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* [Bug other/108623] New: We need to grow the precision field in tree_type_common for PowerPC
@ 2023-02-01  8:00 meissner at gcc dot gnu.org
  2023-02-01  8:18 ` [Bug middle-end/108623] " rguenth at gcc dot gnu.org
                   ` (11 more replies)
  0 siblings, 12 replies; 13+ messages in thread
From: meissner at gcc dot gnu.org @ 2023-02-01  8:00 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108623

            Bug ID: 108623
           Summary: We need to grow the precision field in
                    tree_type_common for PowerPC
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: meissner at gcc dot gnu.org
  Target Milestone: ---

The current patches that have been submitted to the PowerPC back end need to
grow the precision field in the tree_type_common structure (in tree-core.h). 
The current precision field is 10 bits but the type size for the dense math
register is 1,024 bits.

With the current 10 bit value, the precision of the TDO mode becomes 0 instead
of 1,024.  I noticed the tree-ssa-ccp.cc pass was dying with test programs that
use the dense math registers when I run the test on a PowerPC, but they run
fine on the x86_64.

Ultimately I tracked this down to the sext_hwi (sign extend) function in
hwint.h.

The code in hwint.h is:

static inline HOST_WIDE_INT
sext_hwi (HOST_WIDE_INT src, unsigned int prec)
{
  if (prec == HOST_BITS_PER_WIDE_INT)
    return src;
  else
#if defined (__GNUC__)
    {
      /* Take the faster path if the implementation-defined bits it's relying
         on are implemented the way we expect them to be.  Namely, conversion
         from unsigned to signed preserves bit pattern, and right shift of
         a signed value propagates the sign bit.
         We have to convert from signed to unsigned and back, because when left
         shifting signed values, any overflow is undefined behavior.  */
      gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT);
      int shift = HOST_BITS_PER_WIDE_INT - prec;
      return ((HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) src << shift)) >>
shift;
    }
#else
    {
      /* Fall back to the slower, well defined path otherwise.  */
      gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT);
      HOST_WIDE_INT sign_mask = HOST_WIDE_INT_1 << (prec - 1);
      HOST_WIDE_INT value_mask = (HOST_WIDE_INT_1U << prec) - HOST_WIDE_INT_1U;
      return (((src & value_mask) ^ sign_mask) - sign_mask);
    }
#endif
}

If the 'prec' argument is 0, the 'shift' variable will become 64.

In C/C++, a 64-bit value that is shifted either left or right by 64 bits is
undefined.  It turns out that the x86_64 will happen to return the original
value if you shift it left 64-bits and then shift it right with sign extension.
 From within the CCP pass, the original value is -1.  On the other hand, the
PowerPC always returns 0.  Since it isn't returning -1, it leads the CCP pass
to create a 0 constant for TDOmode.  But since TDOmode is opaque, converting it
to 0 generates an error when checking is enabled.

The solution is to grow the precision to 11 bits and reduce the
contains_placeholder_bits field to 1 bit.

Alternatively, we could grow the precision to 16 bits, which will cause all
trees to grow slightly, and keep contains_placeholder_bits at 2 bits. I see
references to contains placeholder bits in tree.cc and cp/module.cc, but I
don't see any place that actually sets the field.

^ permalink raw reply	[flat|nested] 13+ messages in thread
end of thread, other threads:[~2023-05-08 12:05 UTC | newest]

Thread overview: 13+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-02-01  8:00 [Bug other/108623] New: We need to grow the precision field in tree_type_common for PowerPC meissner at gcc dot gnu.org
2023-02-01  8:18 ` [Bug middle-end/108623] " rguenth at gcc dot gnu.org
2023-02-01  8:21 ` rguenth at gcc dot gnu.org
2023-02-01 10:16 ` rsandifo at gcc dot gnu.org
2023-02-01 16:48 ` meissner at gcc dot gnu.org
2023-02-01 17:32 ` segher at gcc dot gnu.org
2023-02-01 17:37 ` meissner at gcc dot gnu.org
2023-02-01 17:40 ` meissner at gcc dot gnu.org
2023-02-01 18:52 ` joseph at codesourcery dot com
2023-02-02  8:22 ` rguenther at suse dot de
2023-02-02  9:04 ` rsandifo at gcc dot gnu.org
2023-05-08 12:03 ` cvs-commit at gcc dot gnu.org
2023-05-08 12:05 ` rguenth at gcc dot gnu.org

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