Re: target/10595: [ia64] use of register variables creates stop bits

Re: target/10595: [ia64] use of register variables creates stop bits

[wilson]

Synopsis: [ia64] use of register variables creates stop bits

Responsible-Changed-From-To: unassigned->wilson
Responsible-Changed-By: wilson
Responsible-Changed-When: Thu May  8 05:21:05 2003
Responsible-Changed-Why:
    IA-64 maintainer
State-Changed-From-To: open->analyzed
State-Changed-By: wilson
State-Changed-When: Thu May  8 05:21:05 2003
State-Changed-Why:
    There is a stop bit before the call because both alloc and br.call write to CFM (current frame marker).  If I modify the example so that there are more instructions before the call, then this problem does not show up.  For instance
    register long int r __asm ("r13");
    int i;
    int
    foo (long a)
    {
      i += 1;
      return bar(r);
    }
    Gives me
            st4 [r15] = r14
            mov r35 = r13
            br.call.sptk.many b0 = bar#
            ;;
    and now there is no stop bit immediately after the global register variable read.
    
    I don't have access to IA-64 hardware currently, so I can't easily examine the Linux thread library.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10595

target/10595: [ia64] use of register variables creates stop bits

[drepper]

>Number:         10595
>Category:       target
>Synopsis:       [ia64] use of register variables creates stop bits
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 02 03:16:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Ulrich Drepper
>Release:        gcc-3.2.3 (and probably later)
>Organization:
>Environment:
Linux/ia64
>Description:
Every use of register variables, variables declared like

  register int foo __asm ("r13")

cause stop bits to be generated right after loading from the register.

The use of register variables is quite frequent in the C runtime and also in the kernel so that this is a problem.
>How-To-Repeat:
Compile this code:

register long int r __asm ("r13");
int
foo (long a)
{
  return bar(r) != a;
}



The result I see looks like this:

   0:   01 18 15 08 80 05       [MII]       alloc r35=ar.pfs,5,4,0
   6:   20 02 00 62 00 80                   mov r34=b0
   c:   04 68 00 84                         mov r36=r13;;
  10:   17 00 00 00 00 08       [BBB]       nop.b 0x0
                        12: PCREL21B    bar
  16:   00 00 00 00 10 00                   nop.b 0x0
  1c:   08 00 00 50                         br.call.sptk.many b0=10
<foo+0x10>;;
[...]


Note the stop bits after mov r36=r13.  Many more examples can be seen by looking at the thread Linux library.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: