When an initializer is provided to a variable, the Linux kernel relied
on the compiler to zero-initialize unspecified fields, as clarified in
https://www.spinics.net/lists/netdev/msg1007244.html.
But clang doesn't guarantee this:
1. For a union type, if an empty initializer is given, clang only
initializes bytes for the first field, left bytes for other (larger)
fields are marked as undef. Accessing those undef bytes can lead
to undefined behaviors.
2. For a union type, if an initializer explicitly sets a field, left
bytes for other (larger) fields are marked as undef.
3. When an initializer is given, clang doesn't zero initialize padding.
So this patch makes the following change:
1. In C, when an initializer is provided for a variable, zero-initialize
undef and padding fields in the initializer.
2. Document the change in LanguageExtensions.rst.
As suggested in
https://github.com/llvm/llvm-project/issues/78034#issuecomment-2183437928,
the change isn't required by C23, but it's standards conforming to do
so.
Fixes: https://github.com/llvm/llvm-project/issues/97459
We have a new policy in place making links to private resources
something we try to avoid in source and test files. Normally, we'd
organically switch to the new policy rather than make a sweeping change
across a project. However, Clang is in a somewhat special circumstance
currently: recently, I've had several new contributors run into rdar
links around test code which their patch was changing the behavior of.
This turns out to be a surprisingly bad experience, especially for
newer folks, for a handful of reasons: not understanding what the link
is and feeling intimidated by it, wondering whether their changes are
actually breaking something important to a downstream in some way,
having to hunt down strangers not involved with the patch to impose on
them for help, accidental pressure from asking for potentially private
IP to be made public, etc. Because folks run into these links entirely
by chance (through fixing bugs or working on new features), there's not
really a set of problematic links to focus on -- all of the links have
basically the same potential for causing these problems. As a result,
this is an omnibus patch to remove all such links.
This was not a mechanical change; it was done by manually searching for
rdar, radar, radr, and other variants to find all the various
problematic links. From there, I tried to retain or reword the
surrounding comments so that we would lose as little context as
possible. However, because most links were just a plain link with no
supporting context, the majority of the changes are simple removals.
Differential Review: https://reviews.llvm.org/D158071
with a string. This case is sort of tricky because we can't modify the
StringLiteral used to represent such initializers.
We are forced to decompose the string into individual characters.
Fixes <rdar://problem/10465114>.
llvm-svn: 183791
