[LLD][COFF] Follow up comments on pr146610 (#147152)

This is a follow-up PR for post-commit comments in
https://github.com/llvm/llvm-project/pull/146610

- Changed "exporteddllmain" references to "importeddllmain".
- Add support for x86 target and test coverage.
- Changed a comment to better express why we're skipping importing
`DllMain`.

lld/COFF/Config.h

@@ -319,7 +319,7 @@ struct Configuration {
   bool warnDebugInfoUnusable = true;
   bool warnLongSectionNames = true;
   bool warnStdcallFixup = true;
-  bool warnExportedDllMain = true;
+  bool warnImportedDllMain = true;
   bool incremental = true;
   bool integrityCheck = false;
   bool killAt = false;

lld/COFF/Driver.cpp

@@ -1643,8 +1643,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
         config->warnLocallyDefinedImported = false;
       else if (s == "longsections")
         config->warnLongSectionNames = false;
-      else if (s == "exporteddllmain")
+      else if (s == "importeddllmain")
-        config->warnExportedDllMain = false;
+        config->warnImportedDllMain = false;
       // Other warning numbers are ignored.
     }
   }

lld/COFF/InputFiles.cpp

@@ -117,8 +117,6 @@ static coff_symbol_generic *cloneSymbol(COFFSymbolRef sym) {
 // Skip importing DllMain thunks from import libraries.
 static bool fixupDllMain(COFFLinkerContext &ctx, llvm::object::Archive *file,
                          const Archive::Symbol &sym, bool &skipDllMain) {
-  if (skipDllMain)
-    return true;
   const Archive::Child &c =
       CHECK(sym.getMember(), file->getFileName() +
                                  ": could not get the member for symbol " +
@@ -128,13 +126,13 @@ static bool fixupDllMain(COFFLinkerContext &ctx, llvm::object::Archive *file,
             file->getFileName() +
                 ": could not get the buffer for a child buffer of the archive");
   if (identify_magic(mb.getBuffer()) == file_magic::coff_import_library) {
-    if (ctx.config.warnExportedDllMain) {
+    if (ctx.config.warnImportedDllMain) {
       // We won't place DllMain symbols in the symbol table if they are
       // coming from a import library. This message can be ignored with the flag
-      // '/ignore:exporteddllmain'
+      // '/ignore:importeddllmain'
       Warn(ctx)
           << file->getFileName()
-          << ": skipping exported DllMain symbol [exporteddllmain]\nNOTE: this "
+          << ": skipping imported DllMain symbol [importeddllmain]\nNOTE: this "
              "might be a mistake when the DLL/library was produced.";
     }
     skipDllMain = true;
@@ -204,14 +202,24 @@ void ArchiveFile::parse() {
     }
   }
 
-  // Read the symbol table to construct Lazy objects.
   bool skipDllMain = false;
+  StringRef mangledDllMain, impMangledDllMain;
+
+  // The calls below will fail if we haven't set the machine type yet. Instead
+  // of failing, it is preferable to skip this "imported DllMain" check if we
+  // don't know the machine type at this point.
+  if (!file->isEmpty() && ctx.config.machine != IMAGE_FILE_MACHINE_UNKNOWN) {
+    mangledDllMain = archiveSymtab->mangle("DllMain");
+    impMangledDllMain = uniqueSaver().save("__imp_" + mangledDllMain);
+  }
+
+  // Read the symbol table to construct Lazy objects.
   for (const Archive::Symbol &sym : file->symbols()) {
-    // If the DllMain symbol was exported by mistake, skip importing it
+    // If an import library provides the DllMain symbol, skip importing it, as
-    // otherwise we might end up with a import thunk in the final binary which
+    // we should be using our own DllMain, not another DLL's DllMain.
-    // is wrong.
+    if (!mangledDllMain.empty() && (sym.getName() == mangledDllMain ||
-    if (sym.getName() == "__imp_DllMain" || sym.getName() == "DllMain") {
+                                    sym.getName() == impMangledDllMain)) {
-      if (fixupDllMain(ctx, file.get(), sym, skipDllMain))
+      if (skipDllMain || fixupDllMain(ctx, file.get(), sym, skipDllMain))
         continue;
     }
     archiveSymtab->addLazyArchive(this, sym);

lld/test/COFF/imported-dllmain-i386.test

@@ -0,0 +1,58 @@
+REQUIRES: x86
+RUN: split-file %s %t.dir && cd %t.dir
+
+RUN: llvm-mc -filetype=obj -triple=i386-windows a.s -o a.obj
+
+RUN: llvm-mc -filetype=obj -triple=i386-windows b1.s -o b1.obj
+RUN: llvm-mc -filetype=obj -triple=i386-windows b2.s -o b2.obj
+
+### This is the line where our problem occurs. Here, we export the DllMain symbol which shouldn't happen normally.
+RUN: lld-link b1.obj b2.obj -out:b.dll -dll -implib:b.lib -entry:DllMain -export:bar -export:DllMain -safeseh:no
+
+RUN: llvm-mc -filetype=obj -triple=i386-windows c.s -o c.obj
+RUN: lld-link -lib c.obj -out:c.lib
+
+### Later, if b.lib is provided before other libs/objs that export DllMain statically, we previously were using the dllimported DllMain from b.lib, which is wrong.
+RUN: lld-link a.obj b.lib c.lib -dll -out:out.dll -entry:DllMain -safeseh:no 2>&1 | FileCheck -check-prefix=WARN %s
+RUN: lld-link a.obj b.lib c.lib -dll -out:out.dll -entry:DllMain -ignore:importeddllmain -safeseh:no 2>&1 | FileCheck -check-prefix=IGNORED --allow-empty %s
+RUN: llvm-objdump --private-headers -d out.dll | FileCheck -check-prefix=DISASM %s
+
+WARN: lld-link: warning: b.lib: skipping imported DllMain symbol [importeddllmain]
+IGNORED-NOT: lld-link: warning: b.lib: skipping imported DllMain symbol [importeddllmain]
+
+DISASM: The Import Tables:
+DISASM: DLL Name: b.dll
+DISASM-NOT: DllMain
+DISASM: bar
+DISASM: Disassembly of section .text:
+DISASM:      b0 01                         movb    $0x1, %al
+DISASM-NEXT: c3                            retl
+
+#--- a.s
+        .text
+        .globl _foo
+_foo:
+        call *__imp__bar
+        ret
+
+#--- b1.s
+        .text
+        .globl _bar
+_bar:
+        ret
+
+#--- b2.s
+        .intel_syntax noprefix
+        .text
+        .globl _DllMain
+_DllMain:
+        xor al, al
+        ret
+
+#--- c.s
+        .intel_syntax noprefix
+        .text
+        .globl _DllMain
+_DllMain:
+        mov al, 1 
+        ret

lld/test/COFF/imported-dllmain.test

@@ -14,11 +14,11 @@ RUN: lld-link -lib c.obj -out:c.lib
 
 ### Later, if b.lib is provided before other libs/objs that export DllMain statically, we previously were using the dllimported DllMain from b.lib, which is wrong.
 RUN: lld-link a.obj b.lib c.lib -dll -out:out.dll -entry:DllMain 2>&1 | FileCheck -check-prefix=WARN %s
-RUN: lld-link a.obj b.lib c.lib -dll -out:out.dll -entry:DllMain -ignore:exporteddllmain 2>&1 | FileCheck -check-prefix=IGNORED --allow-empty %s
+RUN: lld-link a.obj b.lib c.lib -dll -out:out.dll -entry:DllMain -ignore:importeddllmain 2>&1 | FileCheck -check-prefix=IGNORED --allow-empty %s
 RUN: llvm-objdump --private-headers -d out.dll | FileCheck -check-prefix=DISASM %s
 
-WARN: lld-link: warning: b.lib: skipping exported DllMain symbol [exporteddllmain]
+WARN: lld-link: warning: b.lib: skipping imported DllMain symbol [importeddllmain]
-IGNORED-NOT: lld-link: warning: b.lib: skipping exported DllMain symbol [exporteddllmain]
+IGNORED-NOT: lld-link: warning: b.lib: skipping imported DllMain symbol [importeddllmain]
 
 DISASM: The Import Tables:
 DISASM: DLL Name: b.dll