[LLD][COFF] Support marking sections as x86_64 code in ARM64EC object files (#135280)

On ARM64EC, the `IMAGE_SCN_GPREL` flag is repurposed to indicate that
section code is x86_64. This enables embedding x86_64 code within
ARM64EC object files. MSVC uses this for export thunks in .exp files.

lld/COFF/Chunks.cpp

@@ -61,6 +61,16 @@ SectionChunk::SectionChunk(ObjFile *f, const coff_section *h, Kind k)
     live = true;
 }
 
+MachineTypes SectionChunk::getMachine() const {
+  MachineTypes machine = file->getMachineType();
+  // On ARM64EC, the IMAGE_SCN_GPREL flag is repurposed to indicate that section
+  // code is x86_64. This enables embedding x86_64 code within ARM64EC object
+  // files. MSVC uses this for export thunks in .exp files.
+  if (isArm64EC(machine) && (header->Characteristics & IMAGE_SCN_GPREL))
+    machine = AMD64;
+  return machine;
+}
+
 // SectionChunk is one of the most frequently allocated classes, so it is
 // important to keep it as compact as possible. As of this writing, the number
 // below is the size of this class on x64 platforms.

lld/COFF/Chunks.h

@@ -253,8 +253,7 @@ public:
   size_t getSize() const { return header->SizeOfRawData; }
   ArrayRef<uint8_t> getContents() const;
   void writeTo(uint8_t *buf) const;
-
-  MachineTypes getMachine() const { return file->getMachineType(); }
+  MachineTypes getMachine() const;
 
   // Defend against unsorted relocations. This may be overly conservative.
   void sortRelocations();

lld/test/COFF/arm64ec-x86-sec.yaml

@@ -0,0 +1,43 @@
+# REQUIRES: aarch64, x86
+
+# RUN: yaml2obj %s -o %t.obj
+# RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o %t-loadcfg.obj
+# RUN: lld-link -machine:arm64ec -dll -noentry %t.obj %t-loadcfg.obj -out:%t.dll
+# RUN: llvm-objdump -d %t.dll | FileCheck %s
+
+# CHECK:      Disassembly of section .text:
+# CHECK-EMPTY:
+# CHECK-NEXT: 0000000180001000 <.text>:
+# CHECK-NEXT: 180001000: d503201f     nop
+# CHECK-NEXT: 180001004: d65f03c0     ret
+# CHECK-NEXT:                 ...
+# CHECK-NEXT: 180002000: e9 ff ef ff ff               jmp     0x180001004 <.text+0x4>
+# CHECK-NEXT: 180002005: c3                           retq
+# CHECK-NEXT: 180002006: cc                           int3
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_ARM64EC
+  Characteristics: [  ]
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     1F2003D5C0035FD6
+    SizeOfRawData:   8
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_GPREL, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       16
+    SectionData:     E900000000C3CC
+    SizeOfRawData:   7
+    Relocations:
+      - VirtualAddress:  1
+        SymbolName:      func
+        Type:            IMAGE_REL_ARM64_PAGEBASE_REL21  # interpreted as IMAGE_REL_AMD64_REL32
+symbols:
+  - Name:            func
+    Value:           4
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL