[LLD] [COFF] Add tests to observe details about LTO and __imp_ prefixes. NFC. (#72764)

The commit 3ab6209 had the undesired
effect of retaining every `__imp_` symbol, even if it isn't referenced
in any way. Add a testcase to observe this behaviour, to serve as
a reference point if this behaviour were to be improved later.

Port the testcase from b963c0b from
the llvm/LTO testsuite into LLD as a preparation for changing that fix;
the moved testcase has a comment for one case which doesn't work
currently.

The testcase is ported mostly as is, but with symbol mangling
simplified, rewriting function names from MSVC C++ mangling to plain
C, and unnecessary debug info is removed.

Add a case of a dllimported data symbol, in addition to the existing
call of a dllimported function.

Also extend the testcase to test combinations of both regular
object files and LTO objects. Leave out one combination which currently
fails, with a comment.

Author: mstorsjo

lld/test/COFF/lto-dllimport.ll

@@ -0,0 +1,50 @@
+; REQUIRES: x86
+; RUN: split-file %s %t.dir
+
+; RUN: llvm-as %t.dir/main.ll -o %t.main.bc
+; RUN: llvm-as %t.dir/other.ll -o %t.other.bc
+; RUN: llc %t.dir/main.ll -o %t.main.obj --filetype=obj
+; RUN: llc %t.dir/other.ll -o %t.other.obj --filetype=obj
+
+; RUN: lld-link %t.main.obj %t.other.obj -entry:main -out:%t.exe
+; RUN: lld-link %t.main.bc  %t.other.bc  -entry:main -out:%t.exe
+; RUN: lld-link %t.main.bc  %t.other.obj -entry:main -out:%t.exe
+
+;; This test currently fails if combining the regular object file main.obj
+;; with the bitcode object file other.bc.
+; RUN-skipped: lld-link %t.main.obj %t.other.bc  -entry:main -out:%t.exe
+
+;--- main.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.11.0"
+
+define dso_local i32 @main() local_unnamed_addr {
+entry:
+  %call = tail call i32 @foo()
+  %0 = load i32, ptr @variable, align 4
+  %add = add nsw i32 %0, %call
+  ret i32 %add
+}
+
+@variable = external dllimport local_unnamed_addr global i32, align 4
+
+declare dllimport i32 @foo() local_unnamed_addr
+
+!llvm.module.flags = !{!1}
+
+!1 = !{i32 1, !"ThinLTO", i32 0}
+
+;--- other.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.11.0"
+
+define dso_local i32 @foo() local_unnamed_addr {
+entry:
+  ret i32 42
+}
+
+@variable = dso_local local_unnamed_addr global i32 1, align 4
+
+!llvm.module.flags = !{!1}
+
+!1 = !{i32 1, !"ThinLTO", i32 0}

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

@@ -5,7 +5,15 @@
 ; 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
+; RUN: lld-link /entry:entry %t.main.obj %t.other.obj /out:%t.exe /subsystem:console /debug:symtab
+
+;; The current implementation for handling __imp_ symbols retains all of them.
+;; Observe that this currently produces __imp_unusedFunc even if nothing
+;; references unusedFunc in any form.
+
+; RUN: llvm-nm %t.exe | FileCheck %s
+
+; CHECK: __imp_unusedFunc
 
 ;--- 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"
@@ -33,6 +41,13 @@ entry:
   ret void
 }
 
+@__imp_unusedFunc = global ptr @unusedFuncReplacement
+
+define internal void @unusedFuncReplacement() {
+entry:
+  ret void
+}
+
 define void @other() {
 entry:
   ret void