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Wait; what? If C is not a Low-Level Language, then what is a Low-Level Language? "The features that led to these vulnerabilities, along with several others, we

Wait; what? If C is not a Low-Level Language, then what is a Low-Level Language? "The features that led to these vulnerabilities, along with several others, were added to let C programmers continue to believe they were programming in a low-level language when this hasn't been the case for decades." Now C is again the root of all evils... But I'm afraid that's not right, all those CPU optimizations (branch predictions, speculative execution, caches, etc.) are not tied to any specific languages. They have been designed to make existing programs run faster; if all our software stack was written in Java, Lisp or PHP, I think that on the hardware front, most of the same decisions would have been made.

dragonwriter 7y ago
> Wait; what? If C is not a Low-Level Language, then what is a Low-Level Language? Assembly, actual machine code. (Contrary to the article, C was never a low-level language, when it was younger it was literally a textbook high-level language because it allows abstracting from the specific machine, and while it's less likely to be what a textbook points to as an example today, that hasn't changed.)
- DSMan195276 7y ago
  > Assembly, actual machine code Actually, the author is effectively arguing that x86 assembly is not low-level. Which is somewhat true, but none of the levels under x86 assembly are exposed to the programmer, for the most part.
- nwallin 7y ago
  Following the metrics if the article, assembly language isn't low level either. Assembly language only gives you access to 16 integer registers and the 16 (?) sse/avx SIMD registers on x86_64. It doesn't give you access to the 64 or so integer registers or the who knows how many SIMD registers there are. Assembly instructions do not map to uops, no matter how much we pretend they do. We couldn't even program uops of we wanted to. These instructions are not executed in the order we specify them, and some of them are not executed at all: modern CPUs have their own dead code detectors and will drop instructions if it feels like it. Assembly language programmers have less control of the microcode than raw JVM bytecode programmers have over the x86_64 instructions that eventually get executed have.
  - millstone 7y ago
    Right, but that's the hardware interface. The CPU consumes a compressed instruction stream. Compression is achieved by the compiler via a lossy mapping of infinite registers onto a finite register set. This stream is then re-inflated by the CPU through discovering false dependencies in the interference graph via register renaming, and then cleaning up spilling via caching. If this seems absurdly complex, it might be because of the absurd complexity. But the alternative has been tried, and tried and tried (RISC, VLIW), and always a failure. Well fuck.
    - snaky 7y ago
      "Itanium was in-order VLIW, hope people will build compiler to get perf. We came from opposite direction - we use dynamic scheduling. We are not VLIW, every node defines sub-graphs and dependent instructions. We designed the compiler first. We build hardware around the compiler, Intel approach the opposite." https://www.anandtech.com/show/13255/hot-chips-2018-tachyum-prodigy-cpu-live-blog https://www.anandtech.com/show/13255/hot-chips-2018-tachyum-...
    - nwallin 7y ago
      I mean... sure? But what's the point, what low level languages are there? The linked article is arguing that C isn't low level because modern CPUs behave so differently than what their hardware interface suggests they do. If we accept this argument, then assembly language isn't low level either, because it suffers from all these same limitations. If assembly language isn't low level, then why is "low level" even a phrase? My point is that if you're going to argue that C isn't low level, then it's hard to argue that assembly language is. Conversely, if you're going to argue that assembly language is low level, it's hard to argue that C isn't. So it's flippant to argue (in this thread) that assembly language is low level without also rebuking the article or coming up with a persuasive argument as to why C shouldn't be lumped in with assembly language. Personally, I think the article is wrong. C is low level. It is useful to distinguish between C and Python in terms of C is low level and Python is high level. It is a useful mental model, therefore I'm keeping it. But if people in this thread are going to make both arguments that the article is correct and assembly language is low level, you'll need to justify that fairly strongly. I'm also not arguing that RISC or (ew) VLIW are the answer.
blihp 7y ago
Microcode. Back when C became a thing, microcode only existed on 'big iron'. That changed 20+ years ago. The microarchitecture is the 'real' architecture you're running on, the ISA that assembly language and C code is written against is a facade. It has value in that we don't need to rewrite everything every few years when the microarchitecture changes, but the downside is what we consider low level programming languages talking 'directly' to the hardware are now going through another layer of abstraction.
inopinatus 7y ago
A matter of perspective. ”In a low-level language like C...” - applications programmer ”In a high-level language like C...” - chip designer
Tomte 7y ago
Only marginally lower, but a common example is Ada. You can actually describe hardware registers sanely and portably in Ada. You cannot do that in C. (It obviously still works, because C is ubiquitous, and so processor and compiler vendors do their hardest to "make it work", but that's no accomplishment of C)

dragonwriter 7y ago

> Wait; what? If C is not a Low-Level Language, then what is a Low-Level Language? Assembly, actual machine code. (Contrary to the article, C was never a low-level language, when it was younger it was literally a textbook high-level language because it allows abstracting from the specific machine, and while it's less likely to be what a textbook points to as an example today, that hasn't changed.)

DSMan195276 7y ago
> Assembly, actual machine code Actually, the author is effectively arguing that x86 assembly is not low-level. Which is somewhat true, but none of the levels under x86 assembly are exposed to the programmer, for the most part.
nwallin 7y ago
Following the metrics if the article, assembly language isn't low level either. Assembly language only gives you access to 16 integer registers and the 16 (?) sse/avx SIMD registers on x86_64. It doesn't give you access to the 64 or so integer registers or the who knows how many SIMD registers there are. Assembly instructions do not map to uops, no matter how much we pretend they do. We couldn't even program uops of we wanted to. These instructions are not executed in the order we specify them, and some of them are not executed at all: modern CPUs have their own dead code detectors and will drop instructions if it feels like it. Assembly language programmers have less control of the microcode than raw JVM bytecode programmers have over the x86_64 instructions that eventually get executed have.
- millstone 7y ago
  Right, but that's the hardware interface. The CPU consumes a compressed instruction stream. Compression is achieved by the compiler via a lossy mapping of infinite registers onto a finite register set. This stream is then re-inflated by the CPU through discovering false dependencies in the interference graph via register renaming, and then cleaning up spilling via caching. If this seems absurdly complex, it might be because of the absurd complexity. But the alternative has been tried, and tried and tried (RISC, VLIW), and always a failure. Well fuck.
  - snaky 7y ago
    "Itanium was in-order VLIW, hope people will build compiler to get perf. We came from opposite direction - we use dynamic scheduling. We are not VLIW, every node defines sub-graphs and dependent instructions. We designed the compiler first. We build hardware around the compiler, Intel approach the opposite." https://www.anandtech.com/show/13255/hot-chips-2018-tachyum-prodigy-cpu-live-blog https://www.anandtech.com/show/13255/hot-chips-2018-tachyum-...
  - nwallin 7y ago
    I mean... sure? But what's the point, what low level languages are there? The linked article is arguing that C isn't low level because modern CPUs behave so differently than what their hardware interface suggests they do. If we accept this argument, then assembly language isn't low level either, because it suffers from all these same limitations. If assembly language isn't low level, then why is "low level" even a phrase? My point is that if you're going to argue that C isn't low level, then it's hard to argue that assembly language is. Conversely, if you're going to argue that assembly language is low level, it's hard to argue that C isn't. So it's flippant to argue (in this thread) that assembly language is low level without also rebuking the article or coming up with a persuasive argument as to why C shouldn't be lumped in with assembly language. Personally, I think the article is wrong. C is low level. It is useful to distinguish between C and Python in terms of C is low level and Python is high level. It is a useful mental model, therefore I'm keeping it. But if people in this thread are going to make both arguments that the article is correct and assembly language is low level, you'll need to justify that fairly strongly. I'm also not arguing that RISC or (ew) VLIW are the answer.

blihp 7y ago

Microcode. Back when C became a thing, microcode only existed on 'big iron'. That changed 20+ years ago. The microarchitecture is the 'real' architecture you're running on, the ISA that assembly language and C code is written against is a facade. It has value in that we don't need to rewrite everything every few years when the microarchitecture changes, but the downside is what we consider low level programming languages talking 'directly' to the hardware are now going through another layer of abstraction.

inopinatus 7y ago

A matter of perspective. ”In a low-level language like C...” - applications programmer ”In a high-level language like C...” - chip designer

Tomte 7y ago

Only marginally lower, but a common example is Ada. You can actually describe hardware registers sanely and portably in Ada. You cannot do that in C. (It obviously still works, because C is ubiquitous, and so processor and compiler vendors do their hardest to "make it work", but that's no accomplishment of C)