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Date: Wed, 21 Jul 2010 11:23:00 -0000
Subject: [Bug middle-end/45017]  New: miscompile with bitfield and optimization
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From: "borntraeger at de dot ibm dot com" <gcc-bugzilla@gcc.gnu.org>
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The following testcase has a different result with 4.6 and -O1/2/3

gcc4.5: all optimizations:
r1: 15 r2: 2

gcc4.6: -O0
r1: 15 r2: 2

gcc4.6: -O1,-O2..
r1: 47 r2: 2

#include <stdio.h>

int tester(char *bytes)
{
        union {
                struct {
                        unsigned int r1:4;
                        unsigned int r2:4;
                } fmt;
                char value[1];
        } ovl;

        ovl.value[0] = bytes[0];
        printf("r1: %d r2: %d\n", ovl.fmt.r1, ovl.fmt.r2);
        return 0;
}

int main()
{
        char buff = 0x2f;
        return tester(&buff);
}


-- 
           Summary: miscompile with bitfield and optimization
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: borntraeger at de dot ibm dot com
 GCC build triplet: i686-pc-linux-gnu and s390x-ibm-linux-gnu
  GCC host triplet: i686-pc-linux-gnu and s390x-ibm-linux-gnu
GCC target triplet: i686-pc-linux-gnu and s390x-ibm-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45017