vitalybuka marked 2 inline comments as done.
vitalybuka 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)
}
----------------
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.
Repository:
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https://reviews.llvm.org/D68115/new/
https://reviews.llvm.org/D68115
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