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* [Bug c++/43263] New: inline asm with -O2 breaks logic if comarisons are involved
@ 2010-03-04 22:23 s dot v dot savenko at gmail dot com
2010-03-04 22:25 ` [Bug c++/43263] " pinskia at gcc dot gnu dot org
0 siblings, 1 reply; 2+ messages in thread
From: s dot v dot savenko at gmail dot com @ 2010-03-04 22:23 UTC (permalink / raw)
To: gcc-bugs
I put here a simple example to illustrate what looks like a bug to me
#include<iostream>
inline unsigned next_exp2(unsigned x)
{
unsigned n;
asm volatile("bsr %0, %1" : "=r" (n) : "r" (x)); // most significant
bit
return((x == (1u << n)) ? n : n);
};
inline unsigned next_exp2_(unsigned x)
{
unsigned n;
asm volatile("bsr %0, %1" : "=r" (n) : "r" (x)); // most significant
bit
return((x == (1u << n)) ? n : n+1);
};
int main()
{
unsigned x = 1025;
std::cout << "n1 = " << next_exp2(x) << std::endl;
std::cout << "n2 = " << next_exp2_(x) << std::endl;
return(0);
}
The difference between two function is n+1 returned from the underscored
version. Compiled with g++ -O2 ./foo.cpp, my output is:
n1 = 10
n2 = 3078596801
while I would expect n2 = 11.
If I compile it with g++ -g foo.cpp, everything works as expected.
--
Summary: inline asm with -O2 breaks logic if comarisons are
involved
Product: gcc
Version: 4.3.4
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: s dot v dot savenko at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43263
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug c++/43263] inline asm with -O2 breaks logic if comarisons are involved
2010-03-04 22:23 [Bug c++/43263] New: inline asm with -O2 breaks logic if comarisons are involved s dot v dot savenko at gmail dot com
@ 2010-03-04 22:25 ` pinskia at gcc dot gnu dot org
0 siblings, 0 replies; 2+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-03-04 22:25 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2010-03-04 22:24 -------
*** This bug has been marked as a duplicate of 43262 ***
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |DUPLICATE
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43263
^ permalink raw reply [flat|nested] 2+ messages in thread
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2010-03-04 22:23 [Bug c++/43263] New: inline asm with -O2 breaks logic if comarisons are involved s dot v dot savenko at gmail dot com
2010-03-04 22:25 ` [Bug c++/43263] " pinskia at gcc dot gnu dot org
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