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Comment by acegopher | Hacker News Reader

The same could be said of the ARM Cortex-M0+.

The Cortex-M0's Thumb-1 is a really unpleasant instruction set compared to ARM, Thumb-2, RISC-V, or ARM64.

Though no worse than 16 or 32 bit x86 (without FPU), and probably better because the lower 8 registers are general-purpose.

Also you can get something useful from the "spare" five registers r8-r12 as they support MOV, ADD and CMP with any other register, plus BX. Sadly you're on your own with PUSH/POP except for PUSH LR / POP PC.

Thumb-1 (or ARMv6-M) is fairly similar to RISC-V C extension. It's overall a bit more powerful because it has more opcodes available and because RVC dedicates some opcodes to floating point. RVC only lets you do MV and ADD on all 32 (or 16 in RV32) registers, not CMP (not that RISC-V has CMP anyway). Plus, RVC lets you load/store any register into the stack frame. Thumb-1 r8-r14 need to be copied to/from r0-r7 to load or store them.

But on the other hand, RVC is never present without the full-size 4 byte instructions, even on the $0.10 CH32V003, making that a bit more pleasant than the similar price Cortex M0 Puya PY32F002.

It is better to say RVC is almost never present without the full-size 4 byte instructions since we have one counter example:

https://github.com/gsmecher/minimax

This is an experimental rather than practical design that only directly implements the compressed instructions in hardware and then implements the normal RV32I instructions in "microcode" written using the compressed instructions.

Minimax is a super cool design! I think it's not really a counterexample, because it does implement the uncompressed instructions, just more slowly.

The LUT counts do look competitive, until you realise that this doesn't include the cost of the microcode.

Probably fine on FPGA where there's lots of almost free BRAM, but on an ASIC where you'd need to use SRAM or mask ROM, or if you used LUTRAM, it would look very different.

Plus, the speed penalty for the microcoded instructions is huge. perhaps not as huge as SeRV :-)

That sounds reasonable, yeah. Presumably you'd write your inner loops purely in RVC instructions; in the situations where you'd use SeRV, you wouldn't be using it for your computational bottlenecks, which you'd build special-purpose hardware for, but just to sort of orchestrate a sequence of steps. But Minimax seems like it could really reduce the amount of stuff you had to design special-purpose hardware for.

My initial experience with Thumb-1 was like stepping on a series of rakes. Can't use ADD? Why not? Oh, it turns out you have to use ADDS. Wait, why am I getting an error when I try to use ADDS? Turns out that inside an ITTE (etc.) block, you can't use ADDS; you have to use ADD. And the various other irregular restrictions on what you can express are similarly unpredictable. Maybe my gripe isn't really with Thumb-1 but with GAS, but even when you learn the restrictions, it still takes extra mental effort to program under them. I did have some similar experiences with 8086 code (it took me a certain amount of trial and error to learn which registers I could use as base registers and index registers, as I recall) but never 80386 code, where all of its registers are just as general-purpose as on Thumb-1, unless you're looking for sizecoding hacks to get your demo down under 64 bytes or whatever.

I agree that RVC is similar in theory, but being able to mix 4-byte instructions into your RVC code largely eliminates the stepping-on-rakes problem, even on Graham Smecher's redoubtable Minimax which Jecel Assumpção mentioned. I still prefer ARM assembly over RISC-V, but both definitely have their merits.

If you have ITTE (etc.) then you're not on Thumb-1 (e.g. ARM7TDMI) or ARMv6-M (Cortex M0+), you're on Thumb-2.

> but being able to mix 4-byte instructions into your RVC code largely eliminates the stepping-on-rakes problem

Absolutely, which is why I pointed out that no one (at least no one commercial) has ever implemented RVC alone, not even on the 10c CH32V003.

Oh, you're right, of course. I misremembered that rake. I stepped on some others I can't remember now, though.

I wouldn't be surprised to see commercial implementations of Minimax. It seems like it would have a much better cost/benefit ratio than SeRV for some applications.

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