[llvm] [AArch64] Fix extern_weak function calls on Windows (PR #219202)

[Martin Storsjö via llvm-commits]

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@@ -11,7 +11,7 @@ define void @func() nounwind {
 ; CHECK-NEXT:    ldr x8, [x8, :lo12:.refptr.weakfunc]
 ; CHECK-NEXT:    cbz x8, .LBB0_2
 ; CHECK-NEXT:  // %bb.1:
-; CHECK-NEXT:    blr x8
+; CHECK-NEXT:    bl weakfunc
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mstorsjo wrote:

This is wrong. The previous indirection through a `.refptr` is essential here.

If we have an extern weak, there's a chance that this resolves to absolute null, if there's no actual definition of the symbol. A `bl` instruction encodes a PC relative offset. If the target symbol is absolute null, this doesn't work.

So the existing case, of loading the pointer through `.refptr`, is correct in general.

If the symbol is marked `dso_local` then there's no `.refptr` generated. Only then we may be able to deduce that the symbol won't resolve to absolute null, and the `.refptr` indirection isn't needed. But for the existin cases where `.refptr` is used, we must not change the generated code.

https://github.com/llvm/llvm-project/pull/219202