[PATCH] D74361: [Clang] Undef attribute for global variables

[Jon Chesterfield via Phabricator via cfe-commits]

JonChesterfield added a comment.

I've continued thinking about / experimenting with this. Curiously - `X x;` and `X x{};` are considered distinct by clang. The current patch will only accept the former. I'll add some tests for that.

I think there's a reasonable chance that the developers who want to elide the runtime cost of zero initialising will usually also want to avoid dynamic initialisation. That suggests we could accept only trivially default constructible classes, where the undef initializer is correct in that one cannot determine whether a no-op constructor actually ran or not. If we go with that, then the more complicated question of exactly how this should interact with user-controlled disabling of dynamic initialization can be postponed until that feature is introduced. This patch is then reasonably self contained.

This patch is strongly related to the linker script approach, or equivalent asm. It's target-dependent how the uninitialised data gets represented, e.g. x64 will put it in bss anyway. Opencl's __local is similarly uninitialised, and gets annotated with an address space that maps onto magic in llc and/or the loader.


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https://reviews.llvm.org/D74361