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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">OK, thanks. "My program" is actually test/CodeGenCXX/global-init.cpp which is expecting not to see the memset.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Given the '=default-in-C++03' bug, I'll make it work on both 03 and 11, and leave a FIXME about it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> metafoo@gmail.com [mailto:metafoo@gmail.com]
<b>On Behalf Of </b>Richard Smith<br>
<b>Sent:</b> Tuesday, December 20, 2016 5:07 PM<br>
<b>To:</b> Robinson, Paul<br>
<b>Cc:</b> cfe-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [cfe-dev] C++11 doing double zero initialization?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On 20 December 2016 at 16:39, Robinson, Paul via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Consider the following source:<br>
<br>
struct C { C() = default; C(const C&); int n; };<br>
const C c1 = C();<br>
<br>
then run it through `clang -emit-llvm -std=c++11`,<br>
and I see the following relevant bits:<br>
<br>
%struct.C = type { i32 }<br>
@_ZL2c1 = internal global %struct.C zeroinitializer, align 4<br>
define internal void @__cxx_global_var_init() {{.*}} {<br>
entry:<br>
call void @llvm.memset.p0i8.i64(i8* bitcast (%struct.C* @_ZL2c1 to i8*),<br>
i8 0, i64 4, i32 4, i1 false)<br>
ret void<br>
}<br>
<br>
<br>
The question is: Why the memset? Doesn't 'zeroinitializer'<br>
do the exact same thing?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Initialization of C++ globals is performed in two stages:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> * in the first stage ("static initialization"), <o:p></o:p></p>
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<p class="MsoNormal"> -- if the object has a constant initializer (which C() is not, see <a href="http://wiki.edg.com/pub/Wg21issaquah2016/CoreWorkingGroup/cwg_active.html#1452">http://wiki.edg.com/pub/Wg21issaquah2016/CoreWorkingGroup/cwg_active.html#1452</a>),
then object is initialized to that constant value ("constant intiialization")<o:p></o:p></p>
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<p class="MsoNormal"> -- otherwise is initialized to zero ("zero-initialization").<o:p></o:p></p>
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<p class="MsoNormal"> * in the second stage ("dynamic initialization", which is skipped if the first stage used constant initialization or if no initializer is provided for a scalar type), the initializer is run.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">What you're seeing here is a direct application of these rules to your program.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">C++ allows dynamic initialization to be converted to constant initialization if the initialization doesn't have side-effects (more or less), and Clang uses that permission to remove some dynamic initializers, but we don't happen to do so
in this case (in either C++03 or C++11). That's a QoI bug (and core issue 1452 may make it a correctness bug). Either way, we should fix it.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">I don't see the memset with C++03.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The input is not valid C++03 code, due to the "= default;", and our allowance of that construct in C++03 as an extension does something a bit strange: the C++11 initialization rules say that -- because C::C() is not user-provided -- we
first zero-initialize the entire entire object and then call the no-op constructor, which is why you get a memset. However, the C++03 initialization rules are slightly different, and don't require C() to zero-initialize in this case prior to running the constructor
(you only get the implicit zero-initialization for C() in C++03 if the class doesn't declare any constructors at all).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I think our =default-in-C++03 extension is actually broken here, and we should emit the memset in both languages. Given:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"> struct A { A() = default; int n; };<o:p></o:p></p>
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<p class="MsoNormal"> int f() { A a = A(); return a.n; }<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">... this should be guaranteed to return 0 even in C++03 mode. (Right now it compiles to 'ret undef' in C++03.)<o:p></o:p></p>
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