We've learned that if the compiler emits an aligned SSE memory move instruction for an unaligned address, it will cause a SEGV. Will the same occur with AVX? Or in the case of AVX is the extent of the resulting behavior amount to undesirable performance?
I think you answered the question yourself: aligned instructions require alignment!
In more detail:-
The Fine Manual gives details of the properties of each instruction, as does the online intrinsics guide; here you can see the properties of AVX load instructions. You will observe that there are both aligned and unaligned loads, for instance :-
__m256d _mm256_loadu_pd (double const * mem_addr)
Aligned VEX-encoded loads and stores (i.e. vmovdqa) still require aligned memory operands. However, memory operands for other VEX-encoded instructions (e.g. vpaddd) need not be aligned. You will still pay performance penalty for unaligned memory access though. Refer to Intel Software Developer Manual for the description of particular instructions.
@Tim P. AFAIK, legacy SSE instructions (i.e. non-VEX-encoded) haven't changed and still require aligned memory operands where they previously did. Only the VEX-encoded equivalents have relaxed requirements.
I addressed this recently in a different forum topic, but I can't find the reference right now....
In the beginning, SSE supported unaligned 128-bit loads/stores only via the MOVUPS instruction. All 128-bit memory references that were input arguments to other instructions were required to be 128-bit aligned to avoid a protection fault. In the earliest SSE systems, MOVAPS was faster, so it was preferred when the data was known to be aligned. Later systems eliminated the performance penalty of MOVUPS in the case where the data was aligned, so the compiler switched to generating MOVUPS even in the cases where it knew the data was aligned.
AVX relaxed the alignment restrictions for input arguments for both 128-bit and 256-bit loads. BUT, every generation of processor had different performance penalties for executing these memory references without natural alignment.