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. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D68115/new/ https://reviews.llvm.org/D68115 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits