yabinc wrote:
> It probably makes sense to add a helper like the following to CodeGenModule
> or something like that:
>
> ```
> bool shouldZeroInitPadding() {
> // Comment explaining reasoning for this rule
> return !CGM.getLangOpts().CPlusPlus;
> }
> ```
>
> So we don't have `// See comment in getPadding().` all over the place.
Done.
>
> Is there any relevant code using EmitNullConstant()? As far as I can tell,
> this patch doesn't zero-fill implicit padding in null constants (like in
> `struct { int x; long y; }`.
I ran all tests in clang/tests and found two places using it:
1. One is used in CodeGenFunction::EmitNullInitialization(Address DestPtr,
QualType Ty), when !CGM.getTypes().isZeroInitializable(Ty). This seems to only
happen in C++ code (when a class contains a pointer pointing
to data member).
2. Another is used in CodeGenModule::getOrCreateStaticVarDecl(), which is
further called in three places:
a. CodeGenFunction::AddInitializerToStaticVarDecl(): it later calls
ConstantEmitter::tryEmitForInitializer() if an initializer is used.
b. CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) in CGExpr.cpp
c. ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) in
CGExprConstant.cpp
Not sure what b and c are doing.
When running tests for non c++ code, I only found the 2.a path, used in
CodeGenOpenCL/amdgpu-nullptr.cl.
So I didn't find any C code using EmitNullConstant(), that results in
uninitialized fields with initializers.
For the implementation of EmitNullConstant(), it returns
ConstantStruct::get(llvm::ConstantStruct::get(structure, elements). In
ConstantStruct::get(), if all elements are zero, then it returns
ConstantAggregateZero::get(ST).
>From https://llvm.org/docs/LangRef.html:
`The string ‘zeroinitializer’ can be used to zero initialize a value to zero of
any type, including scalar and
[aggregate](https://llvm.org/docs/LangRef.html#t-aggregate) types. This is
often used to avoid having to print large zero initializers (e.g. for large
arrays) and is always exactly equivalent to using explicit zero initializers.`
>From the above words, I feel using `%struct.S2 zeroinitializer` may not
>guarantee the padding in the struct are zero initialized. But if it is used
>for a static variable created by CodeGenModule::getOrCreateStaticVarDecl(),
>it's
as good as a static/global variable without initializer.
https://github.com/llvm/llvm-project/pull/97121
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