OCaml 内部结构：异常
我很想知道 OCaml 运行时如何处理异常以使它们如此轻量。他们是使用 setjmp/longjmp 还是在每个函数中返回一个特殊值并传播它？
在我看来， longjmp 会给系统带来一点压力，但只有在引发异常时，在检查每个函数返回值时才需要在调用函数后检查每个值，在我看来，这会导致大量的检查和跳跃，看起来它的表现会最差。
通过查看 OCaml 如何与 C 交互（http://caml. inria.fr/pub/docs/manual-ocaml/manual032.html#toc142 ），并查看callback.h，似乎是通过使用内存来标记异常的对象对齐（ #define Is_exception_result(v) (((v) & 3) == 2) ）。这似乎表明它的实现不使用 longjmp 并在每次函数调用后检查每个函数结果。是这样吗？或者 C 函数已经尝试捕获任何异常，然后将其转换为这种格式？
谢谢你！
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!
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OCaml 异常处理
它不使用
setjmp/longjmp。当try时当 被评估时，一个“陷阱”被放置在堆栈上，其中包含有关处理程序的信息。最顶层陷阱的地址保存在寄存器中，当您引发时，它会直接跳转到该陷阱，一次性展开多个堆栈帧（这比检查每个返回代码更好）。陷阱还存储前一个陷阱的地址，该地址在上升时间恢复到寄存器中。¹：或全局，在寄存器不足的架构上
您可以在代码中亲自查看：
Kpushtrap/Kpoptrap字节码围绕try..withed 表达式PUSHTRAP（放置陷阱/处理程序），POPTRAP（删除它，非错误情况）和RAISE（跳转到陷阱）与
setjmp的比较Ocaml 使用非标准调用约定，很少或没有被调用者保存寄存器，这使得这个（和尾递归）变得高效。我想（但我不是专家）这就是 C
longjmp/setjmp在大多数架构上效率不高的原因。请参阅示例 此 x86_64 setjmp 实现 看起来与之前的捕获机制加上被调用者寄存器保存完全一样。C/OCaml 接口 中考虑了这一点：调用C 代码中的 Caml 函数
caml_callback不会捕获 OCaml 领域的异常；如果您愿意，您必须使用特定的 caml_callback_exn 来设置其陷阱处理程序并保存/恢复 C 调用约定的被调用者保存的寄存器。参见例如。 amd64 代码，保存寄存器然后跳转到此标签进行设置异常陷阱。OCaml exception handling
It doesn't use
setjmp/longjmp. When atry <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:
Kpushtrap/Kpoptrapbytecodes surround thetry..withed expressionLpushtrap/Lpoptraparound the expressionPUSHTRAP(places the trap/handler),POPTRAP(remove it, non-error case) andRAISE(jump to the trap)Comparison with
setjmpOcaml 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/setjmpisn'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 specificcaml_callback_exnif 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.