[LLD][COFF] Don't dllimport from static libraries (#134443)

This reverts commit 6a1bdd9 and re-instate behavior that matches what
MSVC link.exe does, that is, error out when trying to dllimport a symbol
from a static library.

A hint is now displayed in stdout, mentioning that we should rather dllimport the symbol
from a import library.

Fixes https://github.com/llvm/llvm-project/issues/131807

lld/COFF/Driver.cpp

@@ -2663,10 +2663,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
     createECExportThunks();
 
   // Resolve remaining undefined symbols and warn about imported locals.
-  ctx.forEachSymtab([&](SymbolTable &symtab) {
-    while (symtab.resolveRemainingUndefines())
-      run();
-  });
+  ctx.forEachSymtab(
+      [&](SymbolTable &symtab) { symtab.resolveRemainingUndefines(); });
 
   if (errorCount())
     return;

lld/COFF/SymbolTable.cpp

@@ -231,6 +231,17 @@ void SymbolTable::reportUndefinedSymbol(const UndefinedDiag &undefDiag) {
   }
   if (numDisplayedRefs < numRefs)
     diag << "\n>>> referenced " << numRefs - numDisplayedRefs << " more times";
+
+  // Hints
+  StringRef name = undefDiag.sym->getName();
+  if (name.consume_front("__imp_")) {
+    Symbol *imp = find(name);
+    if (imp && imp->isLazy()) {
+      diag << "\nNOTE: a relevant symbol '" << imp->getName()
+           << "' is available in " << toString(imp->getFile())
+           << " but cannot be used because it is not an import library.";
+    }
+  }
 }
 
 void SymbolTable::loadMinGWSymbols() {
@@ -432,11 +443,10 @@ void SymbolTable::reportUnresolvable() {
   reportProblemSymbols(undefs, /*localImports=*/nullptr, true);
 }
 
-bool SymbolTable::resolveRemainingUndefines() {
+void SymbolTable::resolveRemainingUndefines() {
   llvm::TimeTraceScope timeScope("Resolve remaining undefined symbols");
   SmallPtrSet<Symbol *, 8> undefs;
   DenseMap<Symbol *, Symbol *> localImports;
-  bool foundLazy = false;
 
   for (auto &i : symMap) {
     Symbol *sym = i.second;
@@ -481,11 +491,6 @@ bool SymbolTable::resolveRemainingUndefines() {
             imp = findLocalSym(*mangledName);
         }
       }
-      if (imp && imp->isLazy()) {
-        forceLazy(imp);
-        foundLazy = true;
-        continue;
-      }
       if (imp && isa<Defined>(imp)) {
         auto *d = cast<Defined>(imp);
         replaceSymbol<DefinedLocalImport>(sym, ctx, name, d);
@@ -513,7 +518,6 @@ bool SymbolTable::resolveRemainingUndefines() {
   reportProblemSymbols(
       undefs, ctx.config.warnLocallyDefinedImported ? &localImports : nullptr,
       false);
-  return foundLazy;
 }
 
 std::pair<Symbol *, bool> SymbolTable::insert(StringRef name) {

lld/COFF/SymbolTable.h

@@ -67,10 +67,7 @@ public:
   // Try to resolve any undefined symbols and update the symbol table
   // accordingly, then print an error message for any remaining undefined
   // symbols and warn about imported local symbols.
-  // Returns whether more files might need to be linked in to resolve lazy
-  // symbols, in which case the caller is expected to call the function again
-  // after linking those files.
-  bool resolveRemainingUndefines();
+  void resolveRemainingUndefines();
 
   // Load lazy objects that are needed for MinGW automatic import and for
   // doing stdcall fixups.

lld/test/COFF/imports-static-lib-indirect.test

@@ -0,0 +1,26 @@
+# REQUIRES: x86
+
+# Pulling in on both a dllimport symbol and a static symbol should only warn.
+# RUN: split-file %s %t.dir
+# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/other.s -o %t.other.obj
+# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/main.s -o %t.main.obj
+# RUN: llvm-lib %t.other.obj -out:%t.other.lib
+# RUN: lld-link %t.other.lib %t.main.obj -out:%t.dll -dll 2>&1 | FileCheck %s
+
+CHECK: warning: {{.*}} locally defined symbol imported: foo {{.*}} [LNK4217]
+
+#--- other.s
+.text
+.globl other
+.globl foo
+other:
+  ret
+foo:
+  ret
+#--- main.s
+.text
+.global _DllMainCRTStartup
+_DllMainCRTStartup:
+  call *other(%rip)
+  call *__imp_foo(%rip)
+  ret

lld/test/COFF/imports-static-lib.test

@@ -0,0 +1,33 @@
+# REQUIRES: x86
+
+# Ensure that we don't import dllimport symbols from static (non-import) libraries
+# RUN: split-file %s %t.dir
+# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/foo.s -o %t.foo.obj
+# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/main.s -o %t.main.obj
+# RUN: llvm-lib %t.foo.obj -out:%t.foo.lib
+# RUN: not lld-link %t.foo.lib %t.main.obj -out:%t.dll -dll 2>&1 | FileCheck %s
+
+CHECK: error: undefined symbol: __declspec(dllimport) foo
+CHECK: NOTE: a relevant symbol 'foo' is available in {{.*}}.foo.lib but cannot be used because it is not an import library.
+
+# Now do the same thing, but import the symbol from a import library.
+# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/foo_dll_main.s -o %t.foo_dll_main.obj
+# RUN: lld-link /out:%t.dll /dll %t.foo.obj %t.foo_dll_main.obj /export:foo /implib:%t.foo.imp.lib
+# RUN: lld-link %t.main.obj %t.foo.imp.lib -out:%t.exe -dll
+
+#--- foo.s
+.text
+.globl foo
+foo:
+  ret
+#--- foo_dll_main.s
+.text
+.global _DllMainCRTStartup
+_DllMainCRTStartup:
+  ret
+#--- main.s
+.text
+.global _DllMainCRTStartup
+_DllMainCRTStartup:
+  call *__imp_foo(%rip)
+  ret

lld/test/COFF/undefined_lazy.test

@@ -1,26 +0,0 @@
-# REQUIRES: x86
-
-# RUN: split-file %s %t.dir
-# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/foo.s -o %t.foo.obj
-# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/bar.s -o %t.bar.obj
-# RUN: llvm-mc --filetype=obj -triple=x86_64-windows-msvc %t.dir/qux.s -o %t.qux.obj
-# RUN: llvm-lib %t.foo.obj -out:%t.foo.lib
-# RUN: llvm-lib %t.bar.obj -out:%t.bar.lib
-# RUN: lld-link %t.foo.lib %t.bar.lib %t.qux.obj -out:%t.dll -dll
-#
-#--- foo.s
-.text
-.globl foo
-foo:
-  call bar
-#--- bar.s
-.text
-.globl bar
-bar:
-  ret
-#--- qux.s
-.text
-.global _DllMainCRTStartup
-_DllMainCRTStartup:
-  call *__imp_foo(%rip)
-  ret