c++ - Code alignment dramatically affects performance

Question

Today I have found sample code which slowed down by 50%, after adding some unrelated code. After debugging I have figured out the problem was in the loop alignment. Depending of the loop code placement there is different execution time e.g.:

Address	Time[us]
00007FF780A01270	980us
00007FF7750B1280	1500us
00007FF7750B1290	986us
00007FF7750B12A0	1500us

Answer 1

In the slow cases (i.e., 00007FF7750B1280 and 00007FF7750B12A0), the jne instruction crosses a 32-byte boundary. The mitigations for the "Jump Conditional Code" (JCC) erratum (https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf) prevent such instructions from being cached in the DSB. The JCC erratum only applies to Skylake-based CPUs, which is why the effect does not occur on your i5-3570k CPU.

As Peter Cordes pointed out in a comment, recent compilers have options that try to mitigate this effect. Intel JCC Erratum - should JCC really be treated separately? mentions MSVC's /QIntel-jcc-erratum option; another related question is How can I mitigate the impact of the Intel jcc erratum on gcc?