By: Gabriele Svelto (gabriele.svelto.delete@this.gmail.com), July 6, 2015 2:11 am
Room: Moderated Discussions
Maynard Handley (name99.delete@this.name99.org) on July 5, 2015 6:01 pm wrote:
> Interestingly I did not see any OTHER fusion possibilities in the code. In particular the possibility
> IBM selected (fusing instructions to create a large immediate) is not utilized, which might, of course
> reflect not enough time to add this to the design; but maybe also reflects something about ARM's constant
> generation and so a less frequent need for generating immediates through successive instructions.
In POWER8's case instruction fusion applies to add immediate / add immediate shifted + loads which covers a weakness in the ISA which might not apply to ARM (lack of base + immediate addressing for certain types of load).
IBM has also been doing more aggressive fusion such as turning conditional-jump-over-a-single-instruction sequences into a single predicated µop (the instruction can be an add, and, or, xor plus some immediate forms as well as a store). See the POWER8 user manual section 10.1.4.7.
Beside being different ISAs, IBM doesn't have many constraints as far as power goes so the tradeoffs they're making might not apply to Apple's design.
> Interestingly I did not see any OTHER fusion possibilities in the code. In particular the possibility
> IBM selected (fusing instructions to create a large immediate) is not utilized, which might, of course
> reflect not enough time to add this to the design; but maybe also reflects something about ARM's constant
> generation and so a less frequent need for generating immediates through successive instructions.
In POWER8's case instruction fusion applies to add immediate / add immediate shifted + loads which covers a weakness in the ISA which might not apply to ARM (lack of base + immediate addressing for certain types of load).
IBM has also been doing more aggressive fusion such as turning conditional-jump-over-a-single-instruction sequences into a single predicated µop (the instruction can be an add, and, or, xor plus some immediate forms as well as a store). See the POWER8 user manual section 10.1.4.7.
Beside being different ISAs, IBM doesn't have many constraints as far as power goes so the tradeoffs they're making might not apply to Apple's design.
