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C Is Not a Low-Level Language

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Re: C Is Not a Low-Level Language

#71

Post by greglindahl »

mattnewport wrote: post #23
ge0rg wrote: post #14

Earlier quoted context omitted.

I also had the initial impression that the article is misleading, but later on the author made the point that the C compiler is doing significant work to reorder / parallelize / optimize the code. I agree that x86/x64 is not a low-level language either, but even if it was, with the description the author provided, I'd agree with his point of C not being low-level. Regarding cutting off backwards compatibility to impr…

I think you can make a good case that the failure of the Itanium (and other similar attempts to un-hide some of this stuff like the IBM/Sony Cell used in the PlayStation 3) was precisely because they tried to shift optimization work from the CPU to the compiler / programmer.

Funny, the compiler people I've worked with complain that Itanium tried to do too much in hardware, like the hw support for loop unrolling, which made superpipelining optimizations in existing compilers much more complicated.

Re: C Is Not a Low-Level Language

#72

Post by [deleted] »

PersistentCough wrote: post #32
ge0rg wrote: post #14

Earlier quoted context omitted.

I also had the initial impression that the article is misleading, but later on the author made the point that the C compiler is doing significant work to reorder / parallelize / optimize the code. I agree that x86/x64 is not a low-level language either, but even if it was, with the description the author provided, I'd agree with his point of C not being low-level. Regarding cutting off backwards compatibility to impr…

> the C compiler You mean C compiler X has the feature of Y. There are lots of compilers and that's not part of the language.

[deleted]

Re: C Is Not a Low-Level Language

#73

Post by chubot »

pvg wrote: post #67
admax88q wrote: post #58

Earlier quoted context omitted.

> If C is not a low level language then a low level language does not exist for modern CPUs I think that's a fair conclusion though, I don't think the article is misleading. x86 assembly is a high level language. It's analogous to JVM bytecode. Modern x86 processors are more like a virtual machine for x86 bytecode.

It's analogous to JVM bytecode. That's only true in the way everything is analogous to everything.

I think it's a good analogy because it's saying that nontrivial optimizations happen below the code layer.

In early CPUs, nothing was optimized. They just executed your instructions. Now there are nontrivial optimizations and rewriting, just like the JVM.

Re: C Is Not a Low-Level Language

#74

Post by coldtea »

pvg wrote: post #67
admax88q wrote: post #58

Earlier quoted context omitted.

> If C is not a low level language then a low level language does not exist for modern CPUs I think that's a fair conclusion though, I don't think the article is misleading. x86 assembly is a high level language. It's analogous to JVM bytecode. Modern x86 processors are more like a virtual machine for x86 bytecode.

It's analogous to JVM bytecode. That's only true in the way everything is analogous to everything.

Nope, also in the way the parent explains right after that sentence.

Re: C Is Not a Low-Level Language

#75

Post by pjmlp »

abofh wrote: post #60
nerdponx wrote: post #50

Earlier quoted context omitted.

On the flipside, doesn't it allow more or less direct memory access?

I mean, yes, no, it depends on what you mean? You can write to null in C, your operating system rejects it. You can write past your allocated memory, your OS rejects it. It's not like just because it's written in C it gets to read the kernel memory - it's just that you can try. All memory access in userspace is mediated by the MMU, so nothing in C gets "direct" memory access - but it does allow you to screw up your o…

Many C targets also run bare metal, there is nothing to reject there, unless the hardware has an MMU and the code bothered to configure it on boot.

Re: C Is Not a Low-Level Language

#76

Post by rhacker »

The article itself has 4 definitions or "attributes" for low-level languages that can be considered contradictory:

* "A programming language is low level when its programs require attention to the irrelevant."

* Low-level languages are "close to the metal," whereas high-level languages are closer to how humans think.

* One of the common attributes ascribed to low-level languages is that they're fast.

* One of the key attributes of a low-level language is that programmers can easily understand how the language's abstract machine maps to the underlying physical machine.

So basically the entire article's premise (the title) hinges on the last bullet- which can be contested. All the other mentioned attributes can be applied to Java, C, C#, C++. So failing the last bullet point doesn't apply to just C.

Re: C Is Not a Low-Level Language

#77

Post by mattnewport »

Avshalom wrote: post #39
swsieber wrote: post #36

Earlier quoted context omitted.

Correct - it's an argument for broadening the title. Calling out a specific language as not being something leads one to ask, "Well what is?". In this case, there is no qualifying alternative, so the title might as well be, "There is no low-level language for CPUs".

... any more. It's not a category that can't exist, it just doesn't have any members right now.

It actually can't exist at all for current modern CPUs (x64/ARM/PowerPC) since they don't expose a programming interface for many of the things discussed in the article (speculative / out of order execution, register renaming, full cache control).

Re: C Is Not a Low-Level Language

#78

Post by obl »

United857 wrote: post #53

It's worth noting that chips that were designed for high-performance computing (e.g. the Cell) from the outset generally don't have silicon devoted to things like out of order execution, register renaming, etc. In this case, the bulk of the optimization logic does shift to the programmer (aided by the compiler). The reason is that in these domains (e.g. game consoles, supercomputing), you know ahead of time the preci…

The point of dynamic optimizations (such as ooo) is not only to hide implementation details (such as register file size) but very much to take advantage of dynamic opportunities that simply cannot be known statically. The optimal schedule can be very different depending on whether some load hit L1 vs L2 or even was forwarded from the store buffer.

There are some classes of very regular algorithm where you could probably predict everything (and handle the memory hierarchy) statically, such as GEMM, but it's not very common.

Re: C Is Not a Low-Level Language

#79

Post by mattnewport »

United857 wrote: post #53

It's worth noting that chips that were designed for high-performance computing (e.g. the Cell) from the outset generally don't have silicon devoted to things like out of order execution, register renaming, etc. In this case, the bulk of the optimization logic does shift to the programmer (aided by the compiler). The reason is that in these domains (e.g. game consoles, supercomputing), you know ahead of time the preci…

> The reason is that in these domains (e.g. game consoles, supercomputing), you know ahead of time the precise hardware characteristics of your target, you can assume it won't change, and can thus optimize specifically for that ahead of time.

Cell was a failure in large part because this proved to be less true / less relevant than its designers thought.

Source: many late nights / weekends trying to get PS3 launch titles performing well enough to ship.

Re: C Is Not a Low-Level Language

#80

Post by saagarjha »

abofh wrote: post #60
nerdponx wrote: post #50

Earlier quoted context omitted.

On the flipside, doesn't it allow more or less direct memory access?

I mean, yes, no, it depends on what you mean? You can write to null in C, your operating system rejects it. You can write past your allocated memory, your OS rejects it. It's not like just because it's written in C it gets to read the kernel memory - it's just that you can try. All memory access in userspace is mediated by the MMU, so nothing in C gets "direct" memory access - but it does allow you to screw up your o…

But is there really any programming language that lets you write to arbitrary memory? This is a problem of privileges rather than how low level your language is: it's not like you can write assembly that will give you arbitrary access to kernel memory.
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