Exception Handling ABI for the ARM Architecture

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Exception handling ABI for the ARM architecture Other helper functions are private to the language implementation. When an object built with that implementation is distributed for possible linking with objects built by other implementations, its private (implementation-specific) helper functions must be distributed with it. 3.1.1 The linker must match the run-time support code All this suggests the following principle, which we adopt in relation to exception processing. In a static link step involving relocatable objects generated by different producers, the static linker and the runtime support code must be from the same tool chain. (Aside: This allows a static linker for a standalone execution environment to encode fully linked exception tables in any way acceptable to the matching run-time system. End aside). 3.2 The ELF model 3.2.1 Relocatable ELF A design principle underlying ELF [AAELF] can be caricatured as smart format, dumb linker. That’s not to say that intelligent linking is precluded, or that the linking process is trivial, but to emphasize that the way a collection of relocatable objects should be processed should be explicit in those objects, with no hidden contract between object producers and the static linker. 3.2.2 Executable ELF The execution environment determines the format of an executable or shared object. Historically, ELF as an execution format has been associated with Unix System V-based execution environments (such as ARM Linux). 3.2.3 Principles of usage This suggests the following principles, which we adopt in relation to exception processing. At the interface between relocatable object producers and static linkers we give priority to ease of producing complete, precise exception table descriptions that can be processed straightforwardly by static linkers. At the interface between a fully linked executable (or shared object) and its execution environment, a postprocessor should be able to generate the environment-specific encoding of the exception table from the generic form. (Aside: In practice, we expect such post-processing to be integrated into platform-specific linkers. End aside). ARM IHI 0038A Copyright © 2002-2005, 2007 ARM Limited. All rights reserved. Page 10 of 50
Exception handling ABI for the ARM architecture 4 THE TOP-LEVEL EXCEPTION HANDLING ARCHITECTURE Except where stated otherwise, this section describes the execution architecture that would be created by a dumb linker. Object producers must emit object conformant with these descriptions. A dumb linker can take the objects and the matching runtime libraries and produce a working implementation by performing only standard linking operations. A smart linker might create a different execution architecture or simply a minor variant (for example it might change table index encoding to improve compaction), in which case compatible support code must be available in the associated environment runtime libraries. 4.1 Overview for executables, shared objects, and DLLs This architecture applies to each independently loaded executable segment of a program. A program’s executable segments comprise those of the root executable together with those from the shared objects or DLLs it links to dynamically. (Aside: A static link unit containing multiple executable segments destined for memory at disjoint addresses nonetheless has a single independently loaded executable segment for this purpose because the address relationships among such segments are either fixed or subject to load-time relocation. In any case, in all mainstream execution environments, each static link unit has precisely one executable segment. End aside). With each independent executable segment we associate data structures that support unwinding: Exception handling tables for functions contained within the segment. A binary-searchable index table. Each entry associates a function with its exception-handling table. The data structures are read-only at execution time and should be free of dynamic relocations, so that they are genuinely RO. To this end, references between them, and from them to code, are place-relative and hence position independent. References from them to writable or imported data are implemented in a platform-specific manner (possibly involving indirection through a dynamically relocatable location) which avoids the need to write into the structures at load-time. 4.2 The binary searched index table Exception-handling table entries have a variable size. A handling table entry is found by searching a table of index entries. To support binary search, the index table must consist of contiguous fixed-size entries, each of which identifies a function start address, with the entries ordered by increasing function start address. 4.3 The exception-handling table The exception-handling table (EHT) contains one entry for each non-leaf function that may need to be unwound. (By definition there are no entries for leaf functions because an exception can only be thrown from the site of a function call so a leaf function can never need unwinding). A table entry has a variable size. It encodes, in a vendor- and language-specific way, the actions required to propagate an exception through the function. For C++ functions, this information is: How to unwind a stack frame associated with the function. How to perform any cleanup actions associated with the unwinding. How to locate handlers associated with the function. A description of exception types not blocked by this function. Not all functions have cleanup actions or handlers and most functions simply pass all exceptions not handled. ARM IHI 0038A Copyright © 2002-2005, 2007 ARM Limited. All rights reserved. Page 11 of 50
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Exception Handling ABI for the ARM Architecture

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