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Ok, w/o dipping into machine code, show me a low level language. Any snippet of C-code is transparent in that you know roughly how it is going to be translated into machine code.
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> cost of cache misses How is the C memory model a leaky abstraction here? What better way do you suggest? Are we not fine coding sequential (in memory) datastructures in C?
How about a different example entirely? Memory nowadays is either CPU (and the GPU can access it) or GPU (and the CPU has a window into it). It's terribly important to use the right one: if a chunk of memory is mostly used by the CPU, it needs to be CPU memory, and if it's mostly used by the GPU, it needs to be GPU memory. But there's no good way to specify that. You might argue that a modern computer is more like pr…
When different things are mapped into the address space, that's an abstraction the programmer (or the user) consciously made. It should be possible to figure out the performance characteristics there.
Of course, many programs work on various machines with their own performance characteristics. You should still be able optimize for any one of them by querying the hardware and selecting an appropriate implementation. If you want to put in the work.
I don't think assembler/C is such a big problem here. But then again, I'm not a low level guy (in this sense) for now.
This article makes some valid points but is overall rather misleading I think. Almost all of the reasons given why C is "not a low-level language" also apply to x86/x64 assembly. Register renaming, cache hierarchies, out of order and speculative execution etc are not visible at the assembly / machine code level either on Intel or other mainstream CPU architectures like ARM or Power PC. If C is not a low level languag…
Cache hierarchies are directly accessible with CLFLUSH, INV, WBINVD x86 instructions; we may count also PREFETCHx, but they call it "a hint". FENCE instructions touch even the multicore part of system.
Many low-level CPU concepts leak to higher layer. A bright example is false sharing, which may manifest even in Java or C# programs.
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C cannot read the overflow bit after an ADD, because of it's abstractions ... so I would say modern ASM is still lower in some aspects because it has less constrains and more importantly, it is less expressive, which is the whole idea of this hierarchy. The compiler should offer a macro for that. Then the question is whether to take the specification or the implementation at which point it's an absurd question to beg…
> C cannot read the overflow bit after an ADD ... The compiler should offer a macro for that. If you use GCC then __builtin_add_overflow() is what you are looking for: "The compiler will attempt to use hardware instructions to implement these built-in functions where possible, like conditional jump on overflow after addition, conditional jump on carry etc." https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins…
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It's just practical. Otherwise how do you call java ? Or python ? And what would be the benefit of changing those particular sementics ? In french we define such article as "fucking a fly".
Java and Python are much closer to C than C is to assembly. Managing memory and raw pointers are nothing. Try implementing a recursive function call in x86 assembler to get a real idea of low level.
Isn't that pretty easy? Recursion is just a function calling itself, directly or indirectly.
Here's the simplest possible recursive function in x86 assembler. It simply causes a stack overflow.
recursion:
call recursion
Tail recursive version would be simply a jump:
recursion:
jmp recursion
recursion:
dec eax
jz rec_exit
call recursion
rec_exit:
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I played a little bit with gpgpu on the raspberry pi. I’d imagine it’s relativly primitive compared to whatever shaders are compiled to on modern GPUs, but it was humbling to have to manage things like separate, per core, disjoint register files which can only be read 4 cycles after write. The cores are heterogeneous, so there is special hardware for exchanging register reads between cores if necessary.
What language do you use for GPGPU?
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>403 Error - Access Forbidden We are sorry ... ... but we have temporarily restricted your access to the Digital Library. Your activity appears to be coming from some type of automated process. To ensure the availability of the Digital Library we can not allow these types of requests to continue. The restriction will be removed automatically once this activity stops. We apologize for this inconvenience. Please contac…
I really really liked this article, and reading the comments here is blowing my mind. Did we read the same thing? I think it's a strong insight that insight that chip designers and compiler vendors have spent person-millenia maintaining the illusion that we are targeting a PDP-11-like platform even while the platform has grown less and less like that. And, it turns out, with things like Spectre and the performance co…