[clang] [clang][CodeGen] Zero init unspecified fields in initializers in C (PR #97121)

[John McCall via cfe-commits]

================
@@ -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.
----------------
rjmccall 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. 

No, I mean initializing a local aggregate with a non-constant initializer, e.g.

```
void g(char c) {
  struct A a = { c, 2 };
}
```

Do we already zero the padding correctly in this case?

https://github.com/llvm/llvm-project/pull/97121