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* [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same
@ 2021-01-18 14:41 doko at debian dot org
2021-01-19 7:43 ` [Bug libstdc++/98731] " rguenth at gcc dot gnu.org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: doko at debian dot org @ 2021-01-18 14:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Bug ID: 98731
Summary: s390x: Large classes of std::bitset and
std::vector<bool> hash the same
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: doko at debian dot org
Target Milestone: ---
[forwarded from https://bugs.debian.org/977638]
same behavior with GCC 8, 9 and 10.
On s390x, std::hash returns identical values for large classes of
std::bitset and std::vector<bool>:
$ cat bug.cc
#include <bitset>
#include <functional>
#include <iostream>
#include <vector>
int main() {
std::bitset<2> a("00"), b("01");
std::vector<bool> c = {false, true, false, true};
std::vector<bool> d = {true, false, true, false};
std::bitset<9> e("000000000"), f("010101010");
std::vector<bool> g = {true, true, true, true, true, true, true, true,
true};
std::vector<bool> h = {false, false, false, true, true,
false, false, false, false};
std::hash<std::bitset<2>> h1;
std::hash<std::bitset<9>> h2;
std::hash<std::vector<bool>> h3;
std::cout << h1(a) << '\n'
<< h1(b) << '\n'
<< h3(c) << '\n'
<< h3(d) << "\n\n"
<< h2(e) << '\n'
<< h2(f) << '\n'
<< h3(g) << '\n'
<< h3(h) << '\n';
}
$ g++ -o bug bug.cc
$ ./bug
7857072875483051545
7857072875483051545
7857072875483051545
7857072875483051545
4158372090644325695
4158372090644325695
4158372090644325695
4158372090644325695
It appears that the hash value is completely dependent on the size of
the object in bytes. 1–8-bit values all hash to 7857072875483051545;
9–16-bit values all hash to 4158372090644325695; and though bug.cc
doesn’t demonstrate it, 17-bit values hash to 14756137038141193723.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
@ 2021-01-19 7:43 ` rguenth at gcc dot gnu.org
2021-01-19 10:00 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-01-19 7:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|target |libstdc++
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I bet it's the default specialization.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
2021-01-19 7:43 ` [Bug libstdc++/98731] " rguenth at gcc dot gnu.org
@ 2021-01-19 10:00 ` redi at gcc dot gnu.org
2021-01-19 18:02 ` redi at gcc dot gnu.org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2021-01-19 10:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2021-01-19
Assignee|unassigned at gcc dot gnu.org |redi at gcc dot gnu.org
Status|UNCONFIRMED |ASSIGNED
Ever confirmed|0 |1
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think we just hash the bytes of the storage, but assume the first bytes are
populated. For a bitset<2> we have an unsigned long long where only the two
least significant bits are used. For LE they're in the first byte, but for BE
they're in the last byte.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
2021-01-19 7:43 ` [Bug libstdc++/98731] " rguenth at gcc dot gnu.org
2021-01-19 10:00 ` redi at gcc dot gnu.org
@ 2021-01-19 18:02 ` redi at gcc dot gnu.org
2021-01-21 11:35 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2021-01-19 18:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes the problem is that a bitset<2> uses the two least significant bits of an
unsigned long, so we want to hash a single byte. But we take the address of the
unsigned long and then hash the first byte. For BE we need to hash the last
byte.
With a bitset of more than 64 bits we have a layout like this:
[76543210][...43210]
We assume the value bytes are contiguous and just hash N bytes from the address
of the first long, and then the next 5 bytes. We should be hashing the last 5
bytes of the last word instead.
I haven't looked at hash<vector<bool>> but it's probably the same bug.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
` (2 preceding siblings ...)
2021-01-19 18:02 ` redi at gcc dot gnu.org
@ 2021-01-21 11:35 ` redi at gcc dot gnu.org
2021-04-19 10:40 ` redi at gcc dot gnu.org
2021-09-29 18:15 ` redi at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2021-01-21 11:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target|s390x-linux-gnu |s390x-linux-gnu,
| |powerpc-*-*, powerpc64-*-*
Keywords| |ABI
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Matthias Klose from comment #0)
> It appears that the hash value is completely dependent on the size of
> the object in bytes.
It's not *completely* dependent on the size. Only the last x.size()%64 bits are
hashed incorrectly. For x.size() < 32 it's completely dependent, because we
never look at the right bits. For larger numbers of bits we look at *some* of
them.
Fixing it is an ABI break though, because it would mean that the same value
produces a different hash when compiled with an old GCC or a fixed GCC.
Inserting an element into an unordered_map in one TU and then looking it up in
another TU could fail.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
` (3 preceding siblings ...)
2021-01-21 11:35 ` redi at gcc dot gnu.org
@ 2021-04-19 10:40 ` redi at gcc dot gnu.org
2021-09-29 18:15 ` redi at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2021-04-19 10:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |NEW
Assignee|redi at gcc dot gnu.org |unassigned at gcc dot gnu.org
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug libstdc++/98731] s390x: Large classes of std::bitset and std::vector<bool> hash the same
2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
` (4 preceding siblings ...)
2021-04-19 10:40 ` redi at gcc dot gnu.org
@ 2021-09-29 18:15 ` redi at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: redi at gcc dot gnu.org @ 2021-09-29 18:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98731
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |miladfarca at gmail dot com
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 102531 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-09-29 18:15 UTC | newest]
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2021-01-18 14:41 [Bug target/98731] New: s390x: Large classes of std::bitset and std::vector<bool> hash the same doko at debian dot org
2021-01-19 7:43 ` [Bug libstdc++/98731] " rguenth at gcc dot gnu.org
2021-01-19 10:00 ` redi at gcc dot gnu.org
2021-01-19 18:02 ` redi at gcc dot gnu.org
2021-01-21 11:35 ` redi at gcc dot gnu.org
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