[Bug c++/42810] New: Enumeration with sequential values has its for-loop exit condition optimized out.

["tony3 at GarlandConsulting dot us" <gcc-bugzilla at gcc dot gnu dot org>]

An enumeration with sequentially-assigned values is used as an index of a for
loop. The loop operates properly at optimization levels -O0 and -O1. At level
-O2 the optimizer "optimizes out" the test for the exit condition and the loop
never exits.

I'll be attaching the *.i file, but below is the test source file which shows
the situation.  At -O2, the loop itself never exits and a separate counter
which is part of the test triggers a break itself--after printing that the test
has failed.

/*
Sample program to demonstrate comparison failure involving sequential
enumeration when optimized.  To reproduce the failure:

    g++ -O2 test.cpp && ./a.out

Any one of the following changes correct the problem:

    - Change optimization (omit or use -O0 or -O1).
    - Set ENUM_VALUE_FIRST to 0.
    - Delete one of the enum values between first and last (one less
      value).
    - Insert an addition enum value between first and last (one more
      value).

agarland at fluke dot com

*/

// Compile at -O1 or uncomment any one of the following to "fix" the problem.
//#define FIX1 // Omit an enum value.
//#define FIX2 // Add an extra enum value.
//#define FIX3 // Start first value at zero rather than 1.

#include <stdio.h>
enum ENUM_VALUE
{
#ifdef FIX3
    ENUM_VALUE_FIRST = 0,
#else
    ENUM_VALUE_FIRST = 1,
#endif
    ENUM_VALUE_A,
#ifndef FIX1
    ENUM_VALUE_B,
#endif
    ENUM_VALUE_C,
    ENUM_VALUE_D,
    ENUM_VALUE_E,
#ifdef FIX2
    ENUM_VALUE_F,
#endif
    ENUM_VALUE_LAST
};

main()
{
    printf("Compiled on %s %s\n", __DATE__, __TIME__);
    unsigned int count = 0;
    for (
        ENUM_VALUE id = ENUM_VALUE_FIRST;
        id <= ENUM_VALUE_LAST;
        id = static_cast<ENUM_VALUE>(id+1)
    )
    {
        printf("id=%d\n", id);

        if (100 == count++)
        {
            // Should never get here.
            printf("ENUMERATION COMPARISON FAILED!\n");
            break;
        }
    }
}


-- 
           Summary: Enumeration with sequential values has its for-loop exit
                    condition optimized out.
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tony3 at GarlandConsulting dot us


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42810