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Date: Thu, 04 Mar 2010 22:23:00 -0000
Subject: [Bug c++/43263]  New: inline asm with -O2 breaks logic if comarisons are involved
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From: "s dot v dot savenko at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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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