[llvm-dev] Multiple simplifycfg pass make some loop significantly slower

[Siu Kwan Lam via llvm-dev]

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

-- 
Siu Kwan Lam
Software Engineer
Continuum Analytics
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