[clang-tools-extra] [llvm] [clang] [llvm] Support IFuncs on Darwin platforms (PR #73686)

[Fangrui Song via cfe-commits]

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@@ -1809,6 +1814,255 @@ void AArch64AsmPrinter::emitInstruction(const MachineInstr *MI) {
   EmitToStreamer(*OutStreamer, TmpInst);
 }
 
+void AArch64AsmPrinter::emitGlobalIFunc(Module &M, const GlobalIFunc &GI) {
+  if (!TM.getTargetTriple().isOSBinFormatMachO())
+    return AsmPrinter::emitGlobalIFunc(M, GI);
+
+  // On Darwin platforms, emit a manually-constructed .symbol_resolver that
+  // implements the symbol resolution duties of the IFunc.
+  //
+  // Normally, this would be handled by linker magic, but unfortunately there
+  // are a few limitations in ld64 and ld-prime's implementation of
+  // .symbol_resolver that mean we can't always use them:
+  //
+  //    *  resolvers cannot be the target of an alias
+  //    *  resolvers cannot have private linkage
+  //    *  resolvers cannot have linkonce linkage
+  //    *  resolvers cannot appear in executables
+  //    *  resolvers cannot appear in bundles
+  //
+  // This works around that by emitting a close approximation of what the linker
+  // would have done.
+
+  auto EmitLinkage = [&](MCSymbol *Sym) {
+    if (GI.hasExternalLinkage() || !MAI->getWeakRefDirective())
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MaskRay wrote:

`MAI->getWeakRefDirective()` is always true for Mach-O.

https://github.com/llvm/llvm-project/pull/73686