[Bug c++/33661] New: template methods forget explicit local reg vars

[Bug c++/33661] New: template methods forget explicit local reg vars

[vincent dot riviere at freesbee dot fr]

Hello.

http://gcc.gnu.org/onlinedocs/gcc-4.2.1/gcc/Local-Reg-Vars.html#Local-Reg-Vars
"However, using the variable as an asm operand guarantees that the specified
register is used for the operand."

This does not always work if the code is inside a template method which gets
inlined.

For this testcase, imagine a system call invoked by trap #0. It returns a
pointer into d0, and clobbers d1/a0/a1.

$ cat a.cpp
template<class T>
class Tpl
{
public:
        static long* MySysCall()
        {
                register long retval __asm__("d0");

                asm
                (
                        "trap   #0\n\t"
                        "move.l %0,%0\n\t"
                : "=r"(retval)
                :
                : "d1", "a0", "a1"
                );

                return (long*)retval;
        }
};

void f()
{
        long* p = Tpl<char>::MySysCall();
        *p = 1;
}

Note that I added an extra "move.l %0,%0" to see which register is affected to
%0.

$ g++ -S a.cpp -O1 -o -
#NO_APP
        .file   "a.cpp"
        .text
        .align  2
        .globl  _Z1fv
        .type   _Z1fv, @function
_Z1fv:
.LFB3:
        link.w %fp,#0
.LCFI0:
        move.l %a2,-(%sp)
.LCFI1:
#APP
        trap    #0
        move.l  %a2,%a2

#NO_APP
        moveq #1,%d0
        move.l %d0,(%a2)
        move.l (%sp)+,%a2
        unlk %fp
        rts
.LFE3:
        .size   _Z1fv, .-_Z1fv
        .globl  __gxx_personality_v0
        .ident  "GCC: (GNU) 4.2.1"
        .section        .note.GNU-stack,"",@progbits

We can see that even though we requested "retval" to be mapped to d0, it has
been mapped to a2.

The problem does not appear if the code is in a method of a standard class (not
a method of a template).
Another way to get it work is to add "volatile" to the register variable
declaration.


-- 
           Summary: template methods forget explicit local reg vars
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vincent dot riviere at freesbee dot fr
GCC target triplet: m68k-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2007-12-10 01:55 -------
Confirmed,
It also fails with 3.3 so this is not a regression.
A testcase for PPC:
template<class T>
class Tpl
{
public:
        static long* MySysCall()
        {
                register long retval __asm__("r3");

                asm
                (
                        "trap   #0\n\t"
                        "move.l %0,%0\n\t"
                : "=r"(retval)
                :
                : "r4"
                );

                return (long*)retval;
        }
};

void f()
{
        long* p = Tpl<char>::MySysCall();
        *p = 1;
}

Note I did not change the inline-asm's string though so it will not assemble.




-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
      Known to fail|                            |3.3.3 4.1.0 4.3.0
   Last reconfirmed|0000-00-00 00:00:00         |2007-12-10 01:55:24
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[vincent dot riviere at freesbee dot fr]

------- Comment #2 from vincent dot riviere at freesbee dot fr  2008-02-02 20:48 -------
Still fails in GCC release 4.2.3


-- 

vincent dot riviere at freesbee dot fr changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|3.3.3 4.1.0 4.3.0           |3.3.3 4.1.0 4.2.3 4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2008-02-05 10:28 -------
This works for me on x86_64 with 4.3.0.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[vincent dot riviere at freesbee dot fr]

------- Comment #4 from vincent dot riviere at freesbee dot fr  2008-02-05 19:56 -------
Same problem with GCC 4.3-20080201 on target i686-pc-linux-gnu


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2008-04-29 17:09 -------
*** Bug 36080 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adam at os dot inf dot tu-
                   |                            |dresden dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[adam at os dot inf dot tu-dresden dot de]

------- Comment #6 from adam at os dot inf dot tu-dresden dot de  2008-04-30 13:34 -------
Even if this is not a regression it would be very helpful if g++ would emit a
warning that the register allocation will be ignored. Those bugs are subtle.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[vincent dot riviere at freesbee dot fr]

------- Comment #7 from vincent dot riviere at freesbee dot fr  2008-04-30 15:32 -------
This is not a regression, however it is a bug, it has to be fixed. Inline
assembly coupled with templates is very powerful, however because of this bug
it is unusable :-(
If I remember well, a workaround is to put the assembly in an inline global
function, then call it from the template method.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[adam at os dot inf dot tu-dresden dot de]

------- Comment #8 from adam at os dot inf dot tu-dresden dot de  2008-04-30 20:22 -------
(In reply to comment #7)
> This is not a regression, however it is a bug, it has to be fixed. Inline
> assembly coupled with templates is very powerful, however because of this bug
> it is unusable :-(

I fully agree. I don't know how hard this one is to fix so at least a
warning/error would be good to have. Silently ignoring the allocation just
causes bugs nobody wants to have.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

[Bug c++/33661] template methods forget explicit local reg vars

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |krebbel at gcc dot gnu.org

--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 66393 has been marked as a duplicate of this bug. ***

[Bug c++/33661] template methods forget explicit local reg vars

[adam at os dot inf.tu-dresden.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

--- Comment #11 from Adam Lackorzynski <adam at os dot inf.tu-dresden.de> ---
Confirming issue still exists for 4.7.4, 4.8.4, 4.9.2 and 5.0 (tested on
x86_64).

[Bug c++/33661] template methods forget explicit local reg vars

[Aleksandrs.Saveljevs at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

--- Comment #10 from Aleksandrs Saveljevs <Aleksandrs.Saveljevs at gmail dot com> 2012-05-31 08:16:41 UTC ---
Confirming that the bug still exists on ARM, x86 and x86_64 as of GCC 4.6.3.

[Bug c++/33661] template methods forget explicit local reg vars

[s.schuh85 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33661

Stefan Schuh <s.schuh85 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |s.schuh85 at gmail dot com

--- Comment #9 from Stefan Schuh <s.schuh85 at gmail dot com> 2011-08-24 10:00:48 UTC ---
I can confirm this bug in gcc version 4.5.2 on i686-linux-gnu 
and in gcc version 4.6.0 on s390x-ibm-linux-gnu