[GlobalOpt] Don't replace alias with aliasee if either alias/aliasee may be preemptible
Generalize D99629 for ELF. A default visibility non-local symbol is preemptible in a -shared link. `isInterposable` is an insufficient condition. Moreover, a non-preemptible alias may be referenced in a sub constant expression which intends to lower to a PC-relative relocation. Replacing the alias with a preemptible aliasee may introduce a linker error. Respect dso_preemptable and suppress optimization to fix the abose issues. With the change, `alias = 345` will not be rewritten to use aliasee in a `-fpic` compile. ``` int aliasee; extern int alias __attribute__((alias("aliasee"), visibility("hidden"))); void foo() { alias = 345; } // intended to access the local copy ``` While here, refine the condition for the alias as well. For some binary formats like COFF, `isInterposable` is a sufficient condition. But I think canonicalization for the changed case has little advantage, so I don't bother to add the `Triple(M.getTargetTriple()).isOSBinFormatELF()` or `getPICLevel/getPIELevel` complexity. For instrumentations, it's recommended not to create aliases that refer to globals that have a weak linkage or is preemptible. However, the following is supported and the IR needs to handle such cases. ``` int aliasee __attribute__((weak)); extern int alias __attribute__((alias("aliasee"))); ``` There are other places where GlobalAlias isInterposable usage may need to be fixed. Reviewed By: rnk Differential Revision: https://reviews.llvm.org/D107249
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