[Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0

[Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

            Bug ID: 114327
           Summary: wrong code with _BitInt() modulo at -O0
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 57688
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57688&action=edit
reduced testcase

Output:
$ x86_64-pc-linux-gnu-gcc testcase.c
$ ./a.out 
Aborted

$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-r14-9441-20240312154250-gef79c64cb57-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/14.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --with-cloog --with-ppl --with-isl
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --enable-libsanitizer
--disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-r14-9441-20240312154250-gef79c64cb57-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.0.1 20240312 (experimental) (GCC)

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|wrong code with _BitInt()   |`-CST % 1` is wrong for
                   |modulo at -O0               |_BitInt()
             Status|UNCONFIRMED                 |NEW
                 CC|                            |pinskia at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-03-14

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
For (ignore the strict aliasing issue):
```
typedef signed _BitInt(256) B;

[[gnu::noinline]]
B
foo (signed char c, B b)
{
  return b % c;
}

int
main (void)
{
  B x = foo (1, -3); // -3 % 1 -> 0
 // if (x)
 //   __builtin_abort();
    signed long *t = (signed long *)&x;
  for(int i = 0;i < sizeof(B)/sizeof(long); i++ )
  {
    __builtin_printf("%lx\n", t[i]);
  }
  return 0;
}
```
We get:
```
0
ffffffffffffffff
ffffffffffffffff
ffffffffffffffff
```

Which makes it seem like we are doing the sign extend when the value was the
result was 0.

Even:
> B x = foo (3, -3); // -3 % 3 -> 0

Gives the wrong similar result.

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

--- Comment #3 from Zdenek Sojka <zsojka at seznam dot cz> ---
It's not only % 1; wrong results are also for:
  B x = foo (3, -0x9e9b9fe60);

or for

B
foo (char c, B b)
{
  return b / c;
}


  B x = foo (-0x6, 0); /* 0 / -6 = 0 */

in all these cases, the result is the same: -1 << 64.

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 57697
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57697&action=edit
gcc14-pr114327.patch

Untested fix.

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:a6dab195f7041671166b9aa6a37e0db4236c829d

commit r14-9498-ga6dab195f7041671166b9aa6a37e0db4236c829d
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Mar 15 19:04:33 2024 +0100

    libgcc: Fix quotient and/or remainder negation in __divmodbitint4
[PR114327]

    While for __mulbitint3 we actually don't negate anything and perform the
    multiplication in unsigned style always, for __divmodbitint4 if the
operands
    aren't unsigned and are negative, we negate them first and then try to
    negate them as needed at the end.
    quotient is negated if just one of the operands was negated and the other
    wasn't or vice versa, and remainder is negated if the first operand was
    negated.
    The case which doesn't work correctly is if due to limited range of the
    operands we perform the division/modulo in some smaller number of limbs
    and then extend it to the desired precision of the quotient and/or
    remainder results.  If they aren't negated, the extension is done with
    memset to 0, if they are negated, the extension was done with memset
    to -1.  The problem is that if the quotient or remainder is zero,
    then bitint_negate negates it again to zero (that is ok), but we should
    then extend with memset to 0, not memset to -1.

    The following patch achieves that by letting bitint_negate also check if
    the negated operand is zero and changes the memset argument based on that.

    2024-03-15  Jakub Jelinek  <jakub@redhat.com>

            PR libgcc/114327
            * libgcc2.c (bitint_negate): Return UWtype bitwise or of all the
limbs
            before negation rather than void.
            (__divmodbitint4): Determine whether to fill in the upper limbs
after
            negation based on whether bitint_negate returned 0 or non-zero,
rather
            then always filling with -1.

            * gcc.dg/torture/bitint-63.c: New test.

[Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114327

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed, thanks for the report.