[llvm-dev] clang performing worse than gcc for this loop

Riyaz Puthiyapurayil via llvm-dev llvm-dev at lists.llvm.org
Sun Aug 23 10:53:02 PDT 2020

I am analyzing a clang 10.0.0 vs gcc 7.3 performance difference that I can reproduce in the following test.

unsigned foo(unsigned t1, unsigned t2, int count, int step) {
    unsigned tmp = 0;
    int state = 0;
   for (int i = 0 ; i < count ; i += step) {
        if (state > 5)
            state = 0;
        if (state == 3)
            tmp += t2;
    return  tmp;

Clang output is about 40% slower when the function is called with t2=7, count=2000000000, step=3 (t1 is unimportant in this case as it is unused here). The attached screenshot shows the `perf report` annotated assembly code from clang and gcc (clang is on the left). Gcc generated code takes 0.512 sec vs clang's 0.731 sec. The machine I am running is a Broadwell... Intel(R) Xeon(R) CPU E5-2640 v4 @ 2.40GHz.

The code generated by gcc runs consistently faster for all values for `step` I tried; in some cases, the performance difference is worse than 40% seen with the aforementioned parameter values to `foo`. The code generated by clang is a direct result of simplifycfg that eliminates the inner branches and replaces them with `select` which is then lowered to the two `cmov` instructions.

The code generated by clang takes far fewer branches but executes more instructions. `perf` reports 32.76% front-end cycles idle with the clang code compared to 24.20% for gcc generated code. Clang generated code seems to perform worse in branch-miss and icache events (as reported by `perf`). But it is not clear why. Are the two back-to-back cmove instructions the reason? Any comments on this?

[cid:image002.png at 01D67897.72235000]

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