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I carefully studied the 2.1 and 2.2 versions of the spec. I wrote, in Racket, a miniature interpreter for lists of RISC-V instructions (just symbolic lists, no

I carefully studied the 2.1 and 2.2 versions of the spec. I wrote, in Racket, a miniature interpreter for lists of RISC-V instructions (just symbolic lists, not the byte strings they assemble into), then wrote programs to compute triangular numbers and Fibonacci, and verified they worked. I've built the RISC-V toolchain and compiled a few test C programs and run them in "spike". I don't think I took more than a glance at the "gcc -S" output. I have no familiarity with SPARC or Motorola assembly, but I have decent familiarity with x86-64 assembly, having written what looks like a couple dozen miniature test programs and one medium-sized program, and having tried a few times to write a decent x86-64 assembler. I found the RISC-V spec enjoyable to read, especially with all the rationales; it felt clean, minimal, and well-thought-out. Writing a program in it was probably not much different from writing in x86-64 assembly. The encodings seemed much easier to deal with than x86-64, though as mentioned I didn't go as far as writing an encoder. What kinds of important features are present in SPARC or MC68k but not in RISC-V? Are they absent from x86-64 as well?

Annatar 8y ago
Motorola assembler is what is known as orthogonal instruction set. The flow is logical: move.b for byte, move.w for word, move.l for longword, from source to destination, which reflects real life. The assembler reads almost like a high level programming language. The register scheme is intuitive as well, from a0-a7 being the address, to d0-d7 being the data registers. Now, let's do a mental exercise: I'm going to load an effective address relative to the program counter, 32-bits wide, into the 1st address register. Then, I'm going to load an arbitrary value from an arbitrary memory location into the second address register. Do the same in RISC-V; compare intuitiveness. lea MemoryAddress(pc), a0 move.l $00bfe001, a1 rts MemoryAddress: DC.l 0
- waterhouse 8y ago
  Indeed, in RISC-V, full 32-bit constants must be broken across two instructions. The translation goes: ; 1. lea MemoryAddress(pc), a0 auipc a0, [upper 20 bits of (MemoryAddress - label)] addi a0, a0, [lower 12 bits of (MemoryAddress - label)] label: ; 2. move.l #$00bfe001, a1 lui a1, 0x00bfe000 addi a1, a1, 0x001 ; 3. rts jalr x0, x1, 0 It is cumbersome in that sense. But it will probably be handled by an assembler (as a macro or builtin). The spec contains an appendix of "pseudoinstructions", which gives the first translation above. There isn't even a dedicated "move" instruction—it's just "addi dst, src, 0" by convention! Clearly anyone writing assembly will use at least that pseudoinstruction. If there's a canonical format for "pseudoinstructions", and all assemblers handle them in the same way, and the abstraction doesn't leak in any way (i.e. the only temporary registers you use are ones you overwrite fully by the end; it is true that now some "instructions" have longer encodings, but that comes with the compressed instructions anyway; and it is true that an interrupt could happen between the two halves of the "instruction", but I think that shouldn't make a difference), then I don't think there's much of a problem.
  - Annatar 8y ago
    "Indeed, in RISC-V, full 32-bit constants must be broken across two instructions." Any RISC processor has that issue, because encoding is fixed at 32-bits to keep the hardware simple. That's not what I'm referring to. Look at that retardation, "auipc". Because "auipc" is intuitive, right? (For the record, I'm being sarcastic.) What the hell was the instruction designer smoking? Then there is the square bracket notation, like on intel, and intel has some of the most idiotic, non-conventional assembler syntax -- and this thing mimics something so bad? Every other processor uses parenthesis, that's a well understood norm. Then there is more intel retardation in the form of dst, src (or dst, src, src on RISC). What kind of a warped, twisted mind came up with that? What was going on in that person's head to work in such twisted ways? Then there's the "cherry on top": jalr x0, x1, 0 ; because "jalr" is intuitive as well, it immediately tells you what it does? You'll know a bad processor design by its instruction set. Always and forever.
    - waterhouse 8y ago
      All right, I count myself disappointed. Surely you know that "processors" don't use parentheses; the parentheses and brackets are in assembler syntax, which is long gone by the time the processor is executing byte sequences. There is nothing inherent about Intel-style syntax in physical RISC-V processors; nor in Intel processors, for that matter, as demonstrated by the fact that gcc outputs its x86-64 assembly in AT&T syntax by default. The fact that Intel syntax appears in the RISC-V spec might, at the very worst, demonstrate that the creators have bad taste in that area. If that taste has led to bad decisions elsewhere, it would be more substantive to point out those bad decisions. If not, you can configure your toolchain to use AT&T syntax and take pride in your superiority. Bad names are harder to work around. However, about those names: "auipc" is certainly letter salad, but (a) it stands for "Add Upper Immediate to Program Counter", and its effect is to add an immediate value (multiplied by 2^12) to the program counter and put the result in a register, so the name is entirely logical given its task; and (b) except in the rare case where the PC-relative offset is exactly a multiple of 2^12, an AUIPC will be immediately followed by an add (to load a full PC-relative address), or perhaps a load-with-offset (to load a value at a full PC-relative address), or a jump-with-offset (to jump to a label at a full PC-relative address); and all three of these AUIPC+(add|load|jump) combinations are given as pseudoinstructions as well (e.g. "la" for "load address"), so the programmer will probably never need to write a bare "auipc". As for "jalr", well, it stands for "jump and link register", which jumps to the "register" argument and stores the return address in the "link" argument. There's a set of pseudo-instructions based off x0 being the "always-zero" register and x1 being the conventional "return address" register: Pseudo Real Description j offset jal x0, offset Jump jal offset jal x1, offset Jump and link jr rs jalr x0, rs, 0 Jump register jalr rs jalr x1, rs, 0 Jump and link register ret jalr x0, x1, 0 Return from subroutine Seems logical enough to me. You can just use "jal offset" and "ret" if you prefer. Anyway, if your goal was to convince me there are serious flaws in RISC-V, then criticizing the naming conventions and surface syntax has led me away from the hypothesis that you know any.
brandmeyer 8y ago
There are a few instructions that nearly every modern processor ended up adopting, but RISC-V still lacks. Off the top of my head, (with the RV ISA developers response in parenthesis): - bitwise rotate (let them eat macro-op fusion) - byte and bit swapping (strictly missing from RV, although proposals exist) - leading zero count, trailing zero count, and popcount (strictly missing from RV, proposals exist) - efficient multiword arithmetic (let them eat macro-op fusion, or long dependency chains) - base + [scaled] index addressing modes (you don't really need those) - multi-register save/restore instructions (ARMv8 doesn't have them / RVC is equivalent in density to ARMv8, nanoMIPS is irrelevant, let them eat millicode) So yeah, there are deficiencies. None of them are crippling, but I wouldn't say that RV is super-wham-o-dyne, either.

Annatar 8y ago

Motorola assembler is what is known as orthogonal instruction set. The flow is logical: move.b for byte, move.w for word, move.l for longword, from source to destination, which reflects real life. The assembler reads almost like a high level programming language. The register scheme is intuitive as well, from a0-a7 being the address, to d0-d7 being the data registers. Now, let's do a mental exercise: I'm going to load an effective address relative to the program counter, 32-bits wide, into the 1st address register. Then, I'm going to load an arbitrary value from an arbitrary memory location into the second address register. Do the same in RISC-V; compare intuitiveness. lea MemoryAddress(pc), a0 move.l $00bfe001, a1 rts MemoryAddress: DC.l 0

waterhouse 8y ago
Indeed, in RISC-V, full 32-bit constants must be broken across two instructions. The translation goes: ; 1. lea MemoryAddress(pc), a0 auipc a0, [upper 20 bits of (MemoryAddress - label)] addi a0, a0, [lower 12 bits of (MemoryAddress - label)] label: ; 2. move.l #$00bfe001, a1 lui a1, 0x00bfe000 addi a1, a1, 0x001 ; 3. rts jalr x0, x1, 0 It is cumbersome in that sense. But it will probably be handled by an assembler (as a macro or builtin). The spec contains an appendix of "pseudoinstructions", which gives the first translation above. There isn't even a dedicated "move" instruction—it's just "addi dst, src, 0" by convention! Clearly anyone writing assembly will use at least that pseudoinstruction. If there's a canonical format for "pseudoinstructions", and all assemblers handle them in the same way, and the abstraction doesn't leak in any way (i.e. the only temporary registers you use are ones you overwrite fully by the end; it is true that now some "instructions" have longer encodings, but that comes with the compressed instructions anyway; and it is true that an interrupt could happen between the two halves of the "instruction", but I think that shouldn't make a difference), then I don't think there's much of a problem.
- Annatar 8y ago
  "Indeed, in RISC-V, full 32-bit constants must be broken across two instructions." Any RISC processor has that issue, because encoding is fixed at 32-bits to keep the hardware simple. That's not what I'm referring to. Look at that retardation, "auipc". Because "auipc" is intuitive, right? (For the record, I'm being sarcastic.) What the hell was the instruction designer smoking? Then there is the square bracket notation, like on intel, and intel has some of the most idiotic, non-conventional assembler syntax -- and this thing mimics something so bad? Every other processor uses parenthesis, that's a well understood norm. Then there is more intel retardation in the form of dst, src (or dst, src, src on RISC). What kind of a warped, twisted mind came up with that? What was going on in that person's head to work in such twisted ways? Then there's the "cherry on top": jalr x0, x1, 0 ; because "jalr" is intuitive as well, it immediately tells you what it does? You'll know a bad processor design by its instruction set. Always and forever.
  - waterhouse 8y ago
    All right, I count myself disappointed. Surely you know that "processors" don't use parentheses; the parentheses and brackets are in assembler syntax, which is long gone by the time the processor is executing byte sequences. There is nothing inherent about Intel-style syntax in physical RISC-V processors; nor in Intel processors, for that matter, as demonstrated by the fact that gcc outputs its x86-64 assembly in AT&T syntax by default. The fact that Intel syntax appears in the RISC-V spec might, at the very worst, demonstrate that the creators have bad taste in that area. If that taste has led to bad decisions elsewhere, it would be more substantive to point out those bad decisions. If not, you can configure your toolchain to use AT&T syntax and take pride in your superiority. Bad names are harder to work around. However, about those names: "auipc" is certainly letter salad, but (a) it stands for "Add Upper Immediate to Program Counter", and its effect is to add an immediate value (multiplied by 2^12) to the program counter and put the result in a register, so the name is entirely logical given its task; and (b) except in the rare case where the PC-relative offset is exactly a multiple of 2^12, an AUIPC will be immediately followed by an add (to load a full PC-relative address), or perhaps a load-with-offset (to load a value at a full PC-relative address), or a jump-with-offset (to jump to a label at a full PC-relative address); and all three of these AUIPC+(add|load|jump) combinations are given as pseudoinstructions as well (e.g. "la" for "load address"), so the programmer will probably never need to write a bare "auipc". As for "jalr", well, it stands for "jump and link register", which jumps to the "register" argument and stores the return address in the "link" argument. There's a set of pseudo-instructions based off x0 being the "always-zero" register and x1 being the conventional "return address" register: Pseudo Real Description j offset jal x0, offset Jump jal offset jal x1, offset Jump and link jr rs jalr x0, rs, 0 Jump register jalr rs jalr x1, rs, 0 Jump and link register ret jalr x0, x1, 0 Return from subroutine Seems logical enough to me. You can just use "jal offset" and "ret" if you prefer. Anyway, if your goal was to convince me there are serious flaws in RISC-V, then criticizing the naming conventions and surface syntax has led me away from the hypothesis that you know any.

brandmeyer 8y ago

There are a few instructions that nearly every modern processor ended up adopting, but RISC-V still lacks. Off the top of my head, (with the RV ISA developers response in parenthesis): - bitwise rotate (let them eat macro-op fusion) - byte and bit swapping (strictly missing from RV, although proposals exist) - leading zero count, trailing zero count, and popcount (strictly missing from RV, proposals exist) - efficient multiword arithmetic (let them eat macro-op fusion, or long dependency chains) - base + [scaled] index addressing modes (you don't really need those) - multi-register save/restore instructions (ARMv8 doesn't have them / RVC is equivalent in density to ARMv8, nanoMIPS is irrelevant, let them eat millicode) So yeah, there are deficiencies. None of them are crippling, but I wouldn't say that RV is super-wham-o-dyne, either.