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Got back into my SubV ( assembler / machine-code language modelled after @akkartik's SubX) codebase this weekend.

I figured out pretty quickly that what seemed like a terrible problem and caused me to essentially drop it a year ago is actually... not a problem? So that's good news :)

Had to stop for now, but long jumps (absolute/relative with lui/auipc+jalr) can be expressed now.

@s_ol What was the problem?!

Also, just in case you haven't looked at Mu code recently, here's what it looks like now: akkartik.github.io/mu/html/ex2

@akkartik
I still didn't arrive at Mu, but I'll try to catch up ;)

I was under the impression that the two-instruction relative long-jump aiupc+jalr could only work if SubV would recognize the two-instruction construct and generate the correct offsets in a "smart" way.

I think what I got wrong was that the PC is added in the "auipc" instruction, not when jalr is executed.

@akkartik

Now that I took another look I figure out that it can be expressed very cleanly without grouping opcoded and without any opcode-specific logic in "survey" by allowing offsets in label-references:

# 32-bit relative jump
# load upper 20 bits and add to PC
auipc 5/rd/t0 label[31:12]/off32
# jump, adding lower 12 bits
jalr 0/rd/x0 5/rb/t0 label+4[11:0]/off32

the +4 is to account for the fact that PC was "captured" in the auipc instruction

@akkartik
(typing this blind on my phone, so surely got some details wrong and the opcode instruction bits are missing. Also jalr's immediate argument is actually the [11:1] slice plus a sign bit)

I want to make it so you can optionally specify the exact slices as above, or just write label/off32 and it will get the slices right automatically (including sign extension)

@s_ol Yeah that aiupc construct is a bit strange: stackoverflow.com/questions/55

Even outside of our weird notations 🙂 I'm struggling to understand how a compiler would use it. Are they expecting that functions are aligned at 4KB boundaries or something?

Oh, this is only for far jumps. So that alignment constraint might actually be a reasonable choice.

@akkartik
yeah, auipc is only used to "prime" registers for operations that have an immediate argument with limited range and can use a register input to extend that.

My hangup was that in my mind a "relative jump" was always going to be relative to the jump instruction, which is not true in the case of auipc+jalr; jalr by nature is an absolute jump and using auipc you can get the absolute address relative to *that* instruction.

@akkartik
jalr actual also has a 2-byte alignment constraint on the lower immediate, but that is reasonable because instructions are either two or four byte wide.

@akkartik btw it's very interesting to see you and the person who answered in the SO question to also misspell auipc as aiupc, which I have been struggling with a lot also!

I wonder what it is about those three vowels that makes it hard to remember the right way? To my german brain, "aui" should actually be more familiar ("au" is a bigram with a specific sound, and aui is pronouncable) whereas aiu is just letter soup.

@s_ol Oh interesting! I see in my browser history that I actually searched for 'aiupc', which is probably why I got that page.

Thanks for pointing it out. I get annoyed when people spell Gandhi as Ghandi 🙂

@akkartik @s_ol the trick to remember all this gibberish is to read it by their meaning: add upper immediate to program counter.
I force myself to use the whole meaning because instructions like slli or bqez get complicated to read if you don't have it written with fire in your mind.

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