[PATCH] D68115: Zero initialize padding in unions

[Vitaly Buka via Phabricator via cfe-commits]

vitalybuka created this revision.
vitalybuka added reviewers: rsmith, jfb.
Herald added subscribers: cfe-commits, dexonsmith.
Herald added a project: clang.
vitalybuka edited the summary of this revision.
vitalybuka edited the summary of this revision.

Existing implementation puts undef into padding bits which almost always
compiled into zeroes. However with -ftrivial-auto-var-init=pattern those undefs
became 0xAA pattern and break some code. We need to zero initialized them.

C++ requires zeroes in padding bit.

  11.6 Initializers
  (6.3) — if T is a (possibly cv-qualified) union type, its padding bits (6.7) are initialized to zero bits

Looks like C does not require, as union is not aggregate, but a lot of code already relies on this behavior.

  6.7.9 Initialization
  10. If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.
  If an object that has static or thread storage duration is not initialized explicitly, then:
  — if it is a union, the first named member is initialized (recursively) according to these rules, and
  any padding is initialized to zero bits;
  21. If there are fewer initializers in a brace-enclosed list than there are elements or members of an
  aggregate, or fewer characters in a string literal used to initialize an array of known size than there
  are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as
  objects that have static storage duration.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D68115

Files:
  clang/lib/CodeGen/CGExprConstant.cpp
  clang/test/CodeGen/2008-08-07-AlignPadding1.c
  clang/test/CodeGen/64bit-swiftcall.c
  clang/test/CodeGen/arm-swiftcall.c
  clang/test/CodeGen/designated-initializers.c
  clang/test/CodeGen/init.c
  clang/test/CodeGen/union-init2.c
  clang/test/CodeGen/windows-swiftcall.c
  clang/test/CodeGenCXX/auto-var-init.cpp
  clang/test/CodeGenCXX/const-init-cxx11.cpp
  clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp
  clang/test/CodeGenCXX/designated-init.cpp
  clang/test/CodeGenCXX/static-init.cpp

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D68115.222068.patch
Type: text/x-patch
Size: 21695 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20190927/ed28c7ab/attachment-0001.bin>