Do you make sense of 'Exception tables' and 'Unwind tables"?

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本文探讨了在C++等高级语言中处理异常所必需的异常表和Unwind表的作用。Unwind表包含用于处理异常的调试帧信息，且只有包含Unwind表的函数才能使异常传播。对于由C++代码调用的汇编语言函数，必须确保存在异常表和Unwind表，以便异常能够通过这些函数传播。若函数没有Unwind表，则异常处理运行时环境会终止程序。

Exception tables are necessary to handle exceptions thrown by functions in high-level languages such as C++.

Unwind tables contain debug frame information which is also necessary for the handling of such exceptions. An exception can only propagate through a function with a unwind table. You use FRAME directives to enable the assembler to generate unwind tables.

An assembly language function is code enclosed by either PROC and ENDP or FUNC and ENDFUNC directives. Functions written in C++ have unwind information by default. However, for assembly language functions that are called from C++ code, you must ensure that there are exception tables and unwind tables to enable the exceptions to propagate through them.

An exception can not propagate through a function with a no-unwind table. The exception handling runtime environment terminates the program if it encounters a no-unwind table during exception processing.

The assembler can generate no-unwind table entries for all functions and non-functions. The assembler can generate an unwind table for a function only if the function contains sufficient FRAME directives to describe the use of the stack within the function. To be able to create an unwind table for a function, each POP or PUSH instruction must be followed by a FRAME POP or FRAME PUSH directive respectively. Functions must conform to the conditions set out in the Exception Handling ABI for the ARM Architecture(EHABI), section 9.1 Constraints on Use. If the assembler can not generate an unwind table it generates a no-unwind table.

Overview of exception handling

The process of finding excepting handling information from the current PC is summarized in the diagram below:

Diagram of exception handling process

All tables are in “text” space. The types pointed by the typeinfo pointers are identified by a GP-relative offset.

Note:

For more detailed information, see:
https://itanium-cxx-abi.github.io/cxx-abi/exceptions.pdf
https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#base-data