[lld] Always mark ARM64EC modules as instrumented.

FIXME: The test doesn't seem to work on MSVC, but it's observed in real-world images, needs more investigation.

Author: cjacek

lld/COFF/Writer.cpp

@@ -1213,8 +1213,7 @@ void Writer::createMiscChunks() {
     createSEHTable();
 
   // Create /guard:cf tables if requested.
-  if (config->guardCF != GuardCFLevel::Off)
-    createGuardCFTables();
+  createGuardCFTables();
 
   if (isArm64EC(config->machine))
     createECChunks();
@@ -1973,6 +1972,15 @@ void Writer::markSymbolsWithRelocations(ObjFile *file,
 void Writer::createGuardCFTables() {
   Configuration *config = &ctx.config;
 
+  if (config->guardCF == GuardCFLevel::Off) {
+    if (isArm64EC(config->machine)) {
+      Symbol *flagSym = ctx.symtab.findUnderscore("__guard_flags");
+      cast<DefinedAbsolute>(flagSym)->setVA(
+          uint32_t(GuardFlags::CF_INSTRUMENTED));
+    }
+    return;
+  }
+
   SymbolRVASet addressTakenSyms;
   SymbolRVASet giatsRVASet;
   std::vector<Symbol *> giatsSymbols;

lld/test/COFF/cfguard-flags-arm64ec.s

@@ -0,0 +1,22 @@
+// ARM64EC code is always instrumented, check that CF_INSTRUMENTED flag is set even with -guard:no argument.
+
+// REQUIRES: aarch64
+
+// RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %s -o %t.obj
+// RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o %t-loadconfig.obj
+
+// RUN: lld-link -out:%t1.dll -machine:arm64ec %t.obj %t-loadconfig.obj -dll -noentry
+// RUN: lld-link -out:%t2.dll -machine:arm64ec %t.obj %t-loadconfig.obj -dll -noentry -guard:no
+
+// RUN: llvm-readobj --coff-load-config %t1.dll | FileCheck %s
+// RUN: llvm-readobj --coff-load-config %t2.dll | FileCheck %s
+// CHECK:       GuardFlags [ (0x100)
+// CHECK-NEXT:    CF_INSTRUMENTED (0x100)
+// CHECK-NEXT:  ]
+
+// RUN: llvm-readobj --hex-dump=.test %t1.dll | FileCheck --check-prefix=SYM %s
+// RUN: llvm-readobj --hex-dump=.test %t2.dll | FileCheck --check-prefix=SYM %s
+// SYM: 0x180003000 00010000
+
+        .section .test, "r"
+        .word __guard_flags