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This seems a bit wrong-headed. All code ultimately runs CPU instructions, regardless of the language a program is written in. It doesn’t matter if the Java comp

This seems a bit wrong-headed. All code ultimately runs CPU instructions, regardless of the language a program is written in. It doesn’t matter if the Java compiler or runtime is written in C, because CPUs don’t run C. The Java bytecode compiler might be written in C, but it ultimately emits CPU instructions. When talking about performance you say that “there's literally nothing Java can do that C cannot because C can write Java” but other than the tautology that they are both Turing machines, your conclusion doesn’t follow. The Java runtime dynamically optimises based on the currently executing code, but C compiles to static CPU instructions. A C program is statically optimised so it’s possible that some optimisations in C will perform worse than those in Java because Java knows more about the actual code paths taken at runtime, and this can change between executions. For C to be able to optimise itself the same way that Java does, it would need its own heavyweight runtime and bytecode, like Java does (as I think you suggest). But C doesn’t actually have such a runtime, and although I’m certainly no expert on the C language spec, I assume it has all sorts of runtime guarantees and a memory model that would preclude arbitrarily rearranging the executable code at runtime, so it arguable that the resulting system wouldn’t actually be “C” if it did this. Put another way: although Java is/was written in C, the way Java works is fundamentally different to C, and because of this, certain optimisations are available to Java that are not available to C. (All of that said, in my experience C code is always faster than Java code, these days I use Go and Swift, and I’m very happy to have seen the back of the JVM.)

kovac 5y ago
This is what I used to think about JIT as well. That JIT would have more information than a static compiler to optimise the code better. However, it seems to me that most of the performance depends on how well we can exploit the cache lines and how many fewer instructions the cpu has to execute to get the job done. JIT could perhaps improve on the latter but in many cases it seems to me that it's generally very hard to be cache aware when programming in Java not to mention every user-defined type is allocated on heap. So, I feel like it's quite unlikely that there can be Java programs that outperform C and even then that it can't in principle be done better in C.
- kaba0 5y ago
  Actually, Java gets ridiculously close to C performance when you only do computations on primitives. Of course you do have some pointer chasing in most code bases but in my experience cache is mostly relevant for repetitive workloads. I’m not sure that a typical web application would win all that much from a C rewrite.
- doctor_eval 5y ago
  Yeah I don’t disagree with you, I was really trying to dispel the idea that somehow the implementation language puts an upper limit on the performance of the implemented language. So, consider a perfectly spherical Java language… :)

kovac 5y ago

This is what I used to think about JIT as well. That JIT would have more information than a static compiler to optimise the code better. However, it seems to me that most of the performance depends on how well we can exploit the cache lines and how many fewer instructions the cpu has to execute to get the job done. JIT could perhaps improve on the latter but in many cases it seems to me that it's generally very hard to be cache aware when programming in Java not to mention every user-defined type is allocated on heap. So, I feel like it's quite unlikely that there can be Java programs that outperform C and even then that it can't in principle be done better in C.

kaba0 5y ago
Actually, Java gets ridiculously close to C performance when you only do computations on primitives. Of course you do have some pointer chasing in most code bases but in my experience cache is mostly relevant for repetitive workloads. I’m not sure that a typical web application would win all that much from a C rewrite.
doctor_eval 5y ago
Yeah I don’t disagree with you, I was really trying to dispel the idea that somehow the implementation language puts an upper limit on the performance of the implemented language. So, consider a perfectly spherical Java language… :)