[LLD][COFF] Add EC alias symbols for undefined x86_64 symbols on ARM64EC target (llvm#114466)

Author: cjacek

lld/COFF/InputFiles.cpp

@@ -534,7 +534,22 @@ void ObjFile::initializeSymbols() {
 
 Symbol *ObjFile::createUndefined(COFFSymbolRef sym, bool overrideLazy) {
   StringRef name = check(coffObj->getSymbolName(sym));
-  return ctx.symtab.addUndefined(name, this, overrideLazy);
+  Symbol *s = ctx.symtab.addUndefined(name, this, overrideLazy);
+
+  // Add an anti-dependency alias for undefined AMD64 symbols on the ARM64EC
+  // target.
+  if (isArm64EC(ctx.config.machine) && getMachineType() == AMD64) {
+    auto u = dyn_cast<Undefined>(s);
+    if (u && !u->weakAlias) {
+      if (std::optional<std::string> mangledName =
+              getArm64ECMangledFunctionName(name)) {
+        Symbol *m = ctx.symtab.addUndefined(saver().save(*mangledName), this,
+                                            /*overrideLazy=*/false);
+        u->setWeakAlias(m, /*antiDep=*/true);
+      }
+    }
+  }
+  return s;
 }
 
 static const coff_aux_section_definition *findSectionDef(COFFObjectFile *obj,

lld/test/COFF/arm64ec-entry-mangle.test

@@ -8,6 +8,7 @@ RUN: llvm-mc -filetype=obj -triple=arm64ec-windows mangled-func.s -o mangled-fun
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows ref-demangled.s -o ref-demangled.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows demangled-entry-drectve.s -o demangled-entry-drectve.obj
 RUN: llvm-mc -filetype=obj -triple=x86_64-windows demangled-dll-main.s -o x64-dll-main.obj
+RUN: llvm-mc -filetype=obj -triple=x86_64-windows ref-demangled.s -o ref-x64.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o loadconfig-arm64ec.obj
 
 RUN: llvm-lib -machine:arm64ec -out:func.lib mangled-func.obj
@@ -80,6 +81,14 @@ RUN: lld-link -machine:arm64ec -dll -noentry -out:demangled-export-ref.dll mangl
 RUN:          ref-demangled.obj loadconfig-arm64ec.obj "-export:#func"
 RUN: llvm-objdump -d demangled-export-ref.dll | FileCheck -check-prefix=DISASM %s
 
+Verify that an x86_64 object file can reference ARM64EC mangled functions without requiring an explicit alias.
+RUN: lld-link -machine:arm64ec -dll -noentry -out:x64-ref.dll mangled-func.obj ref-x64.obj loadconfig-arm64ec.obj
+RUN: llvm-objdump -d x64-ref.dll | FileCheck -check-prefix=DISASM2 %s
+
+Verify that an x86_64 object file can reference ARM64EC mangled functions provided by a library.
+RUN: lld-link -machine:arm64ec -dll -noentry -out:x64-lib-ref.dll func.lib ref-x64.obj loadconfig-arm64ec.obj
+RUN: llvm-objdump -d x64-lib-ref.dll | FileCheck -check-prefix=DISASM2 %s
+
 DISASM2:      0000000180001000 <.text>:
 DISASM2-NEXT: 180001000: d65f03c0     ret