[PATCH] D147714: [Attr] Introduce [[clang::nonportable_musttail]] as less strict version of [[clang::musttail]]

[Josh Haberman via Phabricator via cfe-commits]

haberman added a comment.

> Such a definition doesn't make much sense because for some targets tail call optimization isn't available at all.
> Eg. Wasm w/o tail call proposal, xtensa windowed abi.

That is true. But what is the correct behavior if you try to use `[[clang::musttail]]` on such a target? In my case, I want to get a compiler error, because if a tail call cannot be performed, the algorithm is incorrect (it will cause stack overflow at runtime).

There are three possible semantics that I can see:

1. Perform a tail call if possible.
2. Perform a tail call, fail to compile if this is not possible (`[[clang::nonportable_musttail]]`)
3. Perform a tail call, fail to compile if this is not possible //or// if the function signatures between caller and callee differ in ways that make tail calls difficult on some platforms (`[[clang::musttail]]`)

Both (2) and (3) would fail to compile on Wasm without tail calls. If all you want is (1), that doesn't require any attribute (the optimizer will do it automatically).


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https://reviews.llvm.org/D147714