Load address calculation when using AVX2 gather instructions

Question

Looking at the AVX2 intrinsics documentation there are gathered load instructions such as VPGATHERDD:

__m128i _mm_i32gather_epi32 (int const * b         


        

Answer 1

Judging by the description in Intel's AVX programming reference document available here, it looks like the gather instructions use byte addressing. Specifically, see the following quotes from the description of the VPGATHERDD instruction (on page 389):

DISP: optional 1, 2, 4 byte displacement;
DATA_ADDR = BASE_ADDR + (SignExtend(VINDEX[i+31:i])*SCALE + DISP;

Since you can use 1/2/4 byte displacements, I would assume that the overall memory address is a byte address. While it may not be a common application, there could be cases where you would want to read a 32- or 64-bit value from a misaligned address. That's one of the more flexible things about the x86 architecture when compared to something like ARM; you have the flexibility to perform misaligned accesses if you want, instead of triggering a CPU exception as some others do.