MIPS quine

A quine is a program that prints its own source code. This one accomplishes this by disassembling itself.

$ mips-linux-gnu-gcc -static -fno-pic -mno-abicalls -nostdlib quine.S -o quine
$ qemu-mips ./quine | head
.global __start
.set noreorder

__start:
    la $a1, s0
    jal zstr
    addu $0, $0, $0

    la $t0, __executable_start
    lw $t1, 32($t0)
$ diff <(qemu-mips ./quine) quine.S
$

why?

It seemed more enticing than going to sleep.

how do you handle labels?

It parses its own ELF headers and walks its own symbol table.

what's with the cryptic label names?

If the binary goes over 4 KiB, the symbol table lands in a part of the file that doesn't get mapped. The easiest way to work around this was to golf it down. It even was a fun challenge to make it fit. I could probably open the executable and map it into memory, but that seems really finicky.

Trivia

The code assumes that a lui instruction is always part of the assembler's expansion of la and blindly reads out the literal field of the following addiu without checking whether it's actually an addiu. This is fine, as we only need to handle the situations that occur in our code.
The disassembler is table driven, with each entry in the table consisting of a single byte, followed by the opcode name as a null-terminated string. The single byte holds 6 bits to identify the opcode, and 2 bits to describe how the operands should get decoded.

This became a problem when the jr instruction called for a 5th operand mode in the R-encoding table. Instead of handling this properly, I just made the instruction always decode as jr $ra, by having "jr $ra" be the name of the opcode. It's not like anything else occurs in the quine's code.
To avoid writing a separate routine for printing out the tables that drive the disassembler, I encode the opcode bytes using octal escapes. It's hideous, but it gets the job done.
MIPS register names are mere suggestions. "Caller-saved registers"? That sounds like a global variable to me. "Function arguments are passed through $a0-$a3"? Sometimes you just don't feel like it and use $t0. Sometimes the only argument goes through $a1, skipping $a0 entirely. I plead code golf.

Closing remarks