not used with configure yet with make? pass another
CFLAGS to make as argument?
IIRC, we tends to assume configure use same CFLAGS as Makefile (which
generated by configure anyway)



> char (*f) ();

However, this particular problem I can fix with gcc by changing this line to

char (* volatile f) ();

and now gcc won't optimize away the store, even with -flto.
Unfortunately, I can't check LLVM at the moment, as I don't have LLVM
-flto support set up on any of my machines at the moment.

So this can be fixed by not using -flto at configure time, or by
modifying configure scripts to use volatile.  There is no linker or
compiler fix to make here.

Jim
___
bug-binutils mailing list
bug-binutils@gnu.org
https://lists.gnu.org/mailman/listinfo/bug-binutils

"-flto -O2" shouln't opt out "undefined reference" error

[Xuefer]

without -flto or without -O2 produce good (expected) result:

configure:5332: checking for dlsym
configure:5369: clang -o conftest -O2 -pipe -march=ivybridge -O2 -pipe
  conftest.c  >&5
/tmp/conftest-597c62.o:conftest.c:function main: error: undefined
reference to 'dlsym'
clang-3.9: error: linker command failed with exit code 1 (use -v to
see invocation)
configure:5372: $? = 1
configure: failed program was:
#line 5338 "configure"
#include "confdefs.h"
/* System header to define __stub macros and hopefully few prototypes,
which can conflict with char dlsym (); below.  */
#include 
/* Override any gcc2 internal prototype to avoid an error.  */
#ifdef __cplusplus
extern "C"
#endif
/* We use char because int might match the return type of a gcc2
   builtin and then its argument prototype would still apply.  */
char dlsym ();
char (*f) ();

int
main ()
{
/* The GNU C library defines this for functions which it implements
to always fail with ENOSYS.  Some functions are actually named
something starting with __ and the normal name is an alias.  */
#if defined (__stub_dlsym) || defined (__stub___dlsym)
choke me
#else
f = dlsym; /* workaround for ICC 12.0.3 */ if (f == 0) return 1;
#endif

  ;
  return 0;
}


 with both -flto and -O2 produce bad result (tested with clang and
gcc), this is reproduced when i build ncurses which have strange
configure script built with many workaround, unlike other configure
script


configure:5332: checking for dlsym
configure:5369: clang -o conftest -flto -O2 -pipe -march=ivybridge -O2
-pipe   conftest.c  >&5
configure:5372: $? = 0
configure:5375: test -s conftest
configure:5378: $? = 0
configure:5388: result: yes


___
bug-binutils mailing list
bug-binutils@gnu.org
https://lists.gnu.org/mailman/listinfo/bug-binutils