[LLD][COFF] Initial support for ARM64EC importlibs.

Use demangled symbol name for __imp_ symbols and define demangled thunk
symbol as AMD64 thunk.

Author: cjacek

lld/COFF/InputFiles.cpp

@@ -25,6 +25,7 @@
 #include "llvm/DebugInfo/CodeView/TypeDeserializer.h"
 #include "llvm/DebugInfo/PDB/Native/NativeSession.h"
 #include "llvm/DebugInfo/PDB/Native/PDBFile.h"
+#include "llvm/IR/Mangler.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Object/COFF.h"
@@ -1019,9 +1020,16 @@ void ImportFile::parse() {
 
   // Read names and create an __imp_ symbol.
   StringRef buf = mb.getBuffer().substr(sizeof(*hdr));
-  StringRef name = saver().save(buf.split('\0').first);
+  StringRef nameBuf = buf.split('\0').first, name;
+  if (isArm64EC(hdr->Machine)) {
+    if (std::optional<std::string> demangledName =
+            getArm64ECDemangledFunctionName(nameBuf))
+      name = saver().save(*demangledName);
+  }
+  if (name.empty())
+    name = saver().save(nameBuf);
   StringRef impName = saver().save("__imp_" + name);
-  buf = buf.substr(name.size() + 1);
+  buf = buf.substr(nameBuf.size() + 1);
   dllName = buf.split('\0').first;
   StringRef extName;
   switch (hdr->getNameType()) {
@@ -1058,8 +1066,14 @@ void ImportFile::parse() {
   // If type is function, we need to create a thunk which jump to an
   // address pointed by the __imp_ symbol. (This allows you to call
   // DLL functions just like regular non-DLL functions.)
-  if (hdr->getType() == llvm::COFF::IMPORT_CODE)
-    thunkSym = ctx.symtab.addImportThunk(name, impSym, hdr->Machine);
+  if (hdr->getType() == llvm::COFF::IMPORT_CODE) {
+    if (ctx.config.machine != ARM64EC) {
+      thunkSym = ctx.symtab.addImportThunk(name, impSym, hdr->Machine);
+    } else {
+      thunkSym = ctx.symtab.addImportThunk(name, impSym, AMD64);
+      // FIXME: Add aux IAT symbols.
+    }
+  }
 }
 
 BitcodeFile::BitcodeFile(COFFLinkerContext &ctx, MemoryBufferRef mb,

lld/test/COFF/arm64ec-import.test

@@ -0,0 +1,68 @@
+REQUIRES: aarch64, x86
+RUN: split-file %s %t.dir && cd %t.dir
+
+RUN: llvm-mc -filetype=obj -triple=arm64ec-windows test.s -o test.obj
+RUN: llvm-mc -filetype=obj -triple=arm64ec-windows %S/Inputs/loadconfig-arm64ec.s -o loadconfig-arm64ec.obj
+RUN: llvm-lib -machine:arm64ec -def:test.def -out:test-arm64ec.lib
+RUN: llvm-lib -machine:arm64ec -def:test2.def -out:test2-arm64ec.lib
+RUN: llvm-lib -machine:x64 -def:test.def -out:test-x86_64.lib
+
+Link using ARM64EC import library:
+RUN: lld-link -machine:arm64ec -dll -noentry -out:out.dll loadconfig-arm64ec.obj \
+RUN:          test.obj test-arm64ec.lib test2-arm64ec.lib
+
+Link using x86_64 import library:
+RUN: lld-link -machine:arm64ec -dll -noentry -out:out2.dll loadconfig-arm64ec.obj \
+RUN:          test.obj test-x86_64.lib test2-arm64ec.lib
+
+RUN: llvm-readobj --coff-imports out.dll | FileCheck --check-prefix=IMPORTS %s
+RUN: llvm-readobj --coff-imports out2.dll | FileCheck --check-prefix=IMPORTS %s
+IMPORTS:      Import {
+IMPORTS-NEXT:   Name: test.dll
+IMPORTS-NEXT:   ImportLookupTableRVA:
+IMPORTS-NEXT:   ImportAddressTableRVA: 0x2258
+IMPORTS-NEXT:   Symbol: data (0)
+IMPORTS-NEXT:   Symbol: func (0)
+IMPORTS-NEXT:   Symbol: func2 (0)
+IMPORTS-NEXT: }
+IMPORTS-NEXT: Import {
+IMPORTS-NEXT:   Name: test2.dll
+IMPORTS-NEXT:   ImportLookupTableRVA:
+IMPORTS-NEXT:   ImportAddressTableRVA: 0x2278
+IMPORTS-NEXT:   Symbol: t2func (0)
+IMPORTS-NEXT: }
+
+RUN: llvm-objdump -d out.dll | FileCheck --check-prefix=DISASM %s
+RUN: llvm-objdump -d out2.dll | FileCheck --check-prefix=DISASM %s
+
+DISASM:      0000000180001000 <.text>:
+DISASM-NEXT: 180001000: ff 25 5a 12 00 00            jmpq    *0x125a(%rip)           # 0x180002260
+
+RUN: llvm-readobj --hex-dump=.test out.dll | FileCheck --check-prefix=TESTSEC %s
+RUN: llvm-readobj --hex-dump=.test out2.dll | FileCheck --check-prefix=TESTSEC %s
+TESTSEC:      0x180004000 60220000 58220000 68220000 78220000
+TESTSEC-NEXT: 0x180004010 00100000
+
+#--- test.s
+    .section .test, "r"
+    .globl arm64ec_data_sym
+    .p2align 2, 0x0
+arm64ec_data_sym:
+    .rva __imp_func
+    .rva __imp_data
+    .rva __imp_func2
+    .rva __imp_t2func
+    .rva func
+
+#--- test.def
+NAME test.dll
+EXPORTS
+    data DATA
+    func
+    func2
+    unused_func
+
+#--- test2.def
+NAME test2.dll
+EXPORTS
+    t2func