I compiled my c++ program on a machine that supports avx2 (Intel E5-2643 V3). It compiles and runs just fine. I confirm the avx2 instruction is used since after I dissemble the binary, I saw avx2 instructions such as vpbroadcastd.
Then I run this binary on another machine that only has avx instruction set (Intel E5-2643 V2). It runs also fine. Does the binary runs on a backward compatible avx instruction instead? What is this instruction? Do you see any potential issue?
It's not rare to have AVX2 instructions in a binary that uses CPU detection to make sure it only runs them on CPUs that support them. (e.g. via cpuid and setting function pointers).
If the AVX2 instruction actually executed on a CPU without AVX2 support, it raises #UD, so the OS delivers SIGILL (illegal instruction) to your process, or the Windows equivalent.
There are a few cases where an instruction like lzcnt decodes as rep bsr, which runs as bsr on CPUs without BMI1. (Giving a different answer). But VEX-coded AVX2 instructions just fault on older CPUs.