================
@@ -361,6 +368,13 @@ CodeGenFunction::AddInitializerToStaticVarDecl(const
VarDecl &D,
}
return GV;
}
+ if (!getLangOpts().CPlusPlus) {
+ // In C, when an initializer is given, the Linux kernel relies on clang to
+ // zero-initialize all members not explicitly initialized, including
padding
+ // bits.
----------------
yabinc wrote:
> But I assume that similar code is required in order to explicitly zero out
> any padding when dynamically initializing a local variable.
I guess you mean variables assigned by Compound literals after variable
definition. An example is in https://godbolt.org/z/caaEKnjWe. From the ast
dump, it is treated as an assignment from InitListExpr. The source code seems
to be in AggExprEmitter::VisitCXXParenListOrInitListExpr() in
https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGExprAgg.cpp#L1757.
Each field of the InitListExpr is stored to the variable separately. One
solution is to always do memset in CheckAggExprForMemSetUse() in
https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGExprAgg.cpp#L1974.
The memset can be optimized away later if not needed.
Another place is compound literals for global values. It seems handled in
CGM.GetAddrOfConstantCompoundLiteral().
To not introduce a big change at once, Shall I handle them in a follow up patch?
> Also, is adding constant padding fields retroactively really the best way to
> handle this instead of just expecting the constant emitter to fill them in
> itself?
Currently I limit the change to C and reuse code for trivial auto var zero
init. So it's easier to do it outside the constant emitter. If the change can
be extended to C++, I feel it's better to always do it in the constant emitter.
> Comments in the compiler generally shouldn't refer to specific projects. We
> are making a guarantee based on our interpretation of the standard.
The standard is ambiguous on this, so can't be used as a reason for the change.
Please let me know if you have better idea about how to reword the comment.
> Similarly, the PR title is also inappropriate. It is fine to note in the PR
> description that Linux relies on this for correctness, but the PR title
> should describe the semantic change.
Changed the PR title back to the commit title, which describes the semantic
change.
> Also, I think we should do this unconditionally in all language modes. It's
> not like this is forbidden in C++.
I agree. But when I was trying to do it before, I need to fix a lot more tests.
And the Linux kernel only has C code. I don't have strong reason to push this
to C++. So I want to limit the change to C at first. If reviewers think we
should also do it in C++, shall I do it in a follow up change?
https://github.com/llvm/llvm-project/pull/97121
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