Re: Miscompilation of glibc with CVS mainline

[Andreas Jaeger <aj at suse dot de>]

"John David Anglin" <dave@hiauly1.hia.nrc.ca> writes:

> Andreas Jaeger wrote:
>> GCC CVS mainline miscompiles glibc on i686-linux-gnu.  The actual
>> error I noticed is:

>> CPP='/opt/gcc/3.3-devel/bin/gcc -E -x c-header'  /builds/glibc/test-libc/elf/ld-linux.so.2 --library-path /builds/glibc/test-libc:/builds/glibc/test-libc/math:/builds/glibc/test-libc/elf:/builds/glibc/test-libc/dlfcn:/builds/glibc/test-libc/nss:/builds/glibc/test-libc/nis:/builds/glibc/test-libc/rt:/builds/glibc/test-libc/resolv:/builds/glibc/test-libc/crypt:/builds/glibc/test-libc/linuxthreads /builds/glibc/test-libc/sunrpc/rpcgen -Y ../scripts -c rpcsvc/bootparam_prot.x -o /builds/glibc/test-libc/sunrpc/xbootparam_prot.T
>> make[2]: *** [/builds/glibc/test-libc/sunrpc/xbootparam_prot.stmp] Segmentation fault
>> make[2]: Leaving directory `/cvs/libc/sunrpc'
>
> I hit what appears to be the same error trying to build glibc-2.3.1 on
> hppa-linux.  It's hard to debug because the segmentation fault occurs in
> a shared library without any debug symbols.  In my particular case, it
> seemed as if the code was trying to call an undefined weak (indirect
> call to function at address 0x0).

I could reproduce this with a simple hello-world program and also with
some smaller program, it is indeed a bug in handling of weak extern
functions.

Here's a small testcase that has the same behaviour:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
extern void weak_func (void *arg);
asm (".weak weak_func");

void
test (void *arg)
{
  if (&weak_func != (void *)0)
    weak_func (arg);
  
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Is compiled correctly with GCC 3.3 CVS but with GCC 3.4 the test for
NULL is removed:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arthur:~/tmp:[0]$ /opt/gcc/3.3-devel/bin/gcc -O2 -c weak.c 
arthur:~/tmp:[0]$ objdump -Dr weak.o 

weak.o:     file format elf32-i386

Disassembly of section .text:

00000000 <test>:
   0:   55                      push   %ebp
   1:   b8 00 00 00 00          mov    $0x0,%eax
                        2: R_386_32     weak_func
   6:   85 c0                   test   %eax,%eax
   8:   89 e5                   mov    %esp,%ebp
   a:   74 06                   je     12 <test+0x12>
   c:   5d                      pop    %ebp
   d:   e9 fc ff ff ff          jmp    e <test+0xe>
                        e: R_386_PC32   weak_func
  12:   5d                      pop    %ebp
  13:   c3                      ret    
Disassembly of section .data:
arthur:~/tmp:[0]$ /opt/gcc/3.4-devel/bin/gcc -O2 -c weak.c 
arthur:~/tmp:[0]$ objdump -Dr weak.o 

weak.o:     file format elf32-i386

Disassembly of section .text:

00000000 <test>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   5d                      pop    %ebp
   4:   e9 fc ff ff ff          jmp    5 <test+0x5>
                        5: R_386_PC32   weak_func
Disassembly of section .data:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If I change this line:
asm (".weak weak_func");

to 
#pragma weak weak_func
, GCC 3.4 compiles this correctly with the test for NULL.

This one seems to be fine also:
extern void weak_func (void *arg) __attribute ((weak));

GCC changed existing behaviour and miscompiles code that worked before
for ages.

So, how should this miscompilation get fixed? Is this a bug in glibc
or an overoptimizing GCC?

I've CC'ed the libc-alpha list,
Andreas
-- 
 Andreas Jaeger
  SuSE Labs aj@suse.de
   private aj@arthur.inka.de
    http://www.suse.de/~aj

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