[PATCH] D150892: [clang][ExprConstant] fix __builtin_object_size for flexible array members

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added inline comments.


================
Comment at: clang/lib/AST/ExprConstant.cpp:11740
+          LVal.getLValueBase().dyn_cast<const ValueDecl *>());
+      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+    }
----------------
nickdesaulniers wrote:
> erichkeane wrote:
> > nickdesaulniers wrote:
> > > erichkeane wrote:
> > > > Isn't this a possible null-deref?  
> > > I don't think so; in fact, I can use `cast` and `get` rather than `dyn_cast_or_null` and `dyn_cast` here.
> > > 
> > > Just because we have a pointer doesn't mean it's possibly `nullptr`; I don't think we can reach this code patch for evaluating the `__builtin_object_size` of a struct with a flexible array member if the LValue doesn't have a corresponding VarDecl.
> > Of course that is a possibility.  You shouldn't use dyn_cast right before a dereference, this should definitely be using `cast`/`get`, since they assert.
> > 
> > What does the LValue have when the dynamic struct does not have an initializer?  Or just an empty one? 
> s/code patch/code path/
> What does the LValue have when the dynamic struct does not have an initializer? Or just an empty one?

Tested:
```
struct foo {
    int x;
    int y[];
};

static struct foo instance = {
    .x = 3,
    // .y = { 5, 10, 15, },
};

unsigned long foo (void) {
    return __builtin_object_size(&instance, 1);
}
```
With the following values for `.y` in `instance`:
1. `.y = { 5, 10, 15 }`: `foo` returns `16`.
2. `.y = {},`: `foo` returns `4`.
3. <`.y` is not specified or explicitly initialized>: `foo` returns `4`.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D150892/new/

https://reviews.llvm.org/D150892