[Bug target/43249] New: unsigned int arg with no prototype gets full 64-bit reg

[Bug target/43249] New: unsigned int arg with no prototype gets full 64-bit reg

[janis at gcc dot gnu dot org]

For a function with no prototype that defines an argument as unsigned
int, GCC for powerpc64-linux with -m64 and optimization uses the
entire 64-bit argument register rather than masking off the lower
32 bits.  This problem was discoverd with 176.gcc using options
"-m64 -O2 -ftree-vectorize -maltivec -fdata-sections", which changed
the order of 32-element arrays so that accessing element 63 instead of
element 31 started getting garbage that no longer happened to work.

Here's a simple testcase that works with -O0 and fails with -O1.
It's useful to compare this with a a version whose argument is
unsigned short.  For the GCC testsuite, just remove references to
printf.

-------------------------------------------------------------------
extern int printf (const char *, ...);

int
__attribute__((noinline))
floor_log2_wide (x)
  unsigned int x;
{
  int log = -1;
  while (x != 0)
    log++,
    x >>= 1;
  return log;
}

int
main ()
{
  int m;
  unsigned int n = 4294967259U;

  m = floor_log2_wide ((int)n);
  printf ("n = %u, m = %d\n", n, m);
  if (m != 31)
    __builtin_abort ();
  return 0;
}
-------------------------------------------------------------------

elm3c105% /home/janis/tools/gcc-trunk-anonsvn/bin/gcc -m64 -O0 bug.c && ./a.out
n = 4294967259, m = 31
elm3c105% /home/janis/tools/gcc-trunk-anonsvn/bin/gcc -m64 -O1 bug.c && ./a.out
n = 4294967259, m = 63
Aborted

I've tried compilers back to GCC 4.1 and they all have the same problem;
it's probably not a regression.


-- 
           Summary: unsigned int arg with no prototype gets full 64-bit reg
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: janis at gcc dot gnu dot org
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43249

[Bug target/43249] unsigned int arg with no prototype gets full 64-bit reg

[janis at gcc dot gnu dot org]

------- Comment #1 from janis at gcc dot gnu dot org  2010-03-10 18:35 -------
GCC 3.4 also has this bug.


-- 

janis at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bergner at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43249

[Bug target/43249] unsigned int arg with no prototype gets full 64-bit reg

[amodra at gmail dot com]

------- Comment #2 from amodra at gmail dot com  2010-05-26 13:22 -------
I think this testcase may invoke undefined behaviour.  Section 6.5.2.2 of the
ISO C spec says of function calls without a prototype that if "the types of the
arguments after promotion are not compatible with those of the parameters after
promotion, the behavior is undefined, except for the following cases" and the
relevant case is "one promoted type is a signed integer type, the other
promoted type is the corresponding unsigned integer type, and the value is
representable in both types".

The value you are passing, (int)4294967259U, is a negative number so not
representable as an unsigned int.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43249

[Bug target/43249] unsigned int arg with no prototype gets full 64-bit reg

[wschmidt at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43249

William J. Schmidt <wschmidt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |wschmidt at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #3 from William J. Schmidt <wschmidt at gcc dot gnu.org> 2012-04-20 14:45:52 UTC ---
Closing per comment #2.