Hi all,
I am noticing a significant degradation in execution performance in loops with just one backedge than loops with two backedges. Unifying the backedges into one will also cause the slowdown.
To replicate this problem, I used the C code in https://gist.github.com/sklam/11f11a410258ca191e6f263262a4ea65 and checked against clang-3.8 and clang-4.0 nightly. Depending on where I put the “increment” code for a for-loop, I can get 2x performance difference.
The slow (but natural) version:
for (i=0; i<size; ++i) {
ai = arr[i];
if ( ai <= amin ) {
amin = ai;
all_missing = 0;
}
}
The fast version:
for (i=0; i<size;) {
ai = arr[i];
++i; // increment moved here
if ( ai <= amin ) {
amin = ai;
all_missing = 0;
}
}
With the fast version, adding a dummy line after the if-block will make the code slow again:
for (i=0; i<size;) {
ai = arr[i];
++i;
if ( ai <= amin ) {
amin = ai;
all_missing = 0;
}
i; // no effect
}
At first, I noticed the problem with any opt level >= O1. In an attempt to narrow it down, I found that using opt -simplifycfg -sroa -simplifycfg will trigger the slowdown. Removing the second simplifycfg solves it and both versions of the code run fast.
Is there a known issue for this? Or, any idea why?
Regards,
Siu Kwan Lam