Exception Handling ABI for the ARM Architecture

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Exception handling ABI for the ARM architecture vendors. Nevertheless it remains possible (for code size reasons, perhaps) to implement a restricted unwinder that only copes with a subset of possible execution environments (such as those without floating point). ARM IHI 0038A Copyright © 2002-2005, 2007 ARM Limited. All rights reserved. Page 14 of 50
Exception handling ABI for the ARM architecture 5 INDEX TABLE ENTRIES An index table entry consists of 2 words. Note Note Note The first word contains a prel31 offset (see §4.4.2) to the start of a function, with bit 31 clear. The second word contains one of: - The prel31 offset of the start of the table entry for this function, with bit 31 clear. - The exception-handling table entry itself with bit 31 set, if it can be encoded in 31 bits (see §6.3). - The special bit pattern EXIDX_CANTUNWIND (0x1), indicating to run-time support code that associated frames cannot be unwound. On encountering this pattern the language-independent unwinding routines return a failure code to their caller, which should take an appropriate action such as calling terminate() or abort(). See §7.3 and §7.4. It is essential that link-time symbol vectoring (see [AAELF]) does not break the index table’s association between code and corresponding exception-handling tables. An index table entry first word must therefore not be constructed using a relocation (of 0) relative to function symbol since vectoring may attach function symbol to different code. A possible way for object producers to construct the first word is to use the section-relative offset of function symbol, or-d with 1 for a Thumb function, relocated by the place-relative offset to the section symbol for the section containing the function, since section symbol is not global and hence not vectorable. A table entry offset to be stored in the second word can be generated as 0 relocated by the table entry symbol, or as the offset of the table entry in the table section relocated by the table section symbol. EXIDX_CANTUNWIND is language-independent, so a smart linker may be able to group such entries (for smaller runtime table size) without needing to understand language-specific table encodings. ARM IHI 0038A Copyright © 2002-2005, 2007 ARM Limited. All rights reserved. Page 15 of 50
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Exception Handling ABI for the ARM Architecture

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