That is not true - no allocator I know of (and certainly not the default glibc allocator) allocates memory in this way. It only does a syscall when it doesn’t have free userspace memory to hand out but it overallocates that memory and also reuses memory you’ve already freed.
Including checking return codes instead of exceptions. It's even possible for exceptions as implemented by g++ in the Itanium ABI to be cheaper than the code that would be used for consistently checking return codes.
[0] https://news.ycombinator.com/item?id=22483028
[1] https://www.research.ed.ac.uk/portal/files/78829292/low_cost...
Feel free to ask me any questions to break the radio silence!
The codegen switch with the offsets is in everything, first time I saw it was in the Rhino JS bytecode compiler in maybe 2006, written it a dozen times since. Still clever you worked it out from first principles.
There are some modern C++ libraries that do frightening things with SIMD that might give your bytestring stuff a lift on modern stupid-wide high mispredict penalty stuff. Anything by lemire, stringzilla, take a look at zpp_bits for inspiration about theoretical minimum data structure pack/unpack.
But I think you got damn close to what can be done, niiicccee work.
The “value speculation” thing looks completely weird to me, especially with the “volatile” that doesn't do anything at all (volatile is generally a pointer qualifier in C++). If it works, I'm not really convinced it works for the reason the author thinks it works (especially since it refers to an article talking about a CPU from the relative stone age).
> However, in Chata's case, it needs to access a RISC-V assembler from within its C++ code. The alternative is to use some ugly C function like system() to run external software as if it were a human or script running a command in a terminal.
Have you tried LLVM's C++ API [0]?
To be fair, I do think there's merit in writing your own assembler with your own API. But you don't necessarily have to.
I'm not likely to go back to assembly unless my employer needs that extra level of optimization. But if/when I do, and the target platform is RISC-V, then I'll definitely consider Ultraseembler.
> It's not clear when exactly exceptions are slow. I had to do some research here.
There are plenty of cppcon presentations [1] about exceptions, performance, caveats, blah blah. There's also other C++ conferences that have similar presentations (or even, almost identical presentations because the presenters go to multiple conferences), though I don't have a link handy because I pretty much only attend cppcon.
[0]: https://stackoverflow.com/questions/10675661/what-exactly-is...
[1]: https://www.youtube.com/results?search_query=cppcon+exceptio...
I think I read something about this but couldn't figure out how to use it because the documentation is horrible. So, I found it easier to implement my own, and as it turns out, there are a few HORRIBLE bugs in the LLVM assembler (from cross reference testing) probably because nobody is using the C++ API.
> There are plenty of cppcon presentations [1] about exceptions, performance, caveats, blah blah.
I don't have enough time to watch these kinds of presentations.
If I'm honest, I've never looked into pmr, but I always thought that that's where std has arena allocators and stuff
https://en.cppreference.com/w/cpp/header/memory_resource.htm...
I feel like this might mostly be useful as a reference, because currently RISC-V assembly's specification is mostly "what do GCC/Clang do?"
PTX isn’t the assembly language, it is a virtual ISA, so you need a full backend compiler with 10s to 100s of passes to get to machine code
Ofc, I did add my own bugs.
Lots of simple compilers generate object code directly instead of assembly code, so the above is not so bad by comparison.
