inline assembly improvements

[andrewrk]

This inline assembly does exit(0) on x86_64 linux:

    asm volatile ("syscall"
        : [ret] "={rax}" (-> usize)
        : [number] "{rax}" (60),
            [arg1] "{rdi}" (0)
        : "rcx", "r11");

Here are some flaws:

• 60 and 0 are number literals and need to be casted to a type to be valid. This causes an assertion failure in the compiler if you don't cast the number literals. Assembly syntax should include types for inputs.
• [number], [arg1], [ret] unused, and that is awkward.
• need multiple return values (see multiple expression return values, error type redesign, introduction of copyable property of types #83)
• do we really need this complicated restraint syntax? maybe we can operate on inputs and outputs.
• let's go digging into some real world inline assembly code to see the use cases.
• when we get errors from parsing assembly, we don't attach them to the offset from within the assembly string. connect inline assembly errors from zig back to the source #2080

[andrewrk]

One idea:

const result = asm volatile ("rax" number: usize, "rdi" arg1: usize, "rcx", "r11")
    -> ("rax" ret: usize)  "syscall" (60, 0);

This shuffles the syntax around and makes it more like a function call. Clobbers are extra "inputs" that don't have a name and a type. The register names are still clunky.

This proposal also operates on the assumption that all inline assembly can operate on inputs and outputs.

[andrewrk]

@ofelas can I get your opinion on this proposal?

[ofelas]

Right, you really made me thinkg here, haven't done that much asm in zig yet, here are a few that I've used on x86, they primarily struggle with the issue of multiple return values, the below examples may not be correct, I always end up spending some time reading the GCC manuals when doing inline asm in C, it isn't always straight forwards.

I just skimmed through the discussion over at Rust users and Rust inline assembly, they seem to have similar discussions and it seems that the asm feature may not be used that much. If you really need highly optimized or complex asm wouldn't you break out to asm (or possibly llvm ir)?

I guess what we have to play with is what LLVM provides, at least as long as zig has a tight connection to it (It seems there are discussions on also supporting Cretonne in Rust according to the LLVM Weekly).

With the above proposal would I write the PPC eieio (and isync, sync) like this _ = asm volatile () -> () "eieio" (); and old style _ = asm volatile ("eieio");? This may typically be available as an intrinsic barrier, I guess. Think I read somewhere that the _ would be the same as Nims discard, it may not be needed as this asm didn't return anything.

Not sure I answered you question...

inline fn rdtsc() -> u64 {
    var low: u32 = undefined;
    var high: u32 = undefined;
    // ouput in eax and edx, could probably movl edx, fingers x'ed...
    low = asm volatile ("rdtsc" : [low] "={eax}" (-> u32));
    high = asm volatile ("movl %%edx,%[high]" : [high] "=r" (-> u32)); 
    ((u64(high) << 32) | (u64(low)))
}

The above obviously is a kludge, I initially hoped to write it that more like this, it does however feel strange having to specify the outputs twice, both lhs and inside the asm outputs, with the potential of mixing the order which may be important.

inline fn rdtsc() -> u64 {
    // ouput in eax and edx
    var low: u32 = undefined;
    var high:u32 = undefined;
    low, high = asm
        // no sideeffects
        ("rdtsc"
         : [low] "={eax}" (-> u32), [high] "={edx}" (-> u32)
         : // No inputs
         : // No clobbers
         );
    ((u64(high) << 32) | (u64(low)))
}

Or possibly like this, not having to undefined/zeroes/0 the output only parameters;

inline fn rdtsc() -> u64 {
    // ouput in eax and edx
    const (low: u32, high: u32) = asm
        // no sideeffects
        ("rdtsc"
         : [low] "={eax}" (-> u32), [high] "={edx}" (-> u32)
         : // No inputs
         : // No clobbers
         );
    ((u64(high) << 32) | (u64(low)))
}

I've also tinkered with the cpuid instruction which is particularly nasty;

inline fn cpuid(f: u32) -> u32 {
    // See: https://en.wikipedia.org/wiki/CPUID, there's a boatload of variations...
    var id: u32 = 0;
    if (f == 0) {
        // Multiple outputs (as an ASCII string) which we mark as clobbered and just leave untouched
        return asm volatile ("cpuid" : [id] "={eax}" (-> u32): [eax] "{eax}" (f) : "ebx", "ecx", "edx");
    } else {
        return asm volatile ("cpuid" : [id] "={eax}" (-> u32): [eax] "{eax}" (f));
    }
}

[andrewrk]

With the proposal, rdtsc would look like this in zig:

fn rdtsc() -> u64 {
    const low, const high = asm () -> ("eax" low: u32, "edx" high: u32) "rdtsc" ();
    ((u64(high) << 32) | (u64(low)))
}

This seems like an improvement.

cpuid with the proposal. I propose that instead of naming the function after the assembly instruction, we name it after the information we want. So let's choose one of the use cases, get vendor id.

fn vendorId() -> (result: [12]u8) {
    const a: &u32 = (&u32)(&result[0 * @sizeOf(u32)]);
    const b: &u32 = (&u32)(&result[1 * @sizeOf(u32)]);
    const c: &u32 = (&u32)(&result[2 * @sizeOf(u32)]);
   *a, *b, *c = asm () -> ("ebx" a: u32, "ecx" b: u32, "edx" c: u32) "cpuid" ();
}

Once again volatile not necessary here. cpuid doesn't have side effects, we only want to extract information from the assembly.

So far, so good. Any more use cases?

[ofelas]

Yes, that ain't too shabby, so with the correct input in eax it is;

fn vendorId() -> (result: [12]u8) {
    const a: &u32 = (&u32)(&result[0 * @sizeOf(u32)]);
    const b: &u32 = (&u32)(&result[1 * @sizeOf(u32)]);
    const c: &u32 = (&u32)(&result[2 * @sizeOf(u32)]);
   // in eax=0, out: eax=max accepted eax value(clobbered/ignored), string in ebx, ecx, edx
   *a, *b, *c = asm ("eax" func: u32) -> ("ebx" a: u32, "ecx" b: u32, "edx" c: u32, "eax") "cpuid" (0);
}

Would something like this be possible, ignoring my formatting?

result = asm ( // inputs
        "=r" cnt: usize = count,
        "=r" lhs: usize = &left,
        "=r" rhs: usize = &right,
        "=r" res: u8 = result,
        // clobbers
        "al", "rcx", "cc")
        -> ( // outputs
        "=r" res)
        // multiline asm string
        \\movq %[count], %rcx
        \\1:
        \\movb -1(%[lhs], %rcx, 1), %al
        \\xorb -1(%[rhs], %rcx, 1), %al
        \\orb %al, %[res]
        \\decq %rcx
        \\jnz 1b
        // args/parameters
        (count, &left, &right, result);

[andrewrk]

Yes, that ain't too shabby, so with the correct input in eax it is;

Ah right, nice catch.

I like putting the values of the inputs above as you did. Then we don't need them below.

Is the count arg necessary to have the movq instruction? seems like we could pass that as a register.

And then finally result should be an output instead of an input right?

So it would look like this:

const result = asm ( // inputs
        "{rcx}" cnt: usize = count,
        "=r" lhs: usize = &left,
        "=r" rhs: usize = &right,
        // clobbers
        "al", "rcx", "cc")
        -> ( // outputs
        "=r" res: u8)
        // multiline asm string
        \\1b:
        \\movb -1(%[lhs], %rcx, 1), %al
        \\xorb -1(%[rhs], %rcx, 1), %al
        \\orb %al, %[res]
        \\decq %rcx
        \\jnz 1b
);

This is a good example of why we should retain the constraint syntax, since we might want {rcx} or =r.

[ofelas]

Not too familiar with the x86 asm, I nicked that example from the Rust discussions, in this case rcx (and ecx i 32 bit) is a loop counter somewhat similar to ctr on Power PC. So the movq, decq, jnz drives the loop. So as long at that condition is met it probably doesn't matter. Maybe it could have been done with the loop instruction that decrements and tests at the same time.

result is both an input and an output, like if you were updating a cksum or similar where you would feed in an initial or intermediate value that you want to update.

Are you planning to support all the various architecture specific input/output/clobber constraints and indirect inputs/outputs present in LLVM?

[kiljacken]

Another avenue to go down is the MSVC way of doing inline assembly. M$ does a smart augmented assembly, where you can transparently access C/C++ variables from the assembly. An example would be a memcpy implementation:

void
CopyMemory(u8* Dst, u8* Src, memory_index Length)
{
	__asm {
		mov rsi, Src
		mov rdi, Dst
		mov rcx, Length
		rep movsb
	}
}

It provides a really nice experience. However, MSVC isn't smart about the registers, so all registers used are backed up to the stack before emitting the assembly, and are then restored after the assembly. This avoids the mess of having to specify cluttered registers, but at the cost of a fair bit of performance.

The smart syntax is awesome, but it might be hard fit with a LLVM backend, if you do not want to write an entire assembler as well.

[dd86k]

As kiljacken says, I personally really, really enjoy the Intel syntax over GAS as D has done it (except for GDC, which is based on GCC). I'm only assuming it'll be harder to implement a MSVC-styled inline assembly feature.

[andrewrk]

The end game is we will have our own assembly syntax, like D, which will end up being compiled to llvm compatible syntax. It's just a lot of work.

I at first tried to use the Intel syntax but llvm support for it is buggy and some of the instructions are messed up to the point of having silent bugs.