[Bug target/59371] [4.8/4.9/5 Regression] Performance regression in GCC 4.8 and later versions.

[sje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59371

--- Comment #15 from Steve Ellcey <sje at gcc dot gnu.org> ---
I am not sure yet where and how to improve this automatically but I have found
an interesting hand optimization that could point to a way to fix this.  If I
change the original function:

int foo(int *p, unsigned short c)
{
    signed short i;
    int x = 0;
    for (i = 0; i < c; i++) {
        x = x + *p; p++;
    }
    return x;
}

To:

int foo(int *p, unsigned short c)
{
    signed short i;
    unsigned short new_i;
    int x = 0;

    if (c > 32767)
        for (i = 0; i < c; i++) {
            x = x + *p; p++;
        }
    else
        for (new_i = 0; new_i < c; new_i++) {
            x = x + *p; p++;
        }
    return x;
}


Then GCC 5.0 generates an empty infinite loop for the first for loop and
a compact 4 instruction loop (better even then 4.7) for the second for loop.

I am not sure where or if we can do this optimization in GCC but I am going
to investigate some more.