[PATCH] D108479: [Clang] Add __builtin_addressof_nocfi

[Sami Tolvanen via Phabricator via cfe-commits]

samitolvanen planned changes to this revision.
samitolvanen added a comment.

In D108479#3149297 <https://reviews.llvm.org/D108479#3149297>, @rjmccall wrote:

> In D108479#3149228 <https://reviews.llvm.org/D108479#3149228>, @samitolvanen wrote:
>
>> I worked around this for now by explicitly allowing `__builtin_function_start` in `CheckLValueConstantExpression`, but this seems terribly hacky. What would be the correct way to solve this issue?
>
> Try to generalize what we do for `__builtin___CFStringMakeConstantString`.

Thanks, I'll take a look.



================
Comment at: clang/lib/Sema/SemaChecking.cpp:208
+    if (UnaryOp->getOpcode() == UnaryOperator::Opcode::UO_AddrOf)
+      E = UnaryOp->getSubExpr();
+
----------------
rjmccall wrote:
> samitolvanen wrote:
> > rjmccall wrote:
> > > It would be more general to allow any expression that we can constant-evaluate to a specific function / member function reference.  That allows callers to do stuff like `__builtin_function_start((int (A::*)() const) &A::x)` to resolve overloaded function references.
> > > 
> > > You should delay this check if the operand is value-dependent.
> > > It would be more general to allow any expression that we can constant-evaluate to a specific function / member function reference.  That allows callers to do stuff like `__builtin_function_start((int (A::*)() const) &A::x)` to resolve overloaded function references.
> > 
> > I looked into using `Expr::EvaluateAsConstantExpr` here and while it works, I'm not sure if allowing arbitrary expressions as the argument provides any value. We can allow resolving overloaded function references without constant-evaluating the expression (and I added tests for this). Did you have any other use cases in mind where this might be useful?
> I don't see what the advantage of limiting the constant expression would be if we can constant-evaluate it.  `switch` doesn't force you to make case values be integer literals and/or references to enumerators.  What are you trying to achieve with a restriction?
> 
> Not having arbitrary restrictions is particularly useful in C++, where templates and `constexpr` machinery can usefully do a lot of abstraction.
> I don't see what the advantage of limiting the constant expression would be if we can constant-evaluate it.  `switch` doesn't force you to make case values be integer literals and/or references to enumerators.  What are you trying to achieve with a restriction?

I'm trying to understand the benefit of allowing arbitrary expressions like `__builtin_function_start(100 + a)`, which `EvaluateAsConstantExpr` is happy to evaluate into a reference to `a`.

I can obviously understand why these should be allowed for `switch`, but here we have a function whose sole purpose is to return the address of a function and I'm wondering why someone would want pass anything more complicated to it.

> Not having arbitrary restrictions is particularly useful in C++, where templates and `constexpr` machinery can usefully do a lot of abstraction.

Perhaps I'm just not that familiar with the C++ use cases here. Would you be able to provide me an example I could use as a test case?


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