OCaml内部：异常 [英] OCaml internals: Exceptions

问题描述

我很想知道如何在OCaml运行时处理异常，使它们如此轻量级。他们使用setjmp / longjmp还是在每个函数中返回一个特殊的值，并传播它？

在我看来，longjmp会给系统带来一些压力，但只有当引发异常时，检查每个函数返回值需要检查每个和每个值调用一个函数后，这似乎在我看来会做很多检查和跳转，似乎它会执行最糟糕的。

通过查看OCaml如何与C接口（ http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#toc142 ），并查看callback.h，似乎一个异常被标记为使用对象的存储器对齐（#define Is_exception_result（v）（（（v）& 3）== 2））。这似乎表明它的实现不使用longjmp并且在每个函数调用之后检查每个函数结果。是吗？或者C函数已经尝试捕获任何异常，然后将其转换为此格式？

谢谢！

解决方案

OCaml异常处理

不使用 setjmp / longjmp 。当 try< expr>在评估< handle> 时，在堆栈上放置陷阱，其包含关于处理程序的信息。最上面的陷阱的地址保存在寄存器1中，当你提高时，它直接跳到这个陷阱，一次释放几个堆栈帧（这比检查每个返回码更好）。

¹：或者全局，在没有足够寄存器的体系结构上

p>

您可以在代码中看到自己：

• 字节码编译：lines 635-641，两个 Kpushtrap / Kpoptrap 字节码环绕 try..with ed表达式


• native native compilation ：lines 254-260，再次在表达式
周围指示 Lpushtrap / Lpoptrap

• 字节码执行对于字节码 PUSHTRAP （放置陷阱/处理程序）， POPTRAP （删除它，非错误的情况） code> RAISE （跳到陷阱）


• 本机代码发射和在amd64上的.inria.fr / cgi-bin / viewvc.cgi / ocaml / release / 3.12.1 / asmcomp / amd64 / emit.mlp？revision = 11110& view = markup＃l613

与 setjmp

的比较

ocaml使用具有很少或没有被调用保存的寄存器的非标准调用约定，这使得（和尾递归）有效。我想（但我不是专家），这就是为什么C longjmp / setjmp 在大多数架构上不是那么高效。请参见此x86_64 setjmp实现，它看起来完全像以前的陷阱机制加callee寄存器保存。

这在 C / OCaml接口：从C代码调用Caml函数的常用方法， caml_callback ，不捕获OCaml-land异常;您必须使用特定的 caml_callback_exn （如果您希望设置其陷阱处理程序并保存/恢复被调用者保存的C调用约定的寄存器。参见例如。 amd64代码，保存寄存器，然后跳转到此标签以设置异常陷阱。

I'm curious to know how exceptions are dealt with in OCaml runtime to make them so lightweight. Do they use setjmp/longjmp or do they return a special value in each function, and propagate it?

It seems to me that longjmp would put a little strain on the system, but only when an exception is raised, while checking for each function return value would need to check for every and each value after calling a function, which seems to me would put a lot of checks and jumps, and it seems it would perform worst.

By looking at how OCaml interfaces with C ( http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#toc142 ), and looking at callback.h, it seems that an exception is tagged by using the memory alignment of objects ( #define Is_exception_result(v) (((v) & 3) == 2) ). This seems to indicate that its implementation doesn't use longjmp and checks each function result after each function call. Is that it? Or the C function already tries to catch any exception, and then converts it to this format?

Thank you!

解决方案

OCaml exception handling

It doesn't use setjmp/longjmp. When a try <expr> with <handle> is evaluated, a "trap" is placed on the stack, that contains information about the handler. The address of the topmost trap is kept in a register¹, and when you raise, it jumps directly to this trap, unwinding several stack frames in one go (this is better than checking each return code). A trap also stores the address of the previous trap, which is restored in the register at raise time.

¹: or a global, on architectures with not enough registers

You can see for yourself in the code:

• bytecode compilation: lines 635-641, two Kpushtrap/Kpoptrap bytecodes surround the try..withed expression
• native compilation: lines 254-260, again instructions Lpushtrap/Lpoptrap around the expression
• bytecode execution for the bytecode PUSHTRAP (places the trap/handler), POPTRAP (remove it, non-error case) and RAISE (jump to the trap)
• native code emission on mips and on amd64 (for example)

Comparison with setjmp

Ocaml uses a non-standard calling convention with few or no callee-saved registers, which makes this (and tail-recursion) efficient. I suppose (but I'm no expert) that's the reason why C longjmp/setjmp isn't as efficient on most architectures. See for example this x86_64 setjmp implementation that looks exactly like the previous trapping mechanism plus callee-registers save.

This is taken into account in the C/OCaml interface: the usual way to call a Caml function from C code, caml_callback, doesn't catch OCaml-land exceptions; you have to use a specific caml_callback_exn if you wish to, which setups its trap handler and saves/restores callee-saved registers of the C calling convention. See eg. the amd64 code, which saves the registers then jump to this label to setup the exception trap.

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