Rewrite static section bounds discovery in Swift. #1012
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This PR rewrites the static section bounds discovery code in Swift (replacing the current C++ implementation.) We are making use of `@_silgen_name` here to name the section bounds symbols that are defined by the linker, so we'll want to get the core team's approval before merging.
This reverts commit e107fa4.
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Azoy
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Honestly, that "Close with comment" button is too easy to click. |
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Waiting for a toolchain build to complete before merging. |
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@jckarter Famous last words. |
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This PR rewrites the static section bounds discovery code in Swift (replacing the current C++ implementation.) We are making use of
@_silgen_namehere to name the section bounds symbols that are defined by the linker, so we'll want to get the core team's approval before merging.Why do we need to rewrite this code in Swift?
In Embedded Swift, there is only one (statically linked) image in the process containing Swift code, and if the target is a microcontroller or similar, it probably doesn't even have a dynamic linker/loader in the first place for us to query. So we need, on Embedded Swift targets, to perform static discovery of exactly one test content section. (This is potentially something we can make configurable via the target's toolset in a future PR.)
The bounds of the test content section are currently defined in C++ using
__asm__to reference linker-defined symbols. However, our C++ target can't reliably tell if it is being built for Embedded Swift. That knowledge is treated by the Swift toolchain as a Swift language feature and can be checked in Swift withhasFeature(Embedded), but there's no C++ equivalent. If the implementation is written in Swift, we can know at compile time whether or not we're dynamically or statically linking to a test target (and therefore whether we need to do dynamic or static lookup of images, customized toolset aside.)Rewriting this logic in Swift reduces our C++ code size (and our direct dependencies on the C++ STL) which is also a nice bonus.
Resolves rdar://147430270.
Checklist: