MIPS is dead, and Loongson was not commercially successful in the past ten years. But I do admire their work on developing CPUs with good performance independently. It seems to be necessary for China, especially after US sanctions. Hope they'll finally get on the right way.
With inexistent documentation comes an awful open-source toolchain. Their official community "Loongnix" cgit is a collection of extremely outdated, long-winded forks of gcc and the kernel. They do provide patches to GCC and Binutils, but only barely enough for the errata (and the old MMX copycat). LLVM is overlooked here, so good luck with that ll/sc lockup when you try Rust. [The "grassroots" @loongson-community on GitHub does try to figure out some more instructions.]
And that brings me to the topic of their fancy ISA extensions. Accelerated X86/ARM emulation, 256-bit SIMD (with a mnemonic of "VBCMULADDPD"), everything! But to hobbists who bought their commercial samples, all that silicon area is just expensive padding with no way to use them.
If you can tolerate Google Translate, I recommend reading https://zh.wikipedia.org/wiki/LoongISA and some of the refs. Especially the comment article by "windows1089" on Zhihu.