Add @_used and @_section attributes for global variables and top-level functions

[kubamracek]

Add @_used and @_section attributes for global variables and top-level functions

This adds:

• @_used attribute that flags as a global variable or a top-level function as "do not dead-strip" via llvm.used, roughly the equivalent of __attribute__((used)) in C/C++.
• @_section("...") attribute that places a global variable or a top-level function into a section with that name, roughly the equivalent of __attribute__((section("..."))) in C/C++.

[eeckstein]

We need to think about inlining behavior. What should happen if a function with a section attribute is inlined into a function without (or a different) section attribute? Should inlining be prevented in this case?

[kubamracek]

Good point. My sense is that we should mirror what __attribute__((section)) does in Clang: It does not prevent inlining or interact with it in any way. What the attribute does is that it makes sure that the standalone/top-level function is emitted into that section, but other functions are still free to inline the code from it (as long as we also emit the standalone function that is in the right section). If the user wants to prevent inlining, they can use another attribute for that, __attribute__((noinline)) / @inline(never).

WDYT?

[kubamracek]

@swift-ci please test

[kubamracek]

As @jckarter pointed out to me, it's needed to use different section names for different platforms / object file formats, but luckily this can already be done by conditionalizing the attribute:

#if os(...)
@_section("...")
#elif os(...)
@_section("...")
#endif
func foo()

[kubamracek]

Notes to self: I need to also update (1) UnderscoredAttributes.md, (2) the attribute list in SwiftSyntax.

[hborla]

I don't see a new experimental feature for these attributes. Can you please gate them behind a new EXPERIMENTAL_FEATURE identifier (declared in Features.def)?

[eeckstein]

lgtm!

[kubamracek]

I don't see a new experimental feature for these attributes. Can you please gate them behind a new EXPERIMENTAL_FEATURE identifier (declared in Features.def)?

Added.

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[dreampiggy]

Finally😂