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I will apply this patch to my Android armv7 build of Swift 5.4 and see if it fixes it. The problem is definitely in _cacheCanonicalSpecializedMetadata, as if I comment out the two calls to that method, building Swift executables starts working again on Android armv7. Someone is going to have to step through the generated assembly for that function and see if some tweaks to its source will get the codegen optimizations working properly again, as the method works fine when unoptimized by LLVM.

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I applied this pull to the Mar. 10 Swift 5.4 snapshot for Android armv7 and hello world is still erroring out: @drexin, can you confirm this actually fixes the problem for you on linux armv7?

With the Swift stdlib built in release mode, I get this backtrace for hello world:

* thread #1, name = 'hello_toplevel', stop reason = signal SIGSEGV: invalid address (fault address: 0x1)
  * frame #0: 0xf742afd8 libswiftCore.so`swift::TargetMetadata<swift::InProcess>::getGenericArgs() const + 76
    frame #1: 0xf744535c libswiftCore.so`cacheCanonicalSpecializedMetadata(swift::TargetTypeContextDescriptor<swift::InProcess> const*, std::__ndk1::once_flag*)::$_21::__invoke(void*) + 468
    frame #2: 0xf6ce36ac libc++_shared.so`std::__ndk1::__call_once(unsigned long volatile&, void*, void (*)(void*)) + 76
    frame #3: 0xf74617d0 libswiftCore.so`swift_once + 64
    frame #4: 0xf743967c libswiftCore.so`swift_getCanonicalPrespecializedGenericMetadata + 36
    frame #5: 0xf741ee5c libswiftCore.so`__swift_instantiateCanonicalPrespecializedGenericMetadata + 48
    frame #6: 0xf701c854 libswiftCore.so`$ss23_ContiguousArrayStorageCMa + 44
    frame #7: 0xf6eba07c libswiftCore.so`$ss27_allocateUninitializedArrayySayxG_BptBwlF + 140
    frame #8: 0xaaaaa99c hello_toplevel`main + 48
    frame #9: 0xf4e101ae libc.so`__libc_init + 50
    frame #10: 0xaaaaa738 hello_toplevel`_start_main + 68

I ran the Swift compiler in lldb and checked the result of this modified shouldPrespecializeGenericMetadata() method and it always returns false now, so I don't think this change is enough.

That __swift_instantiateCanonicalPrespecializedGenericMetadata is being called from ss23_ContiguousArrayStorageCMa indicates that the metadata for ContiguousArray is still being prespecialized.

Oh, my mistake, I'm cross-compiling the Swift stdlib on a linux x86_64 host with a prebuilt official dev snapshot compiler that doesn't have this patch, then running that stlib on Android armv7. I wasn't thinking through how the stdlib was actually compiled without this patch, never mind.