rjmccall added inline comments.
================ Comment at: clang/lib/Sema/SemaChecking.cpp:208 + if (UnaryOp->getOpcode() == UnaryOperator::Opcode::UO_AddrOf) + E = UnaryOp->getSubExpr(); + ---------------- samitolvanen wrote: > 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? > 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. The idea is that you might have a complicated way of picking which function you mean. I agree that this should probably disallow non-zero offsets. > 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? A simple example: `fn` after `constexpr void (*fn)() = &foo;`. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D108479/new/ https://reviews.llvm.org/D108479 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits