<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Fri, Dec 19, 2014 at 1:13 PM, Kaylor, Andrew <span dir="ltr"><<a href="mailto:andrew.kaylor@intel.com" target="_blank">andrew.kaylor@intel.com</a>></span> wrote:<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal">* Add support for unwinding in frames with no exception handlers<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">I’m not clear as to what needs to be done for this. I know clang is intentionally generating errors when you compile code with an MSVC-environment target. I’m guessing that just
removing this block will be sufficient for clang to generate the IR we want. Then it will be up to the back end to produce the correct unwinding support, which I think it may already be doing.<u></u><u></u></p>
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<p class="MsoNormal"><span style="color:#1f497d">>> </span>Do we not do this already? This shouldn't be hard, we already know how to emit the required pdata/xdata call frame information (CFI).<u></u><u></u></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Like I said, clang spits out an error if you try to compile with exceptions enabled for an MSVC-environment target. If you compile without exceptions enabled,
it doesn’t generate the IR necessary to cleanup during unwind if an exception is thrown across a function. I tested it by compiling functions with MSVC that call through a function compiled with clang and then throwing an exception across the clang-compiled
function. It unwinds correctly, but doesn’t do any cleanup.</span></p></div></div></div></div></div></div></blockquote><div><br></div><div>Ah, OK, I misunderstood what you meant. I guess you're asking about emitting handlers for C++ destructors when no explicit EH construct (try or __try) is present.</div><div><br>This shouldn't be hard. This should use the same IR as __finally and should be outlined in the same way. The only difference is what personality function gets used.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-US" link="blue" vlink="purple"><div><div><div><div><div>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I know that llvm has code to handle this with some personality functions (mingw, at least), but I don’t think it supports the “native” MSVC personality functions
yet (__CxxFrameHandler3/4 in this case). I suppose that will come out of the C++EH work with nothing extra to be done especially for SEH in the back end. I just wanted to call it out in case there was something else I had overlooked.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Here’s the test case I have in mind.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">==========================================<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">#include <windows.h><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">#include <stdio.h><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">class MyClass {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">public:<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> MyClass() { printf("Hello\n"); }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> ~MyClass() { printf("Goodbye\n"); }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">};<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">void bar() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> RaiseException(1, 0, 0, NULL);<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">void foo() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> MyClass Fubar;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> bar();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">DWORD filter() {
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> printf("In filter\n");<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> return EXCEPTION_EXECUTE_HANDLER;
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">int main() {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> __try {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> __try {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> foo();<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> __except (filter()) {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> printf("In exception handler\n");<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> __finally {<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> printf("Finally!\n\n");
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> }<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> return 0;<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">}<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">==========================================<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Compiling that with MSVC, it doesn’t execute the MyClass destructor if I compile with no /EH option, but it does execute the destructor with /EHa, having called
it from __CxxFrameHandler3 while unwinding, on its way to the exception handler. I understand that you aren’t worried about faithfully reproducing the behavior of /EHa, but we do at least want to clean up in cases like this, right?</span></p></div></div></div></div></div></div></blockquote><div><br></div><div>I think MSVC might default to EHs and not EHa because most code isn't written to handle recovery from random access violations. The destructors might clean things up successfully, or they might fault. Once we've gone to async exceptions land, it's almost safer to not run C++ cleanups and just run __finally blocks that were explicitly written to clean up after faults.</div><div><br></div><div>However, if someone does want to run C++ destructors, it's easy for us to support it at a best-effort level by using the normal _CxxFrameHandler3/4 personality function and flipping the flag in the EH table that controls this behavior. Some of the cleanups will get run, but probably not the ones in the function that trapped. In the case that you've outlined, I think everything will work as intended because RaiseException() is a potentially throwing function call.</div><div><br></div><div>If we want to support this level of EHa, it's probably worth thinking about how to represent it for LTO. My first idea is "use a function attribute", but those can get lost with cross-module inlining. Then again, it reflects the limitations of the _CxxFrameHandler3 flag, which operates at a whole-function level.</div></div></div></div>