[Bug middle-end/87528] Popcount changes caused 531.deepsjeng_r run-time regression on Skylake

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87528

--- Comment #5 from Martin Jambor <jamborm at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> Can you point me to the source for which we generate the popcount call(s)? 
> It might be not final value replacement but instead code-generating a niter
> analysis result.

It turns out it is as simple as:

----------------------------------------
typedef unsigned long long BITBOARD;

int PopCount (BITBOARD b) {
    int c = 0;

    while (b) {
        b &= b - 1;
        c++;
    }

    return c;
}
----------------------------------------
with -O3.  Trunk generates a call to __popcountdi2 when GCC 7 (or
trunk with -march=skylake) does not.