[Bug math/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[Bug math/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[aj at suse dot de]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

Andreas Jaeger <aj at suse dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at redhat dot com
         AssignedTo|aj at suse dot de           |rth at redhat dot com

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |joseph at codesourcery dot
                   |                            |com
          Component|math                        |soft-fp

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|rth at redhat dot com       |unassigned at sourceware
                   |                            |dot org

--- Comment #1 from Joseph Myers <jsm28 at gcc dot gnu.org> 2012-03-06 21:40:43 UTC ---
Confirmed still present in current sources.  The proposed fix seems reasonable.

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[rth at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

Richard Henderson <rth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |rth at gcc dot gnu.org

--- Comment #2 from Richard Henderson <rth at gcc dot gnu.org> 2012-03-06 22:51:34 UTC ---
In addition, the "sticky bit term" is

  (__builtin_constant_p(N) && (N) == 1                \
   ? X##_f0 & 1                                       \
   : (X##_f0 << (_FP_W_TYPE_SIZE - (N))) != 0));

which also must be zero for a zero shift.

The same pattern occurs in the _FP_FRAC_SRST_* macros.

All that said, when do we *ever* call the SRS macros
with a zero shift count?  Perhaps my poor grepping 
skills, but all I see is _FP_WFRACBITS_##fs and 
2*wfracbits, where wfracbits is _FP_WFRACBITS...

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

--- Comment #3 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Indeed, I don't actually see how the _SRS macros can be called with a shift
count of 0.  In the interest of efficiency, I'm inclined to say we should
define the interfaces to all the shift macros to require a shift count that is
strictly positive and strictly less than the total number of bits in the given
number of words, then review all shift counts for cases that might violate
this; I expect checking for 0 at any rare call sites where it might occur will
be more efficient than making these macros handle the case of 0.

In one such review attempt, I can see the use of *left* shift for converting
integers to floating point may shift by 0.  The following testcase illustrates
that on x86_64 for conversion from unsigned __int128 to __float128 (soft-fp as
used in libgcc):

#include <stdlib.h>

#ifdef COMPILE_TIME
# define VOL /* Empty.  */
#else
# define VOL volatile
#endif

int
main (void)
{
  VOL unsigned long long a = 0x1000000000000ULL;
  VOL unsigned long long b = 0xffffffffffffffffULL;
  unsigned __int128 c = (((unsigned __int128) a) << 64) | b;
  __float128 d = c;
  if (d != 0x1.000000000000ffffffffffffffffp112q)
    abort ();
  exit (0);
}

If you define COMPILE_TIME and build with optimization, the test passes as the
conversion is done at compile time; otherwise, it is done at runtime and a
right-shift by 64 in _FP_FRAC_SLL_2 called to shift left by 0 results in an
incorrect result.

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=7006

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
What I believe was the only remaining case of a zero shift count being passed
to one of the shift macros is fixed for 2.18 by:

commit 8fdda7afb883589b46e423f91f32988373670bfa
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri Jun 21 19:00:43 2013 +0000

    Fix bad shift in soft-fp (bug 7006).

If you find other cases of zero shift counts being passed to these macros,
please file new bugs for them.

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[Bug soft-fp/7006] Bad left shift in _FP_FRAC_SRS_2 macro in soft-fp/op-2.h on SH-4

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=7006

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
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              Flags|                            |security-

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