[LLD][COFF] Require explicit specification of ARM64EC target (#116281)

Inferring the ARM64EC target can lead to errors. The `-machine:arm64ec`
option may include x86_64 input files, and any valid ARM64EC input is
also valid for `-machine:arm64x`. MSVC requires an explicit `-machine`
argument with informative diagnostics; this patch adopts the same
behavior.

Author: cjacek

lld/COFF/Config.h

@@ -114,6 +114,7 @@ struct Configuration {
   bool is64() const { return llvm::COFF::is64Bit(machine); }
 
   llvm::COFF::MachineTypes machine = IMAGE_FILE_MACHINE_UNKNOWN;
+  bool machineInferred = false;
   size_t wordsize;
   bool verbose = false;
   WindowsSubsystem subsystem = llvm::COFF::IMAGE_SUBSYSTEM_UNKNOWN;

lld/COFF/SymbolTable.cpp

@@ -46,6 +46,8 @@ static bool compatibleMachineType(COFFLinkerContext &ctx, MachineTypes mt) {
     return COFF::isArm64EC(mt) || mt == AMD64;
   case ARM64X:
     return COFF::isAnyArm64(mt) || mt == AMD64;
+  case IMAGE_FILE_MACHINE_UNKNOWN:
+    return true;
   default:
     return ctx.config.machine == mt;
   }
@@ -74,14 +76,26 @@ void SymbolTable::addFile(InputFile *file) {
   }
 
   MachineTypes mt = file->getMachineType();
-  if (ctx.config.machine == IMAGE_FILE_MACHINE_UNKNOWN) {
-    ctx.config.machine = mt;
-    ctx.driver.addWinSysRootLibSearchPaths();
-  } else if (!compatibleMachineType(ctx, mt)) {
+  // The ARM64EC target must be explicitly specified and cannot be inferred.
+  if (mt == ARM64EC &&
+      (ctx.config.machine == IMAGE_FILE_MACHINE_UNKNOWN ||
+       (ctx.config.machineInferred &&
+        (ctx.config.machine == ARM64 || ctx.config.machine == AMD64)))) {
+    error(toString(file) + ": machine type arm64ec is ambiguous and cannot be "
+                           "inferred, use /machine:arm64ec or /machine:arm64x");
+    return;
+  }
+  if (!compatibleMachineType(ctx, mt)) {
     error(toString(file) + ": machine type " + machineToStr(mt) +
           " conflicts with " + machineToStr(ctx.config.machine));
     return;
   }
+  if (ctx.config.machine == IMAGE_FILE_MACHINE_UNKNOWN &&
+      mt != IMAGE_FILE_MACHINE_UNKNOWN) {
+    ctx.config.machineInferred = true;
+    ctx.config.machine = mt;
+    ctx.driver.addWinSysRootLibSearchPaths();
+  }
 
   ctx.driver.parseDirectives(file);
 }

lld/test/COFF/arm64ec.test

@@ -4,6 +4,7 @@ RUN: split-file %s %t.dir && cd %t.dir
 RUN: llvm-mc -filetype=obj -triple=aarch64-windows arm64-data-sym.s -o arm64-data-sym.obj
 RUN: llvm-mc -filetype=obj -triple=arm64ec-windows arm64ec-data-sym.s -o arm64ec-data-sym.obj
 RUN: llvm-mc -filetype=obj -triple=x86_64-windows x86_64-data-sym.s -o x86_64-data-sym.obj
+RUN: llvm-mc -filetype=obj -triple=i686-windows x86_64-data-sym.s -o i686-data-sym.obj
 RUN: llvm-cvtres -machine:arm64x -out:arm64x-resource.obj %S/Inputs/resource.res
 
 RUN: lld-link -out:test.dll -machine:arm64ec arm64ec-data-sym.obj -dll -noentry
@@ -46,6 +47,26 @@ RUN: not lld-link -out:test.dll -machine:arm64 arm64-data-sym.obj x86_64-data-sy
 RUN:              -dll -noentry 2>&1 | FileCheck -check-prefix=INCOMPAT3 %s
 INCOMPAT3: lld-link: error: x86_64-data-sym.obj: machine type x64 conflicts with arm64
 
+arm64ec machine type can't be inferred, it must be specified explicitly.
+RUN: not lld-link -out:test.dll arm64ec-data-sym.obj \
+RUN:              -dll -noentry 2>&1 | FileCheck -check-prefix=INCOMPAT4 %s
+INCOMPAT4: lld-link: error: arm64ec-data-sym.obj: machine type arm64ec is ambiguous and cannot be inferred, use /machine:arm64ec or /machine:arm64x
+
+RUN: not lld-link -out:test.dll x86_64-data-sym.obj arm64ec-data-sym.obj \
+RUN:              -dll -noentry 2>&1 | FileCheck -check-prefix=INCOMPAT4 %s
+
+RUN: not lld-link -out:test.dll arm64-data-sym.obj arm64ec-data-sym.obj \
+RUN:              -dll -noentry 2>&1 | FileCheck -check-prefix=INCOMPAT4 %s
+
+RUN: not lld-link -out:test.dll i686-data-sym.obj arm64ec-data-sym.obj \
+RUN:              -dll -noentry 2>&1 | FileCheck -check-prefix=INCOMPAT5 %s
+INCOMPAT5: lld-link: error: arm64ec-data-sym.obj: machine type arm64ec conflicts with x86
+
+arm64x can be inferred and when mixed with ARM64, the first one wins
+RUN: lld-link -out:test.dll -dll -noentry arm64x-resource.obj arm64-data-sym.obj x86_64-data-sym.obj arm64ec-data-sym.obj
+RUN: not lld-link -out:test.dll -dll -noentry arm64-data-sym.obj arm64x-resource.obj x86_64-data-sym.obj 2>&1 | FileCheck -check-prefix=INCOMPAT3 %s
+RUN: not lld-link -out:test.dll -dll -noentry arm64-data-sym.obj arm64x-resource.obj arm64ec-data-sym.obj 2>&1 | FileCheck -check-prefix=INCOMPAT4 %s
+
 #--- arm64ec-data-sym.s
     .data
     .globl arm64ec_data_sym