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* [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0
@ 2024-03-13 19:06 zsojka at seznam dot cz
2024-03-14 0:00 ` [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt() pinskia at gcc dot gnu.org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: zsojka at seznam dot cz @ 2024-03-13 19:06 UTC (permalink / raw)
To: gcc-bugs
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)
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
@ 2024-03-14 0:00 ` pinskia at gcc dot gnu.org
2024-03-14 0:10 ` pinskia at gcc dot gnu.org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-03-14 0:00 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
2024-03-14 0:00 ` [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt() pinskia at gcc dot gnu.org
@ 2024-03-14 0:10 ` pinskia at gcc dot gnu.org
2024-03-14 5:31 ` zsojka at seznam dot cz
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-03-14 0:10 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
2024-03-14 0:00 ` [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt() pinskia at gcc dot gnu.org
2024-03-14 0:10 ` pinskia at gcc dot gnu.org
@ 2024-03-14 5:31 ` zsojka at seznam dot cz
2024-03-14 12:57 ` jakub at gcc dot gnu.org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: zsojka at seznam dot cz @ 2024-03-14 5:31 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
` (2 preceding siblings ...)
2024-03-14 5:31 ` zsojka at seznam dot cz
@ 2024-03-14 12:57 ` jakub at gcc dot gnu.org
2024-03-15 18:05 ` cvs-commit at gcc dot gnu.org
2024-03-15 18:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: jakub at gcc dot gnu.org @ 2024-03-14 12:57 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
` (3 preceding siblings ...)
2024-03-14 12:57 ` jakub at gcc dot gnu.org
@ 2024-03-15 18:05 ` cvs-commit at gcc dot gnu.org
2024-03-15 18:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2024-03-15 18:05 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt()
2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
` (4 preceding siblings ...)
2024-03-15 18:05 ` cvs-commit at gcc dot gnu.org
@ 2024-03-15 18:07 ` jakub at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: jakub at gcc dot gnu.org @ 2024-03-15 18:07 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 7+ messages in thread
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2024-03-13 19:06 [Bug libgcc/114327] New: wrong code with _BitInt() modulo at -O0 zsojka at seznam dot cz
2024-03-14 0:00 ` [Bug libgcc/114327] `-CST % 1` is wrong for _BitInt() pinskia at gcc dot gnu.org
2024-03-14 0:10 ` pinskia at gcc dot gnu.org
2024-03-14 5:31 ` zsojka at seznam dot cz
2024-03-14 12:57 ` jakub at gcc dot gnu.org
2024-03-15 18:05 ` cvs-commit at gcc dot gnu.org
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