Re:Intel's disruption is now complete
by cesarb on Saturday November 14, 2020 @04:08PM
(#25095638)
> Meanwhile with arm, you have constant instruction size (other than thumb, but that’s more straightforward than variable size),
That's only for 32-bit ARM; for 64-bit ARM, the instruction size is always constant (there's no Thumb/Thumb2/ThumbEE/etc). It won't surprise me at all if Apple's new ARM processor is 64-bit only (being 64-bit only is allowed for ARM, unlike x86), which means that not only the decoder does not have to worry about the 2-byte instructions from Thumb, but also the decoder does not have to worry about the older 32-bit ARM instructions (including the quirky ones like LDM/STM).
That would also explains why Apple can have wide decoders while their competitors can't: these competitors want to keep compatibility with 32-bit ARM, while Apple doesn't care.
That's only for 32-bit ARM; for 64-bit ARM, the instruction size is always constant (there's no Thumb/Thumb2/ThumbEE/etc). It won't surprise me at all if Apple's new ARM processor is 64-bit only (being 64-bit only is allowed for ARM, unlike x86), which means that not only the decoder does not have to worry about the 2-byte instructions from Thumb, but also the decoder does not have to worry about the older 32-bit ARM instructions (including the quirky ones like LDM/STM).
That would also explains why Apple can have wide decoders while their competitors can't: these competitors want to keep compatibility with 32-bit ARM, while Apple doesn't care.