Add a descriptive comment to coefficient reordering
@@ -400,6 +400,10 @@ void kvz_quant_flat_avx2(const encoder_state_t * const state, coeff_t * __restri
scan[subpos] + width * 3,
};
+ // NOTE: Upper means "higher in pixel order inside block", which implies
+ // lower addresses (note the difference: HIGH and LOW vs UPPER and LOWER),
+ // so upper 128b vector actually becomes the lower part of a 256-bit coeff
+ // vector and lower vector the higher part!
__m128d coefs_d_upper;
__m128d coefs_d_lower;
__m128d q_coefs_d_upper;
