[__s]

yes, was curious why compression format didn't require

1. non compressed instructions are always 4 byte aligned (pad a 2 byte NOP if necessary, or use uncompressed 4 byte instruction to fix sizing)

2. jump targets are always 4 byte aligned (which exists without C, but C relaxes)

This avoids cache line issues & avoids jumps landing inside an instruction. Can consider each 2 compressed instructions as a single 4 byte instruction

Bit redundant to encode C prefix twice, so there's room to make use of that (take up less encoding space at least by having prefix be 2x as long), but not important

[fwsgonzo]

I completely agree. Not that everything has to be relaxed, but at least the things that made it impossible to decode RISC-V when C is enabled. The amount of code needed to detect when and how instructions are laid out is much larger than it should be.

[brucehoult]

"impossible"?

It's a little easier than ARMv7, and orders of magnitude easier than x86, which doesn't seem to be preventing high performance x86 CPUs (at an energy use and surely silicon size penalty admittedly).

Everyone else in the RISC-V community except Qualcomm thinks the "C" extension is a good trade-off, and the reason Qualcomm don't is very likely because they're adapting Nuvia's Arm core to run RISC-V code instead, and of course that was designed for 4-byte instructions only.

[Pet_Ant]

That is a trade-off towards density that seems not worth it where all it would take is a 16 bit NOP to pad and a few more bytes of memory to save on transistors of implementation.

Maybe they did the actual math and figured it's still cheaper? Might be worth it.

[__s]

SiFive slides: https://s3-us-west-1.amazonaws.com/groupsioattachments/10389...

Their argument is that since eventually there'll be 8 byte instructions, those will have the same cache line issues (tho that could be addressed by requiring 8 byte instructions be 8 byte aligned)

[Pet_Ant]

Check your link? It isn't working for me.

[__s]

https://lists.riscv.org/g/tech-profiles/topic/slides_on_reta...