We have found aproblem in the Linux/ia64 kernel related to sibling call
optimization and the syscall_linkage attribute. On this platform, the attribute
indicates that the function paramaters must be treated as read-only
by the compiler. Any modification requires making a copy. 

The following code give an example of the problem we are describing:

void
bar(void)
{
        printf("in syscall_trace\n");
}

void foo(long value) __attribute__((syscall_linkage));
void
foo (long value)
{
        bar();
}

Because foo() is so simple, and bar() is the last call, the compiler
generates a direct branch to bar as opposed to a normal function call.
The bar() function will return directly to the caller of foo().

The generated code at -O2 looks as follows:

0000000000000000 <bar>:
   0:   [MII]       alloc r33=ar.pfs,4,3,0
   6:               mov r32=b0
   c:               addl r35=0,r1
  10:   [MMI]       mov r34=r1;;
  16:               ld8 r35=[r35]
  1c:               nop.i 0x0
  20:   [MIB]       nop.m 0x0
  26:               nop.i 0x0
  2c:               br.call.sptk.many b0=20 <bar+0x20>;;
  30:   [MII]       mov r1=r34
  36:               mov.i ar.pfs=r33
  3c:               mov b0=r32
  40:   [MIB]       nop.m 0x0
  46:               nop.i 0x0
  4c:               br.ret.sptk.many b0;;

0000000000000050 <foo>:
  50:   [MMI]       alloc r16=ar.pfs,8,8,0;;
  56:               alloc r2=ar.pfs,8,0,0
  5c:               nop.i 0x0
  60:   [MIB]       nop.m 0x0
  66:               nop.i 0x0
  6c:               br.many 60 <foo+0x10>;;
  70:   [MFB]       nop.m 0x0
  76:               break.f 0x0
  7c:               nop.b 0x0;;

foo() cannot modify r32 (the first argument) because of the syscall_linkage
attibute. However bar() is allowed to. 

The sibling optimization makes a direct branch (br.many) to bar().
That branch does not actually "protect" r32 because no RSE movement occurs.
The alloc in bar() simply resizes its current frame but the same r32 that
was passed to foo() is also accessible and can be modified at will.

In functions with the syscall_linkage attribute, sibling call optimization
should not be done because it could break the guarantee offered by the
attribute.