[clang] [clang] constexpr atomic builtins (__c11_atomic_OP and __atomic_OP) (PR #98756)

[Aaron Ballman via cfe-commits]

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In-Reply-To: <llvm.org/llvm/llvm-project/pull/98756 at github.com>


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@@ -1744,97 +1744,97 @@ def SyncSwapN : Builtin, SyncBuiltinsTemplate {
 // C11 _Atomic operations for <stdatomic.h>.
 def C11AtomicInit : AtomicBuiltin {
   let Spellings = ["__c11_atomic_init"];
-  let Attributes = [CustomTypeChecking];
+  let Attributes = [CustomTypeChecking, Constexpr];
   let Prototype = "void(...)";
 }
 
 def C11AtomicLoad : AtomicBuiltin {
   let Spellings = ["__c11_atomic_load"];
-  let Attributes = [CustomTypeChecking];
+  let Attributes = [CustomTypeChecking, Constexpr];
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AaronBallman wrote:

I'm worried about changing the C11 atomic builtins for a C++26 feature; I think there are more changes needed as a result. For example, changing this to be constexpr means we now run the risk of accepting this code in C: https://godbolt.org/z/hc3oqYx6b

We likely are missing test coverage for this kind of thing in C, so I would recommend 1) add test coverage to this PR and if it breaks as a result of these changes, then 2) I would probably add some `getLangOpts().CPlusPlus` checks in the constant expression evaluator. We could invent new tablegen to say "this is constexpr only in C++", but it's not clear to me whether there will be enough builtins that need such functionality or not, but that's another (cleaner,  but a heavier lift for a new contributor) option.

Hmm, but I just remembered that `__has_constexpr_builtin` (https://clang.llvm.org/docs/LanguageExtensions.html#has-constexpr-builtin) is a thing, so we may need that tablegen solution after all because otherwise we change the behavior here in confusing ways: https://godbolt.org/z/qGY575MGr FWIW, here's an example of some recent changes to tablegen that you could model your changes after: https://github.com/llvm/llvm-project/pull/91894/files

https://github.com/llvm/llvm-project/pull/98756