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
For a definition (of most linkage types), dso_local is set for ELF -fno-pic/-fpie
and COFF, but not for Mach-O. This nuance causes unneeded binary format differences.
This patch replaces (function) `define ` with `define{{.*}} `,
(variable/constant/alias) `= ` with `={{.*}} `, or inserts appropriate `{{.*}} `
if there is an explicit linkage.
* Clang will set dso_local for Mach-O, which is currently implied by TargetMachine.cpp. This will make COFF/Mach-O and executable ELF similar.
* Eventually I hope we can make dso_local the textual LLVM IR default (write explicit "dso_preemptable" when applicable) and -fpic ELF will be similar to everything else. This patch helps move toward that goal.
This reverts commit r220169 which reverted r220153. However, it also
contains additional changes:
- We may need to add padding *after* we've packed the struct. This
occurs when the aligned next field offset is greater than the new
field's offset. When this occurs, we make the struct packed.
*However*, once packed the next field offset might be less than the
new feild's offset. It is in this case that we might further pad the
struct.
- We would pad structs which were perfectly sized! This behavior is
immensely old. This behavior came from blindly subtracting
NextFieldOffsetInChars from RecordSize. This doesn't take into
account the fact that the struct might have a greater overall
alignment than the last field.
llvm-svn: 220175
- This is designed to make it obvious that %clang_cc1 is a "test variable"
which is substituted. It is '%clang_cc1' instead of '%clang -cc1' because it
can be useful to redefine what gets run as 'clang -cc1' (for example, to set
a default target).
llvm-svn: 91446
