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2016 Dec 26
1
Multiple simplifycfg pass make some loop significantly slower
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...