What happens when I compile on machine that supports avx2 and run the binary on another machine that only supports avx?

Question

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?

I'm not sure why this question received a down vote. I searched but cannot find the answer. Where can we find the reason for down vote, so that we can improve in the future? — werk

Peter Cordes Peter Cordes · Accepted Answer · 2018-06-29T01:01:31

If the AVX2 instruction actually executed, 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 lder CPUs.

If the program never runs the function that contains the AVX2 instruction, then maybe it would with other inputs, or maybe it does runtime CPU detection.

What happens when I compile on machine that supports avx2 and run the binary on another machine that only supports avx?

1 Answers