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It could be added with an extension, but software would continue to be compiled without it. For many, many years.
by pritambaral 3y ago
It could be added with an extension, but software would continue to be compiled without it. For many, many years.
- stavros 3y agoSure, but that's fine, no? Better late than never.
- pritambaral 3y agoNow that op fusion has been implemented for this idiom, the cost of a new instruction does not seem worth the benefit. The benefit would be only a tiny improvement in code size, and that's restricted to when the compiler can be absolutely certain that the target CPU is going to be modern enough to support it. So, in practice, for many, many years, this benefit would be almost never reaped. This is not a new topic in the design of ISAs and their implementations in silicon. Idiom recognition and optimization has long existed in x86, ARM, and I'm sure other old ISA implementations. RISC-V can avoid it by adding dedicated instructions because it's just getting started (relatively speaking), but even so, I'm sure there will be opportunities for idiom recognition and optimization that RISC-V implementers will take.
- snvzz 3y ago>Idiom recognition and optimization On RISC-V's fusion idioms, it's not just recognition; they're designed in. Compilers are meant to try and produce them reliably, as if they were a dedicated single opcode.
- snvzz 3y agoThere's also the issue of code size. x86 encoding space already has a lot of clutter in it. The cost of low code density is that less code fits in cache. This cost is paid in performance, power, area or a combination of them.