Question

I saw a sentence in a paper “Transforming branches into data dependencies to avoid mispredicted branches.” (Page 6)

I wonder how to change the code from branches into data dependencies.

This is the paper: http://www.adms-conf.org/p1-SCHLEGEL.pdf

Update: How to transform branches in the binary search?

Answer 1

The basic idea (I would presume) would be to change something like:

if (a>b)
    return "A is greater than B";
else
    return "A is less than or equal to B";

into:

static char const *strings[] = {
    "A is less than or equal to B", 
    "A is greater than B"
};

return strings[a>b];

For branches in a binary search, let’s consider the basic idea of the "normal" binary search, which typically looks (at least vaguely) like this:

while (left < right) {
    middle = (left + right)/2;
    if (search_for < array[middle])
        right = middle;
    else
        left = middle;
}

We could get rid of most of the branching here in pretty much the same way:

size_t positions[] = {left, right};

while (left < right) {
    size_t middle = (left + right)/2;

    positions[search_for >= array[middle]] = middle;
}

[For general purpose code use left + (right-left)/2 instead of (left+right)/2.]

We do still have the branching for the loop itself, of course, but we’re generally a lot less concerned about that–that branch is extremely amenable to prediction, so even if we did eliminate it, doing so would gain little as a rule.