[PATCH] D68115: Zero initialize padding in unions

[Hubert Tong via Phabricator via cfe-commits]

hubert.reinterpretcast added inline comments.


================
Comment at: clang/test/CodeGen/init.c:197
   // CHECK-LABEL: @nonzeroPaddedUnionMemset(
-  // CHECK-NOT: store
-  // CHECK-NOT: memcpy
-  // CHECK: call void @llvm.memset.p0i8.i32(i8* {{.*}}, i8 -16, i32 36, i1 false)
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 4 {{.*}}, i8* align 4 {{.*}} [[INIT_PADDEDUNION]], {{.*}}, i32 36, i1 false)
 }
----------------
vitalybuka wrote:
> hubert.reinterpretcast wrote:
> > vitalybuka wrote:
> > > vitalybuka wrote:
> > > > hubert.reinterpretcast wrote:
> > > > > This is C++ aggregate initialization and not value-initialization. The wording you quoted from the C++ standard is for zero-initialization, which might be part of value initialization, but you have not shown that aggregate initialization of a union involves zero-initialization of that union.
> > > > reading this more I don't see any evidence that either C++ or C requires padding initialization.
> > > > Reading this I expect that all function here should be equivalent https://godbolt.org/z/1O_9-e
> > > > But they are not. Clang and GCC initialized padding after the first member.
> > > So if go trough "aggregates" then nothing is said about padding in union.
> > > If we go trough "list-initialization" then value initialization should be applied, part of which is zero initialization. 
> > > If so union padding should be initialized.
> > > 
> > > 
> > In C++14, the list in [dcl.init.list] starts with:
> > If T is an aggregate, aggregate initialization is performed.
> > 
> > The bullet in C++17 follows only a case for copying and then a case for string literals.
> > 
> > The bullet in the C++2a CD likewise, with an additional earlier bullet that also becomes aggregate initialization.
> > 
> > It is in C++11 where an empty list gets value-initialization treatment, and the next bullet goes to aggregate initialization.
> > 
> > The inline comment was not added to an empty list case.
> So if I understand you (and what I see in C++17 and C++11) then only C++11 will essentially requires zeros via value-initialization.
> And the rest (including C), requires only the first field to be initialized and the tail is undefined.
> 
> 
That is my understanding. C++11 will require zeroes for `U u = { };` and `U u{}`. The tail is undefined in other cases with automatic storage duration.


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  https://reviews.llvm.org/D68115/new/

https://reviews.llvm.org/D68115