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

[Hal Finkel via llvm-dev]

On 12/26/2016 01:54 PM, Siu Kwan Lam via llvm-dev wrote:
> 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?

Can you please file a bug report for this (https://llvm.org/bugs/), 
attaching the IR? I suspect we'll need to look at the generated code.

  -Hal

>
> Regards,
> Siu Kwan Lam
>
> -- 
> Siu Kwan Lam
> Software Engineer
> Continuum Analytics
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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