[LLD] [COFF] Handle manually defined __imp_ pointers in LTO

lld/COFF/InputFiles.cpp

@@ -1061,6 +1061,12 @@ void BitcodeFile::parse() {
     } else {
       sym = ctx.symtab.addRegular(this, symName, nullptr, fakeSC, 0,
                                   objSym.isWeak());
+      // Model all symbols with the __imp_ prefix as having external
+      // references. If one LTO object defines a __imp_<foo> symbol, and
+      // another LTO object refers to <foo> with dllimport, make sure the
+      // __imp_ symbol is kept.
+      if (symName.starts_with("__imp_"))
+        sym->isUsedInRegularObj = true;
     }
     symbols.push_back(sym);
     if (objSym.isUsed())

lld/test/COFF/lto-imp-prefix.ll

@@ -0,0 +1,39 @@
+; REQUIRES: x86
+
+; RUN: rm -rf %t.dir
+; RUN: split-file %s %t.dir
+; RUN: llvm-as %t.dir/main.ll -o %t.main.obj
+; RUN: llvm-as %t.dir/other.ll -o %t.other.obj
+
+; RUN: lld-link /entry:entry %t.main.obj %t.other.obj /out:%t.exe /subsystem:console
+
+;--- main.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-w64-windows-gnu"
+
+define void @entry() {
+entry:
+  tail call void @importedFunc()
+  tail call void @other()
+  ret void
+}
+
+declare dllimport void @importedFunc()
+
+declare void @other()
+
+;--- other.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-w64-windows-gnu"
+
+@__imp_importedFunc = global ptr @importedFuncReplacement
+
+define internal void @importedFuncReplacement() {
+entry:
+  ret void
+}
+
+define void @other() {
+entry:
+  ret void
+}