[CodeGen][ARM64EC] Use MCSymbolRefExpr::VK_None for function aliases.

[cjacek]

VK_WEAKREF is meant for .weakref assembly expressions, but we use a regular .set here.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: Jacek Caban (cjacek)

Changes

Depends on #92099.

VK_WEAKREF is meant for .weakref assembly expressions, but we use a regular .set here.

---

Full diff: https://github.com/llvm/llvm-project/pull/92100.diff

2 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp (+36-39)
• (added) llvm/test/CodeGen/AArch64/arm64ec-symbols.ll (+25)

diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index ee39c6355c298..8ce7f87abca00 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -1161,52 +1161,49 @@ void AArch64AsmPrinter::emitFunctionEntryLabel() {
     TS->emitDirectiveVariantPCS(CurrentFnSym);
   }
 
+  AsmPrinter::emitFunctionEntryLabel();
+
   if (TM.getTargetTriple().isWindowsArm64EC() &&
       !MF->getFunction().hasLocalLinkage()) {
     // For ARM64EC targets, a function definition's name is mangled differently
-    // from the normal symbol. We emit the alias from the unmangled symbol to
-    // mangled symbol name here.
-    if (MDNode *Unmangled =
-            MF->getFunction().getMetadata("arm64ec_unmangled_name")) {
-      AsmPrinter::emitFunctionEntryLabel();
-
-      if (MDNode *ECMangled =
-              MF->getFunction().getMetadata("arm64ec_ecmangled_name")) {
-        StringRef UnmangledStr =
-            cast<MDString>(Unmangled->getOperand(0))->getString();
-        MCSymbol *UnmangledSym =
-            MMI->getContext().getOrCreateSymbol(UnmangledStr);
-        StringRef ECMangledStr =
-            cast<MDString>(ECMangled->getOperand(0))->getString();
-        MCSymbol *ECMangledSym =
-            MMI->getContext().getOrCreateSymbol(ECMangledStr);
-        OutStreamer->emitSymbolAttribute(UnmangledSym, MCSA_WeakAntiDep);
-        OutStreamer->emitAssignment(
-            UnmangledSym,
-            MCSymbolRefExpr::create(ECMangledSym, MCSymbolRefExpr::VK_WEAKREF,
-                                    MMI->getContext()));
-        OutStreamer->emitSymbolAttribute(ECMangledSym, MCSA_WeakAntiDep);
-        OutStreamer->emitAssignment(
-            ECMangledSym,
-            MCSymbolRefExpr::create(CurrentFnSym, MCSymbolRefExpr::VK_WEAKREF,
-                                    MMI->getContext()));
-        return;
+    // from the normal symbol, emit required aliases here.
+    auto emitFunctionAlias = [&](MCSymbol *Src, MCSymbol *Dst) {
+      OutStreamer->beginCOFFSymbolDef(Src);
+      OutStreamer->emitCOFFSymbolType(COFF::IMAGE_SYM_DTYPE_FUNCTION
+                                      << COFF::SCT_COMPLEX_TYPE_SHIFT);
+      OutStreamer->endCOFFSymbolDef();
+      OutStreamer->emitSymbolAttribute(Src, MCSA_WeakAntiDep);
+      OutStreamer->emitAssignment(
+          Src, MCSymbolRefExpr::create(Dst, MCSymbolRefExpr::VK_None,
+                                       MMI->getContext()));
+    };
+
+    auto getSymbolFromMetadata = [&](StringRef Name) {
+      MCSymbol *Sym = nullptr;
+      if (MDNode *Node = MF->getFunction().getMetadata(Name)) {
+        StringRef NameStr = cast<MDString>(Node->getOperand(0))->getString();
+        Sym = MMI->getContext().getOrCreateSymbol(NameStr);
+      }
+      return Sym;
+    };
+
+    if (MCSymbol *UnmangledSym =
+            getSymbolFromMetadata("arm64ec_unmangled_name")) {
+      MCSymbol *ECMangledSym = getSymbolFromMetadata("arm64ec_ecmangled_name");
+
+      if (ECMangledSym) {
+        // An external function, emit the alias from the unmangled symbol to
+        // mangled symbol name and the alias from the mangled symbol to guest
+        // exit thunk.
+        emitFunctionAlias(UnmangledSym, ECMangledSym);
+        emitFunctionAlias(ECMangledSym, CurrentFnSym);
       } else {
-        StringRef UnmangledStr =
-            cast<MDString>(Unmangled->getOperand(0))->getString();
-        MCSymbol *UnmangledSym =
-            MMI->getContext().getOrCreateSymbol(UnmangledStr);
-        OutStreamer->emitSymbolAttribute(UnmangledSym, MCSA_WeakAntiDep);
-        OutStreamer->emitAssignment(
-            UnmangledSym,
-            MCSymbolRefExpr::create(CurrentFnSym, MCSymbolRefExpr::VK_WEAKREF,
-                                    MMI->getContext()));
-        return;
+        // A function implementation, emit the alias from the unmangled symbol
+        // to mangled symbol name.
+        emitFunctionAlias(UnmangledSym, CurrentFnSym);
       }
     }
   }
-
-  return AsmPrinter::emitFunctionEntryLabel();
 }
 
 /// Small jump tables contain an unsigned byte or half, representing the offset
diff --git a/llvm/test/CodeGen/AArch64/arm64ec-symbols.ll b/llvm/test/CodeGen/AArch64/arm64ec-symbols.ll
new file mode 100644
index 0000000000000..ab01e1f58fd79
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/arm64ec-symbols.ll
@@ -0,0 +1,25 @@
+; RUN: llc -mtriple=arm64ec-pc-windows-msvc < %s | FileCheck %s
+
+declare void @func() nounwind;
+
+define void @caller() nounwind {
+  call void @func()
+  ret void
+}
+
+; CHECK:      .def    caller;
+; CHECK-NEXT: .type   32;
+; CHECK-NEXT: .endef
+; CHECK-NEXT: .weak_anti_dep  caller
+; CHECK-NEXT: .set caller, "#caller"{{$}}
+
+; CHECK:      .def    func;
+; CHECK-NEXT: .type   32;
+; CHECK-NEXT: .endef
+; CHECK-NEXT: .weak_anti_dep  func
+; CHECK-NEXT: .set func, "#func"{{$}}
+; CHECK-NEXT: .def    "#func";
+; CHECK-NEXT: .type   32;
+; CHECK-NEXT: .endef
+; CHECK-NEXT: .weak_anti_dep  "#func"
+; CHECK-NEXT: .set "#func", "#func$exit_thunk"{{$}}

[efriedma-quic]

Maybe consider pre-committing arm64ec-symbols.ll.

I'd like to see the tests show the effect on the emitted symbols using llvm-objdump, in addition to the asm... from what I recall, the weakref hack was necessary to make the emitted symbols have the right COFF attributes. If that test shows it's not necessary, that's fine, I guess.

[cjacek]

The new version adds the test in a separate commit. It passes both with and without the second commit. Should I create a separate MR for that?

It seems fine in my testing and things like https://reviews.llvm.org/D157547#4650727 seem to work as expected.

[efriedma-quic]

LGTM