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C++: Letting compiler know asm block can call function that can throw?


Hi all,

In LibreOffice's ever-beloved low-level code to synthesize calls to C++ virtual functions, I'm having the following problem (on Linux x86_64). The function callVirtualMethod at <http://cgit.freedesktop.org/libreoffice/core/tree/bridges/source/cpp_uno/gcc3_linux_x86-64/uno2cpp.cxx?id=571876c1234ae55aab0198c7e2caf9049fcd230e#n61> effectively does the following:

First, call dummy_can_throw_anything that can potentially throw (see below for why that's there). Second, in an asm block, call some virtual function (that can potentially throw). Third, call x86_64::fill_struct that can potentially throw (it doesn't, but nobody bothered to annotate it as "throw ()").

Now, at least GCC 4.7.0 with -O0 produces a .gcc_except_table section for callVirtualMethod, with two call-site table entries each spanning the first (dummy_can_throw_anything) and third (x86_64::fill_struct) calls, resp., but none spanning the second (asm block) call. These entries are effectively nop, simply calling back into _Unwind_Resume, and compiling at higher optimization levels leaves them out anyway (leading to callVirtualMethod having no corresponding .gcc_except_table section).

The problem is that if the virtual function called through the asm block throws an exception, that then immediately leads to std::terminate. My understanding is that because the ip is at the call instruction in the asm block that is between the two call-site table entries, the unwind machinery thinks this cannot happen and bails out. (When compiled -O2, the code happens to work fine, as there is no .gcc_except_table section for this frame at all, so unwinding simply passes through it without calling the personality function.)

Making sure that there are no calls to (compiler-visible) functions that can throw within callVirtualMethod would happen to make the code also work with -O0. But that would remain a fragile solution.

Is there a way to let the compiler know that the asm block potentially calls functions that can throw? So that it could emit correct code, regardless of whether callVirtualMethod happens to have a corresponding .gcc_except_table section or not.

(The call to dummy_can_throw_anything, copied from the corresponding older code for 32-bit x86, is there for the following reason: At least with some compiler version and some optimization level, on x86 it was discovered that the compiler did not emit the .eh_frame data necessary for unwinding to successfully pass through this frame at all. As the corresponding x86 code does not have a call to x86_64::fill_struct, the compiler apparently considered callVirtualMethod a leaf function and optimized accordingly. The dummy_can_throw_anything hack happened to make it do the right thing again, but again this is a fragile solution, anyway, that could be replaced with something robust if there were a way to annotate the asm block as "calls functions that can throw.")

Stephan


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