Asmttpd: Web server for Linux written in amd64 assembly (2017)

But for web servers, it's not about micro-optimisations, but having an architecture capable of efficiently handling concurrency, no?

nginx and Apache are both written in C, but nginx can outperform/outscale Apache. That's because of high-level architectural differences, not from tweaking assembly code. An HTTP server isn't doing much in data-transformation/algorithmic terms, it just has to scale efficiently.

If your code is synchronous and non-parallel, it's going to lose every time, even if it's highly tuned assembly.

> it's not uncommon for even a simple/naive/asymptotically less efficient algorithm written in Asm to outperform a more complex/clever/asymptotically efficient one in an HLL at the problem sizes encountered in practice, simply by virtue of the constant factor being much smaller

You're conflating two different things: constant factors based on algorithm choice and the merits of hand-written assembler. It's true that sometimes asymptotically worse algorithms beat asymptotically better ones in practice. It's a lot less clear that humans can regularly beat compilers for typical scalar code.

As I've mentioned in another comment, I wouldn't place any bets on the efficiency of this code. It's written with easy of understanding in mind at the expense of code size and speed.

Really? Picolisp was build with asm? not C as SBCL

Re-read the comment you're replying to. It's a joke.

I don't think they've given reasons for it, but it's likely because they just completely destroy any kind of out of order execution. They'd end up requiring a fence before either instruction to ensure that you only push or pop the correct values and you couldn't speculate at all past them. Combine that with more than double the size of needed space (double number of registers, and double the size of them) it seems pretty wasteful. And then there's the fact that modern compilers are likely already avoiding those instructions because of the timing of saving extra registers that you aren't using in a given function, it probably just doesn't make much sense to keep them anymore.

ARMv7 (not AArch64) has the STM instruction that lets you push a register set, selected by bitmask. eg:

    STMFD sp!, {r3-r7,lr}

(I believe the "FD" suffix is to do with the stack growing down - "full descending")

Of course this is just implemented with microcode so it's not really any more efficient than a series of PUSHes, except there's a bit less I-cache pressure.

Itanium does this automatically. You declare what registers you're using in the the function prologue and it handles popping/pushing/renaming. No register window exceptions too.

ARM has stmdb / ldmia which appear in practically every function prolog/epilog. It also has "banking" systems to swap to a different set of registers on interrupts, which saves time and stack space.

Another one: the old Z-80 CPU had two sets of registers, and a single instruction to swap between the main and the alternate register set.

http://landley.net/history/mirror/cpm/z80.html (search for "alternate registers").

Even if there was one, you'd still be saving registers unnecessarily. If you only clobber one register in a procedure there's no need to save and restore all of them.

x86 has LOADALL and SAVEALL.

https://en.wikipedia.org/wiki/LOADALL

BeagleBone’s processor the AM335x PRU processors by TI has two “PRU” coprocessors. They have “xin” and “xout” instructions that can copy a register bank in 1 cycle. Pretty handy for quick data gathering but tricky to use in C.

Sure. It's called windowed registers and you have one instruction each to move the window right/left.

They have their advantages over config files and command line options. They're better than config files because you can ask for different behavior in different invocations, and they're better than command line options because it allows you to control a program even when you don't invoke it directly.

Consider, for example, wanting different options for less between when it's called by you, or by man, or by git, or by journalctl, or any other utility.

    alias man='LESS="X$LESS" man'
    alias journalctl='LESS="S$LESS" journalctl'

It's also useful when you want all processes that follow from an invocation to share a certain configuration, and when they follow from another invocation to have another. An example of this is $DISPLAY. I startx on my tty1 and that invokes the Xorg server, exports DISPLAY with the display of the server, typically :0, and executes my window manager. When I press certain shortcut keys, applications will be called, and they, in turn, might call other applications, and they all need to know that when they open up a window it should be on display :0. If I login remotely through ssh, forwarding my X11 service, the applications that I call on the same machine where other processes of the same applications opened their windows on :0, should now open their windows on localhost:10, so I get to see them on the laptop I'm logging in from. If someone else wants to use their account on the same computer, they might switch to tty2 and startx themselves, and all the applications they open should open on their Xorg server process with display :1, not mixing with the other windows I opened locally or on the laptop.

This is also why $LANG, $HOME, $USER, and $TERM are useful. They're stuff that should shared by process trees. Their purpose can't be fulfilled by configuration files or command line options.

It's harder to accidentally commit a running process's environment to version control ;)

> Why would anyone want environment variables when you can have config files or explicit options instead?

If you have an underlying .so used by an application that hasn't implemented optioning at init of said library it can then let you control things in something that has no runtime configuration source. I've written several such libraries. Also good for controlling things you override with LD_PRELOAD

Config files set per user/system options whether they are set locally or globally, while env variables set per options per shell.

Global options > user options > env variables > command line parameters

I am very pro environment vars and consider files an anti pattern. Files are like globals, sibling processes end up sharing state, plus they survive longer than their use case.

Env variables cascade, which is why they have wider uses than program args. You can organize a system as a group of processes and envs are a good way to share state and abstract out the details like prod vs staging without affecting the program implementation.

Explicit options require the cooperation of every intermediate program. If program D is called by C, which is called by B, which is called by A, and D gets a new option that we need to use, we have to modify C to pass it to D, then B to pass it to C, and A to pass it to B. Or we could just set an environment variable.

Configuration files don't let us override individual variables easily, in different invocations of the program. We have to generate a custom version of the configuration file for that job and pass that file's name to it.

Environment variables are only visible to children, so they are inherently secure; we don't worry whether permissions are too loose on an environment variable so that another user could see it or modify it. There is no such thing.

In addition to the other advantages mentioned here, environment variables serve as a common language that all programs can understand, in theory. They're also less complex to access than reading and parsing a file (less important for, say, Node.js than C++, but).

> Why would anyone want environment variables

> when you can have config files

You still need a way to tell your program where the config file is.

Environment variables are just like a configuration file that's automatically opened by the OS when a process start and automatically parsed as a set of named strings by the libc. In the many cases where one does not need more complex data structures than that, then to introduce a config file is just making things harder.

That being said, I wish envvars had namespaces of some sort.

Why?

Embedded x86_64? Is that a common thing?

Security is enough of a consideration that any well-established webserver without corporate or foundational backing is instantly unsuitable for serious use.

Definitely looks like it. No library dependencies such like libc, makes it an ideal use case for embedded software.

Wow, even dependently typed, that's pretty brilliant. I can see this being enormously helpful when writing very low level, for example early boot stage, assembly code.

Unfortunately it appears that the linked project is not free software :(

Virtually no undefined behavior, but few security issues in C is because of compiler bugs or backdoors. And integer overflows, buffer overflows, and race conditions don't need any undefined behavior in C, and are the same problem in assembly.

But many programmer mistakes that a C compiler would at the very least warn about will only be apparent at runtime in assembly: Treating something as the wrong type, forgetting to dereference a pointer (or accidentally dereferencing it), mixing up values of different types, mutating something that's meant to be constant in a context, assigning an integer quantity to a pointer without an explicit cast...

Then on top of that you get a myriad of new potential mistakes that are impossible or at least very unlikely in C: Mixing up registers, accessing the wrong offset in a structure (e.g. easier to mix up reg+12 and reg+14 than foo.enabled and foo.name), messing up the stack layout... And now think about what each of those mean when you're actually changing code: Did you update every register? Did you update every offset?

As for the simpler code being simpler to understand and simpler to keep bug-free, that's kind of a tautology that applies independently of the language. And besides, wouldn't those qualities be even easier to achieve for the same code written in C?

"no chances of compiler bugs/backdoors"

That's not true if running complex assembly through an assembler and/or linker. Aside from totally changing the program, there's definitely potential for it to be modified to do things like drop safety checks for optimizations or use instruction patterns that increase covert channels. That's on top of not working correctly at all. I'll add to that point that the Intel documentation is huge with every processor having errata. That's why high-assurance security always considers those things inside the Trusted Computing Base (TCB).

Curious: how does Rust cope in this regard?