[LLD][COFF] Add support for EXPORTAS import name type. (#86541)

#78772 added similar support for .def file parser and import library
writer. This PR adds missing bits in LLD to propagate EXPORTAS name and
allow it in `/export` parser. This is syntax is used by MSVC for ARM64EC
`__declspec(dllexport)` handling.

lld/COFF/Config.h

@@ -54,6 +54,7 @@ enum class EmitKind { Obj, LLVM, ASM };
 struct Export {
   StringRef name;       // N in /export:N or /export:E=N
   StringRef extName;    // E in /export:E=N
+  StringRef exportAs;   // E in /export:N,EXPORTAS,E
   StringRef aliasTarget; // GNU specific: N in "alias == N"
   Symbol *sym = nullptr;
   uint16_t ordinal = 0;
@@ -73,10 +74,9 @@ struct Export {
   StringRef exportName; // Name in DLL
 
   bool operator==(const Export &e) const {
-    return (name == e.name && extName == e.extName &&
-            aliasTarget == e.aliasTarget &&
-            ordinal == e.ordinal && noname == e.noname &&
-            data == e.data && isPrivate == e.isPrivate);
+    return (name == e.name && extName == e.extName && exportAs == e.exportAs &&
+            aliasTarget == e.aliasTarget && ordinal == e.ordinal &&
+            noname == e.noname && data == e.data && isPrivate == e.isPrivate);
   }
 };
 

lld/COFF/Driver.cpp

@@ -945,6 +945,7 @@ void LinkerDriver::createImportLibrary(bool asLib) {
     e2.Name = std::string(e1.name);
     e2.SymbolName = std::string(e1.symbolName);
     e2.ExtName = std::string(e1.extName);
+    e2.ExportAs = std::string(e1.exportAs);
     e2.AliasTarget = std::string(e1.aliasTarget);
     e2.Ordinal = e1.ordinal;
     e2.Noname = e1.noname;
@@ -1044,6 +1045,7 @@ void LinkerDriver::parseModuleDefs(StringRef path) {
       e2.name = saver().save(e1.Name);
       e2.extName = saver().save(e1.ExtName);
     }
+    e2.exportAs = saver().save(e1.ExportAs);
     e2.aliasTarget = saver().save(e1.AliasTarget);
     e2.ordinal = e1.Ordinal;
     e2.noname = e1.Noname;

lld/COFF/DriverUtils.cpp

@@ -585,7 +585,8 @@ Export LinkerDriver::parseExport(StringRef arg) {
     }
   }
 
-  // Optional parameters "[,@ordinal[,NONAME]][,DATA][,PRIVATE]"
+  // Optional parameters
+  // "[,@ordinal[,NONAME]][,DATA][,PRIVATE][,EXPORTAS,exportname]"
   while (!rest.empty()) {
     StringRef tok;
     std::tie(tok, rest) = rest.split(",");
@@ -607,6 +608,13 @@ Export LinkerDriver::parseExport(StringRef arg) {
       e.isPrivate = true;
       continue;
     }
+    if (tok.equals_insensitive("exportas")) {
+      if (!rest.empty() && !rest.contains(','))
+        e.exportAs = rest;
+      else
+        error("invalid EXPORTAS value: " + rest);
+      break;
+    }
     if (tok.starts_with("@")) {
       int32_t ord;
       if (tok.substr(1).getAsInteger(0, ord))
@@ -683,7 +691,9 @@ void LinkerDriver::fixupExports() {
   }
 
   for (Export &e : ctx.config.exports) {
-    if (!e.forwardTo.empty()) {
+    if (!e.exportAs.empty()) {
+      e.exportName = e.exportAs;
+    } else if (!e.forwardTo.empty()) {
       e.exportName = undecorate(ctx, e.name);
     } else {
       e.exportName = undecorate(ctx, e.extName.empty() ? e.name : e.extName);

lld/test/COFF/exportas.test

@@ -9,6 +9,77 @@ RUN: lld-link -out:out1.dll -dll -noentry test.obj test.lib
 RUN: llvm-readobj --coff-imports out1.dll | FileCheck --check-prefix=IMPORT %s
 IMPORT: Symbol: expfunc
 
+Pass -export argument with EXPORTAS.
+
+RUN: llvm-mc -filetype=obj -triple=x86_64-windows func.s -o func.obj
+RUN: lld-link -out:out2.dll -dll -noentry func.obj -export:func,EXPORTAS,expfunc
+RUN: llvm-readobj --coff-exports out2.dll | FileCheck --check-prefix=EXPORT %s
+EXPORT: Name: expfunc
+
+RUN: llvm-readobj out2.lib | FileCheck --check-prefix=IMPLIB %s
+IMPLIB:      Name type: export as
+IMPLIB-NEXT: Export name: expfunc
+IMPLIB-NEXT: Symbol: __imp_func
+IMPLIB-NEXT: Symbol: func
+
+Use .drectve section with EXPORTAS.
+
+RUN: llvm-mc -filetype=obj -triple=x86_64-windows drectve.s -o drectve.obj
+RUN: lld-link -out:out3.dll -dll -noentry func.obj drectve.obj
+RUN: llvm-readobj --coff-exports out3.dll | FileCheck --check-prefix=EXPORT %s
+RUN: llvm-readobj out3.lib | FileCheck --check-prefix=IMPLIB %s
+
+Use a .def file with EXPORTAS.
+
+RUN: lld-link -out:out4.dll -dll -noentry func.obj -def:test.def
+RUN: llvm-readobj --coff-exports out4.dll | FileCheck --check-prefix=EXPORT %s
+RUN: llvm-readobj out4.lib | FileCheck --check-prefix=IMPLIB %s
+
+Use a .def file with EXPORTAS in a forwarding export.
+
+RUN: lld-link -out:out5.dll -dll -noentry func.obj -def:test2.def
+RUN: llvm-readobj --coff-exports out5.dll | FileCheck --check-prefix=FORWARD-EXPORT %s
+FORWARD-EXPORT:      Export {
+FORWARD-EXPORT-NEXT:   Ordinal: 1
+FORWARD-EXPORT-NEXT:   Name: expfunc
+FORWARD-EXPORT-NEXT:   ForwardedTo: otherdll.otherfunc
+FORWARD-EXPORT-NEXT: }
+
+RUN: llvm-readobj out5.lib | FileCheck --check-prefix=FORWARD-IMPLIB %s
+FORWARD-IMPLIB:      Name type: export as
+FORWARD-IMPLIB-NEXT: Export name: expfunc
+FORWARD-IMPLIB-NEXT: Symbol: __imp_func
+FORWARD-IMPLIB-NEXT: Symbol: func
+
+Pass -export argument with EXPORTAS in a forwarding export.
+
+RUN: lld-link -out:out6.dll -dll -noentry func.obj -export:func=otherdll.otherfunc,EXPORTAS,expfunc
+RUN: llvm-readobj --coff-exports out6.dll | FileCheck --check-prefix=FORWARD-EXPORT %s
+RUN: llvm-readobj out6.lib | FileCheck --check-prefix=FORWARD-IMPLIB %s
+
+Pass -export argument with EXPORTAS in a data export.
+
+RUN: lld-link -out:out7.dll -dll -noentry func.obj -export:func,DATA,@5,EXPORTAS,expfunc
+RUN: llvm-readobj --coff-exports out7.dll | FileCheck --check-prefix=ORD %s
+ORD:      Ordinal: 5
+ORD-NEXT: Name: expfunc
+
+RUN: llvm-readobj out7.lib | FileCheck --check-prefix=ORD-IMPLIB %s
+ORD-IMPLIB:      Type: data
+ORD-IMPLIB-NEXT: Name type: export as
+ORD-IMPLIB-NEXT: Export name: expfunc
+ORD-IMPLIB-NEXT: Symbol: __imp_func
+
+Check invalid EXPORTAS syntax.
+
+RUN: not lld-link -out:err1.dll -dll -noentry func.obj -export:func,EXPORTAS, 2>&1 | \
+RUN:     FileCheck --check-prefix=ERR1 %s
+ERR1: error: invalid EXPORTAS value: {{$}}
+
+RUN: not lld-link -out:err2.dll -dll -noentry func.obj -export:func,EXPORTAS,expfunc,DATA 2>&1 | \
+RUN:     FileCheck --check-prefix=ERR2 %s
+ERR2: error: invalid EXPORTAS value: expfunc,DATA
+
 #--- test.s
     .section ".test", "rd"
     .rva __imp_func
@@ -17,3 +88,20 @@ IMPORT: Symbol: expfunc
 LIBRARY test.dll
 EXPORTS
     func EXPORTAS expfunc
+
+#--- test2.def
+LIBRARY test.dll
+EXPORTS
+    func=otherdll.otherfunc EXPORTAS expfunc
+
+#--- func.s
+    .text
+    .globl func
+    .p2align 2, 0x0
+func:
+    movl $1, %eax
+    retq
+
+#--- drectve.s
+    .section .drectve, "yn"
+    .ascii " -export:func,EXPORTAS,expfunc"