[Bug tree-optimization/61304] New: Missed vectorization: control flow in loop

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61304

            Bug ID: 61304
           Summary: Missed vectorization: control flow in loop
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
            Target: x86_64-linux-gnu

(taken from a stackoverflow question about a bug in llvm, replace -1 with -2 if
you want to test llvm and avoid the bug)

gcc -O3 fails to vectorize the following program because it sees control flow
in the loop. If I move i++ before the "if", which becomes i == 0, we still fail
to vectorize because we get confused about the number of iterations. Finally,
if I stop at i == 2048, we do vectorize, but the generated code could do with
some improvements (that would be for a different PR though).

#include <stdint.h>
#include <string.h>

int main()
{
    uint32_t i = 0;
    uint32_t count = 0;

    while (1)
    {
        float n;
        memcpy(&n, &i, sizeof(float));
        if(n >= 0.0f && n <= 1.0f)
            count++;
        if (i == -1)
            break;
        i++;
    }

    return count;
}