llvm-link option and test for recent metadata mapping bug

[tejohnson]

Summary

Add a -preserve-modules option to llvm-link that simulates LTO
clients that don't destroy modules as they are linked. This enables
reproduction of a recent bug introduced by a metadata linking change
that was only caught when the modules weren't destroyed before
writing bitcode (LTO on Windows).

See http://llvm.org/viewvc/llvm-project?view=revision&revision=253170
for more details on the original bug and the fix.

Confirmed the new test added here reproduces the failure using the new
option when I suppress the fix.

[tejohnson]

• Fix output file in new test.

[pcc]

LGTM

llvm/trunk/test/Linker/only-needed-named-metadata.ll

 ; ONLYNEEDED-NOT:@unused_linkonce()
 ; ONLYNEEDED-NOT:@linkoncealias
 ; ONLYNEEDED-NOT:@linkoncefunc2()
 ; ONLYNEEDED-NOT:@weakalias
 ; ONLYNEEDED-NOT:@globalfunc1()
 ; ONLYNEEDED-NOT:@analias
 ; ONLYNEEDED-NOT:@globalfunc2()
 
+; Test -only-needed link with the modules preserved instead of freeing to
+; catch any cross-module references to metadata, which the bitcode writer
+; will assert on.
+; RUN: llvm-link -preserve-modules -o %t3.bc -only-needed %t2.bc %t.bc
+
 @X = global i32 5
 @U = global i32 6
 @U_linkonce = linkonce_odr hidden global i32 6
 define i32 @foo() { ret i32 7 }
 define i32 @unused() { ret i32 8 }
 define linkonce_odr hidden i32 @unused_linkonce() { ret i32 8 }
 @linkoncealias = alias void (...), bitcast (void ()* @linkoncefunc2 to void (...)*)
 

llvm/trunk/tools/llvm-link/llvm-link.cpp

 
 static cl::opt<bool>
 DumpAsm("d", cl::desc("Print assembly as linked"), cl::Hidden);
 
 static cl::opt<bool>
 SuppressWarnings("suppress-warnings", cl::desc("Suppress all linking warnings"),
                  cl::init(false));
 
+static cl::opt<bool>
+    PreserveModules("preserve-modules",
+                    cl::desc("Preserve linked modules for testing"));
+
 static cl::opt<bool> PreserveBitcodeUseListOrder(
     "preserve-bc-uselistorder",
     cl::desc("Preserve use-list order when writing LLVM bitcode."),
     cl::init(true), cl::Hidden);
 
 static cl::opt<bool> PreserveAssemblyUseListOrder(
     "preserve-ll-uselistorder",
     cl::desc("Preserve use-list order when writing LLVM assembly."),
for (const auto &File : Files) {
 
     if (Verbose)
       errs() << "Linking in '" << File << "'\n";
 
     if (L.linkInModule(M.get(), ApplicableFlags, Index.get()))
       return false;
     // All linker flags apply to linking of subsequent files.
     ApplicableFlags = Flags;
+
+    // If requested for testing, preserve modules by releasing them from
+    // the unique_ptr before the are freed. This can help catch any
+    // cross-module references from e.g. unneeded metadata references
+    // that aren't properly set to null but instead mapped to the source
+    // module version. The bitcode writer will assert if it finds any such
+    // cross-module references.
+    if (PreserveModules)
+      M.release();
   }
 
   return true;
 }
 
 int main(int argc, char **argv) {
   // Print a stack trace if we signal out.
   sys::PrintStackTraceOnErrorSignal();