[Bug c/34115] New: atomic builtins not supported on i686?

[Bug c/34115] New: atomic builtins not supported on i686?

[scovich at gmail dot com]

Linking fails for the program below, with the error:

undefined reference to `___sync_val_compare_and_swap_4'

// gcc -Wall atomic.c
int main() {
  int *a, b, c;
  return __sync_val_compare_and_swap(a, b, c);
}

According to the atomic builtins docs (), "Not all operations are supported by
all target processors. If a particular operation cannot be implemented on the
target processor, a warning will be generated and a call an external function
will be generated. The external function will carry the same name as the
builtin, with an additional suffix `_n' where n is the size of the data type."

If CAS is not supported, how come I don't get a warning? Why would i686 *not*
support compare and swap? The cmpxchg instruction has been around since 80486,
according to the intel IA-32 processor manual. 

Also, does an unsupported builtin mean the user is responsible to write that
function, or simply that the compiler must make a function call to synthesize
its behavior?

FWIW, my x86_64 cross-compile gcc 4.2.0 handles it fine, emitting a
"lock"+"cmpxchg" pair.


-- 
           Summary: atomic builtins not supported on i686?
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scovich at gmail dot com
  GCC host triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2007-11-15 22:19 -------
Because the default arch for i686-linux-gnu is i386.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[pcarlini at suse dot de]

------- Comment #2 from pcarlini at suse dot de  2007-11-15 22:30 -------
I think this is essentially invalid. Note that now we also have the various 
__GCC_HAVE_SYNC_COMPARE_AND_SWAP_* macros:

  http://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[joseph at codesourcery dot com]

------- Comment #3 from joseph at codesourcery dot com  2007-11-15 23:53 -------
Subject: Re:  atomic builtins not supported on i686?

On Thu, 15 Nov 2007, pinskia at gcc dot gnu dot org wrote:

> Because the default arch for i686-linux-gnu is i386.

Which is a stupid inconsistency and arguably a bug.  sparcv9-linux-gnu 
defaults to -mcpu=v9 (where -mcpu means -march for SPARC), likewise 
various other architectures; i686-linux-gnu should be consistent with 
those other targets and default to -march=i686.  libstdc++ will use 
i486-specific code if you configure GCC for i[456]86, even when GCC itself 
restricts itself to i386 code, making this default to i386 even less 
useful; likewise glibc will use i486/i586/i686 code when configured for 
those processors (and no longer supports i386).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[pcarlini at suse dot de]

------- Comment #4 from pcarlini at suse dot de  2007-11-16 00:07 -------
Yeah, the wind is changing!


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[scovich at gmail dot com]

------- Comment #5 from scovich at gmail dot com  2007-11-16 01:00 -------
Subject: Re:  atomic builtins not supported on i686?

On 15 Nov 2007 23:53:06 -0000, joseph at codesourcery dot com
<gcc-bugzilla@gcc.gnu.org> wrote:
> > Because the default arch for i686-linux-gnu is i386.
> Which is a stupid inconsistency and arguably a bug.

++

BTW, -march=i686 works beautifully. Close the bug? or rename it as a
RFE to have i686-* default to -march=i686?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[scovich at gmail dot com]

------- Comment #6 from scovich at gmail dot com  2007-11-16 01:04 -------
(In reply to comment #5)
> Subject: Re:  atomic builtins not supported on i686?
> 
> On 15 Nov 2007 23:53:06 -0000, joseph at codesourcery dot com
> <gcc-bugzilla@gcc.gnu.org> wrote:
> > > Because the default arch for i686-linux-gnu is i386.
> > Which is a stupid inconsistency and arguably a bug.
> 
> ++
> 
> BTW, -march=i686 works beautifully. Close the bug? or rename it as a
> RFE to have i686-* default to -march=i686?
> 

Oh, and is there supposed to be a warning about unsupported atomic ops or not?
If not the docs should say to expect a linker error instead (and also
mention/link those macros Paolo pointed out).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[scovich at gmail dot com]

------- Comment #7 from scovich at gmail dot com  2007-11-28 01:56 -------
(In reply to comment #2)
> I think this is essentially invalid. Note that now we also have the various 
> __GCC_HAVE_SYNC_COMPARE_AND_SWAP_* macros:
> 
>   http://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html
> 

Too bad they aren't defined for any machine I've tried so far...
ia64-linux-gnu (4.1.2 Debian)
x86_64-unknown-linux-gnu (4.2.0)
sparc-sun-solaris2.10 (4.1.1)
powerpc64-unknown-linux-gnu (4.1.2 Gentoo)
i686-pc-cygwin (4.2.2)

All these actually *do* support CAS, and emit perfectly respectable .asm... as
long as you don't wrap them in any #ifdef's.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[pcarlini at suse dot de]

------- Comment #8 from pcarlini at suse dot de  2007-11-28 02:05 -------
(In reply to comment #7)
> Too bad they aren't defined for any machine I've tried so far...

The explanation is very simple: the new macros are implemented only in mainline
(would be 4.3.0).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[scovich at gmail dot com]

------- Comment #9 from scovich at gmail dot com  2007-11-28 14:20 -------
(In reply to comment #8)
> (In reply to comment #7)
> > Too bad they aren't defined for any machine I've tried so far...
> 
> The explanation is very simple: the new macros are implemented only in mainline
> (would be 4.3.0).
> 
Any chance of backporting? (I know, probably not)

The only question left is whether the compiler is supposed to emit a warning
when it doesn't support the intrinsics (like the docs say) or whether the user
should just be ready for linker errors.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[joseph dot h dot garvin at gmail dot com]

------- Comment #10 from joseph dot h dot garvin at gmail dot com  2009-06-08 21:18 -------
After encountering this issue and some testing, I think this is definitely a
bug. The problem ONLY occurs when directly returning a call to an atomic
builtin! Assuming no march flags:

Links:

int main()
{
        unsigned long test;
        return __sync_add_and_fetch(&test, 1);

        return (int)test;
}

Does not link:

int main()
{
        unsigned long test;
        return __sync_add_and_fetch(&test, 1);
}

If you do specify -march=i686, then they both link! If this somehow isn't a
bug, the design is poor, because it's very hard to debug behavior.

$  gcc -v
Reading specs from
/opt/app/g++lib6/gcc-4.2/lib/gcc/i386-pc-solaris2.10/4.2.2/specs
Target: i386-pc-solaris2.10
Configured with: ../configure --prefix=/opt/app/g++lib6/gcc-4.2
--enable-languages=c,c++,fortran,objc --disable-nls --with-included-gettext
--with-gnu-as --with-as=/usr/sfw/bin/gas --with-target-tools=/usr/sfw/bin/
--with-gmp=/opt/app/nonc++/gmp-4.2 --with-mpfr=/opt/app/nonc++/mpfr-2.3
Thread model: posix
gcc version 4.2.2


-- 

joseph dot h dot garvin at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |joseph dot h dot garvin at
                   |                            |gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

[Bug target/34115] atomic builtins not supported on i686?

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-10-15 20:48:37 UTC ---
is this still a problem now that GCC defaults to setting the default march
based on the configure target?

[Bug target/34115] atomic builtins not supported on i686?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=34115

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.5.0

--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The defualt arch changed with r0-98640 such that i686 defaults to i686 which
fixes this issue all together.