[clang] [clang] Constexpr for __builtin_shufflevector and __builtin_convertvector (PR #76615)

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Pol Marcet =?utf-8?q?Sardà?= <polmarcetsarda at gmail.com>,
Pol Marcet =?utf-8?q?Sardà?= <polmarcetsarda at gmail.com>,Pol M
 <polmarcetsarda at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/76615 at github.com>


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@@ -10895,6 +10899,132 @@ bool VectorExprEvaluator::VisitUnaryOperator(const UnaryOperator *E) {
   return Success(APValue(ResultElements.data(), ResultElements.size()), E);
 }
 
+static bool EvaluateVectorOrLValue(APValue &Result, EvalInfo &Info,
----------------
sethp wrote:

I've dealt with lvalues in the interpreter, but maybe only a little more than you have. I've found it more than a little confusing, especially because interpreter seemingly uses "RValue" as a synonym for "actual value" (as you're using it here), and "LValue" as "opaque blob of bytes" (with metadata, and accessors, but enough wrinkles that it's just easier to convert to an RValue to work with the actual data).

That said, I think there's two ways to go here to meet the need instead of this function:

1. Teach the parser that these builtins ought to take their arguments as rvalues, so that it will insert a `LValueToRValue` cast in the expression tree wrapping the src value, and so you can use `EvaluateVector` (probably loosening its assertion to allow `xvalue`s too). This probably would have downstream consequences, though, and I lack the expertise to know whether they'd be good or bad. But it might be interesting!
2. Use, as much of the interpreter seems to, `EvaluateAsRValue` as shorthand for "do an in-place 'dereference' of the lvalue": the main difference I see from `EvaluateVectorOrLValue` here is that the latter checks for an impossible condition (SemaChecking.cpp already prevents construction of any  [ConvertVectorExpr](https://github.com/llvm/llvm-project/blob/2e08e821b7ea5bf7c0fe0775feb94a7fdb5204c7/clang/lib/Sema/SemaChecking.cpp#L9428-L9435) or [ShuffleVectorExpr](https://github.com/llvm/llvm-project/blob/2e08e821b7ea5bf7c0fe0775feb94a7fdb5204c7/clang/lib/Sema/SemaChecking.cpp#L9351-L9356) that doesn't have vector-typed arguments in the appropriate slots).

For the latter path, it might be wise to add a test case to exercise the checker, too; perhaps something like this for `__builtin_convertvector`?

```c++
static_assert([]{
    __builtin_convertvector(0, vector8char); // expected-error {{must be a vector}}
    __builtin_convertvector((vector8char){}, int); // expected-error {{must be of vector type}}
    return 1;
}());
```

(and similar for `__builtin_shufflevector`)

https://github.com/llvm/llvm-project/pull/76615