How to make emscripten to generate better code

Question

I have read lots about how fast Emscripten code can be, thus I decided to convert my js application to C++ and then compile to Emscripten. I got about +30% speed increase and it's not the thing I expected! (I thought it will be at least 5x speed increase) So I decided to check the compiled code and I saw this strange thing!

This is an original method code (C++, but it looks almost the same in the original "slow" JS application).

void Op2B()
{
    sm = (sm - 1) & 65535;
    t->time += 2;
}

This is an emscripten-compiled asm.js version of the same code

function __ZN16EpsilonGames_VGS3Z804Op2BEv($0) {
    $0 = $0|0;
    var $1 = 0, $10 = 0, $2 = 0, $3 = 0, $4 = 0, $5 = 0, $6 = 0, $7 = 0, $8 = 0, $9 = 0, label = 0, sp = 0;
    sp = STACKTOP;
    STACKTOP = STACKTOP + 16|0; if ((STACKTOP|0) >= (STACK_MAX|0)) abortStackOverflow(16|0);
    $1 = $0;
    $2 = $1;
    $3 = ((($2)) + 24|0);
    $4 = load4($3);
    $5 = (($4) - 1)|0;
    $6 = $5 & 65535;
    $7 = ((($2)) + 24|0);
    store4($7,$6);
    $8 = load4($2);
    $9 = load4($8);
    $10 = (($9) + 2)|0;
    store4($8,$10);
    STACKTOP = sp;return;
}

As you can see, It's far away from being optimal.

Is there any possibility to make this code a bit more optimal? I still hope to get at least x5 increase in speed.

Thanks!