[PATCH] D101872: [AArch64][ELF] Prefer to lower MC_GlobalAddress operands to .Lfoo$local

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D101872#2740815 <https://reviews.llvm.org/D101872#2740815>, @efriedma wrote:

> I'm not sure I understand the practical effect of this.  I mean, I get the immediate effect on the object file, but I'm not sure I understand how this impacts linking.  Without this patch, will we end up with runtime relocations in some cases?  If so, which cases?

Currently -fpic -fno-semantic-interposition is a no-op on non-x86.
On x86, -fpic -fno-semantic-interposition sets dso_local on default visibility external linkage definitions.

This change by itself does not affect clang behavior. With D101873 <https://reviews.llvm.org/D101873>, -fpic -fno-semantic-interposition will be effective on aarch64.

If D101873 <https://reviews.llvm.org/D101873> is enabled without this patch, we may produce assembly like

  ; CHECK-NEXT:    adrp x0, gv0  # instead of .Lgv0$local
  ; CHECK-NEXT:    ldr w0, [x0, :lo12:gv0]

This will be rejected by `{ld.bfd,gold,ld.lld} -shared` because gv0 (STV_DEFAULT STB_GLOBAL) is by default preemptible and direct access relocations referencing a preemptible symbol cannot be used in -shared mode.



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Comment at: llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp:592
   assert(MO.isGlobal() && "caller should check MO.isGlobal");
-  getSymbol(MO.getGlobal())->print(OS, MAI);
+  getSymbolPreferLocal(*MO.getGlobal())->print(OS, MAI);
   printOffset(MO.getOffset(), OS);
----------------
efriedma wrote:
> This is target-independent code; what targets does it affect?
Only aarch64. No-op for others.

With D101873, aarch64 will behave like x86: -fpic -fno-semantic-interposition will set dso_local on default visibility external linkage definitions.



Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101872/new/

https://reviews.llvm.org/D101872