3 ms·
Mostly. ARM actually has instructions that allow a full 64-bit shift then add. But I checked the Cortex A78 optimisation manual. They take 1 cycle if the shift
by phire 2y ago
Mostly. ARM actually has instructions that allow a full 64-bit shift then add.
But I checked the Cortex A78 optimisation manual. They take 1 cycle if the shift is 4 or less and 2 cycles in other cases.
- brucehoult 2y ago0-4 shift not 0-3? That is a little bit weird.
- phire 2y agoArm64 has fast 128-bit loads. Not just with NEON, but with regular integer instructions, you can quickly load 128 bits into a pair of 64-bit registers. So it kind of makes sense to support fast shift by four. Though, it's more likely they just profiled a bunch of code and decided fast shifts by four was worth budgeting for.