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Gather 'round to hear tell of the saga of autolinking in incremental mode. In the beginning, there was Swift code, and there was Objective-C code. To make one import bind two languages, a twinned Swift module named the same as an Objective-C module could be imported as an overlay. But all was not well, for an overlay could be created which had no Swift content, yet required Swift symbols. And there was much wailing and gnashing of teeth as loaders everywhere disregarded loading these content-less Swift libraries. So, a solution was found - a magical symbol _swift_FORCE_LOAD_$_<MODULE> that forced the loaders to heed the dependency on a Swift library regardless of its content. It was a constant with common linkage, and it was good. But, along came COFF which needed to support autolinking but had no support for such matters. It did, however, have support for COMDAT sections into which we placed the symbol. Immediately, a darkness fell across the land as the windows linker loudly proclaimed it had discovered a contradiction: "_swift_FORCE_LOAD_$_<MODULE> cannot be a constant!", it said, gratingly, "for this value requires rebasing." Undeterred, we switched to a function instead, and the windows linker happily added a level of indirection to its symbol resolution procedure and all was right with the world. But this definition was not all right. In order to support multiple translation units emitting it, and to prevent the linker from dead stripping it, Weak ODR linkage was used. Weak ODR linkage has the nasty side effect of pessimizing load times since the dynamic linker must assume that loading a later library could produce a more definitive definition for the symbol. A compromise was drawn up: To keep load times low, external linkage was used. To keep the linker from complaining about multiple strong definitions for the same symbol, the first translation unit in the module was nominated to recieve the magic symbol. But one final problem remained: Incremental builds allow for files to be added or removed during the build procedure. The placement of the symbol was therefore dependent entirely upon the order of files passed at the command line. This was no good, so a decree was set forth that using -autolink-force-load and -incremental together was a criminal offense. So we must compromise once more: Return to a symbol with common linkage, but only on Mach-O targets. Preserve the existing COMDAT-friendly approach everywhere else. This concludes our tale. rdar://77803299
28 lines
1.7 KiB
Swift
28 lines
1.7 KiB
Swift
// RUN: %empty-directory(%t)
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// RUN: %swift -disable-legacy-type-info -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-WMO %s
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// CHECK-WMO: source_filename = "<swift-imported-modules>"
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// CHECK-WMO: @"_swift_FORCE_LOAD_$_TEST"
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// CHECK-WMO-NOT: source_filename
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// RUN: %swift -disable-legacy-type-info -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir -num-threads 1 %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-WMO-THREADED %s
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// CHECK-WMO-THREADED: source_filename = "<swift-imported-modules>"
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// CHECK-WMO-THREADED: @"_swift_FORCE_LOAD_$_TEST"
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// CHECK-WMO-THREADED: source_filename = "<swift-imported-modules>"
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// CHECK-WMO-THREADED: @"_swift_FORCE_LOAD_$_TEST"
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// RUN: %swift -disable-legacy-type-info -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir -primary-file %s %S/../Inputs/empty.swift | %FileCheck -check-prefix=CHECK-SINGLE-FILE-FIRST %s
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// RUN: %swift -disable-legacy-type-info -target x86_64-apple-macosx10.9 -parse-stdlib -autolink-force-load -module-name TEST -module-link-name TEST -emit-ir %S/../Inputs/empty.swift -primary-file %s | %FileCheck -check-prefix=CHECK-SINGLE-FILE-SECOND %s
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// CHECK-SINGLE-FILE-FIRST: source_filename = "<swift-imported-modules>"
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// CHECK-SINGLE-FILE-FIRST: @"_swift_FORCE_LOAD_$_TEST"
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// CHECK-SINGLE-FILE-FIRST-NOT: source_filename
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// CHECK-SINGLE-FILE-SECOND: source_filename = "<swift-imported-modules>"
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// CHECK-SINGLE-FILE-SECOND: @"_swift_FORCE_LOAD_$_TEST"
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// CHECK-SINGLE-FILE-SECOND-NOT: source_filename
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