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* [Bug c++/106542] New: -O2 sign-extended uint32 to uint64
@ 2022-08-06 2:46 wuz73 at hotmail dot com
2022-08-06 3:32 ` [Bug c++/106542] " pinskia at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: wuz73 at hotmail dot com @ 2022-08-06 2:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106542
Bug ID: 106542
Summary: -O2 sign-extended uint32 to uint64
Product: gcc
Version: 9.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: wuz73 at hotmail dot com
Target Milestone: ---
#include <iostream>
unsigned Msb(uint64_t x) { return 63 - __builtin_clzl(x); }
template<typename T>
void f(T r, T p)
{
if (p > r)
{
auto d = p - r;
unsigned k = Msb(sizeof(T) == 4 ? unsigned(d) : d);
std::cout << sizeof(T) << " k=" << k << "\n";
}
}
int main()
{
int data[] = {-1610499096,2086724600};
f(data[0],data[1]);
}
When compiled with "g++ -O2" the output is "4 k=63". Apparently before calling
Msb(), g++ sign-extended unsigned(d). static_cast<unsigend>(d) yields the same
result. However, "g++ -O0" is correct (k=31), so are earlier versions of g++
such as 4.8.5 and 5.3.1.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/106542] -O2 sign-extended uint32 to uint64
2022-08-06 2:46 [Bug c++/106542] New: -O2 sign-extended uint32 to uint64 wuz73 at hotmail dot com
@ 2022-08-06 3:32 ` pinskia at gcc dot gnu.org
2022-08-06 3:51 ` wuz73 at hotmail dot com
2022-08-06 3:58 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-08-06 3:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106542
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The code is undefined and can be detected with -fsanitize=undefined:
/app/example.cpp:15:14: runtime error: signed integer overflow: 2086724600 -
-1610499096 cannot be represented in type 'int'
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/106542] -O2 sign-extended uint32 to uint64
2022-08-06 2:46 [Bug c++/106542] New: -O2 sign-extended uint32 to uint64 wuz73 at hotmail dot com
2022-08-06 3:32 ` [Bug c++/106542] " pinskia at gcc dot gnu.org
@ 2022-08-06 3:51 ` wuz73 at hotmail dot com
2022-08-06 3:58 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: wuz73 at hotmail dot com @ 2022-08-06 3:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106542
wuz73 at hotmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|INVALID |---
Status|RESOLVED |UNCONFIRMED
--- Comment #2 from wuz73 at hotmail dot com ---
(In reply to Andrew Pinski from comment #1)
> The code is undefined and can be detected with -fsanitize=undefined:
>
> /app/example.cpp:15:14: runtime error: signed integer overflow: 2086724600 -
> -1610499096 cannot be represented in type 'int'
I'm only interested in the lower 32 bits, hence the unsigned(d).
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c++/106542] -O2 sign-extended uint32 to uint64
2022-08-06 2:46 [Bug c++/106542] New: -O2 sign-extended uint32 to uint64 wuz73 at hotmail dot com
2022-08-06 3:32 ` [Bug c++/106542] " pinskia at gcc dot gnu.org
2022-08-06 3:51 ` wuz73 at hotmail dot com
@ 2022-08-06 3:58 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-08-06 3:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106542
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |INVALID
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The code is undefined as the subtraction is done in the int type.
You either need to use -fwrapv or cast to an unsigned type before doing the
subtraction.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2022-08-06 3:58 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2022-08-06 2:46 [Bug c++/106542] New: -O2 sign-extended uint32 to uint64 wuz73 at hotmail dot com
2022-08-06 3:32 ` [Bug c++/106542] " pinskia at gcc dot gnu.org
2022-08-06 3:51 ` wuz73 at hotmail dot com
2022-08-06 3:58 ` pinskia at gcc dot gnu.org
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