[LLD] [COFF] Avoid loading objects for mingw autoimport, when a defined alias exists

[mstorsjo]

Summary

This avoids a spurious and confusing log message in cases where both e.g. "alias" and "__imp_alias" exist.

[ruiu]

LGTM

COFF/SymbolTable.cpp

 void SymbolTable::loadMinGWAutomaticImports() {
   for (auto &i : symMap) {
     Symbol *sym = i.second;
     auto *undef = dyn_cast<Undefined>(sym);
     if (!undef)
       continue;
     if (!sym->isUsedInRegularObj)
       continue;
+    if (undef->getWeakAlias())
+      continue;
 
     StringRef name = undef->getName();
 
     if (name.startswith("__imp_"))
       continue;
     // If we have an undefined symbol, but we have a Lazy representing a
     // symbol we could load from file, make sure to load that.
     Lazy *l = dyn_cast_or_null<Lazy>(find(("__imp_" + name).str()));

test/COFF/alias-implib.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-mingw32 -o %t.o %s
+# RUN: lld-link -lldmingw -entry:main -out:%t.exe %t.o %S/Inputs/alias-implib.lib -verbose 2>&1 | FileCheck %s
+
+# Check that the undefined (weak) alias symbol (with an existing lazy
+# __imp_alias) doesn't trigger trying to load __imp_alias for autoimport,
+# when the weak alias target actually does exist.
+
+# CHECK-NOT: Loading lazy{{.*}}for automatic import
+
+    .text
+    .globl main
+main:
+    call alias
+    ret
+
+# alias-implib.lib was created with "llvm-dlltool -m i386:x86-64
+# -l alias-implib.lib -d alias-implib.def" with this def snippet:
+# LIBRARY lib.dll
+# EXPORTS
+# realfunc
+# alias == realfunc