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<p class="MsoNormal">clang generally does not emit unwind tables into a loadable section on ARM (.eh_frame on 64-bit, or .ARM.exidx on 32-bit), unless they are explicitly requested with -funwind-tables, or they are required for C++ unwinding.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Separately, it looks like there’s a compiler bug involving __clang_call_terminate specifically, where the compiler doesn’t honor -funwind-tables. Filed
<a href="https://bugs.llvm.org/show_bug.cgi?id=41351">https://bugs.llvm.org/show_bug.cgi?id=41351</a> .<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Separately, if you turn on debug info, and you’re also using DWARF unwinding (which is the default on AArch64, but not 32-bit ARM), and any function in a file requires an unwind table for C++ unwinding, all functions in that file get DWARF
unwind tables in the .eh_frame section. This isn’t really intentional; it’s a limitation of the .cfi_sections assembler directive we use for unwind tables. Maybe worth looking into at some point to try to shrink the size of the .eh_frame section.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Eli<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><b>From:</b> cfe-dev <cfe-dev-bounces@lists.llvm.org>
<b>On Behalf Of </b>Serhii Huralnik via cfe-dev<br>
<b>Sent:</b> Tuesday, April 2, 2019 11:47 AM<br>
<b>To:</b> cfe-dev@lists.llvm.org<br>
<b>Subject:</b> [EXT] [cfe-dev] ARM unwind info for __clang_call_terminate<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Hello,<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">I'm working on a simple unwinder for Android that relies on common C++ exception handling tables (.ARM.exidx / .eh_frame) and recently I found out that for armeabi-v7a it is impossible to unwind stack for abort
that happens when someone's destructor throws (seems like it is the case for any nothrow function). In that case std::terminate() is called via shim function __clang_call_terminate() which is emitted by compiler and has EXIDX_CANTUNWIND associated entry that
prevents unwinder to pass through this function:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">$ readelf --unwind test.o | grep __clang_call_terminate<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Unwind section '.ARM.exidx.text.__clang_call_terminate' at offset 0x98 contains 1 entry:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">0x0 <__clang_call_terminate>: 0x1 [cantunwind]<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Here is mentioned shim generated with -funwind-tables -O3 (source code is trivial so I omitted it):<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">__clang_call_terminate:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .fnstart<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .save {r7, lr}<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> push {r7, lr}<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> bl __cxa_begin_catch<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> bl _ZSt9terminatev<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">.Lfunc_end1:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .size __clang_call_terminate, .Lfunc_end1-__clang_call_terminate<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .cantunwind<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"> .fnend<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">At the same time unwinder passes through it on arm64-v8a. I failed to find any explanations of this difference. So asking here seems to be the last resort.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">My question is: why for 32-bit ARM clang emits .cantunwind for __clang_call_terminate() instead of normal unwind instructions?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Thanks.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">-- <o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Best regards<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Serhii<o:p></o:p></p>
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