[llvm-bugs] [Bug 24402] New: Loop is incorrectly optimized away

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https://llvm.org/bugs/show_bug.cgi?id=24402

            Bug ID: 24402
           Summary: Loop is incorrectly optimized away
           Product: clang
           Version: 3.6
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bugz at madler.net
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The C code below gives different results with -O0/-O1 vs. -O2/-O3.  -O1 gives
the correct result, whereas -O2 gives an incorrect result.  See the comments in
the code.

/*
    Example of a clang optimization bug.
    Mark Adler, August 8, 2015.

    Using -O0 or -O1 takes a little while and gives the correct result:

        47 bits set (4294967296 loops)

    Using -O2 or -O3 optimizes out the loop, returning immediately with:

        0 bits set (4294967296 loops)

    Of course, there weren't really that many loops.  The number of loops was
    calculated, correctly, by the compiler when optimizing.  But it got the
    number of bits set wrong.

    This is with:

        Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
        Target: x86_64-apple-darwin14.4.0

 */

#include <stdio.h>
#include <inttypes.h>

/* bit vector of 1<<32 bits, initialized to all zeros */
static uint64_t vec[1 << 26] = {0};

int main(void)
{
    /* set 47 of the bits. */
    vec[31415927] = UINT64_C(0xb9fe2f2fedf7ebbd);

    /* count the set bits */
    uint64_t count = 0;
    uint64_t loops = 0;
    uint32_t x = 0;
    do {
        if (vec[x >> 6] & ((uint64_t)1 << (x & 0x3f)))
            count++;
        x++;
        loops++;
    } while (x);
    printf("%" PRIu64 " bits set (%" PRIu64 " loops)\n", count, loops);
    return 0;
}

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