[clang] [lld] [llvm] [LTO][LLD] Prevent invalid LTO libfunc transforms (PR #164916)

[Daniel Thornburgh via cfe-commits]

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@@ -279,9 +279,31 @@ static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
   RegisterPassPlugins(Conf.PassPlugins, PB);
 
   std::unique_ptr<TargetLibraryInfoImpl> TLII(
-      new TargetLibraryInfoImpl(TM->getTargetTriple(), TM->Options.VecLib));
+      new TargetLibraryInfoImpl(TM->getTargetTriple()));
   if (Conf.Freestanding)
     TLII->disableAllFunctions();
+
+  TargetLibraryInfo TLI(*TLII);
+  for (unsigned I = 0, E = static_cast<unsigned>(LibFunc::NumLibFuncs); I != E;
+       ++I) {
+    LibFunc F = static_cast<LibFunc>(I);
+    StringRef Name = TLI.getName(F);
+    GlobalValue *Val = Mod.getNamedValue(Name);
+
+    // LibFuncs present in the current TU can always be referenced.
+    if (Val && !Val->isDeclaration())
+      continue;
+
+    // LibFuncs not implemented in bitcode can always be referenced.
+    if (!BitcodeLibFuncs.contains(Name))
+      continue;
+
+    // FIXME: Functions that are somewhere in a ThinLTO link (just not imported
+    // in this module) should not be disabled, as they have already been
+    // extracted.
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mysterymath wrote:

IIRC it's something that would have to be added; the symbol table seen by the LTO backend has only symbols that directly concern a given TU; it doesn't have a global view of the module. Another way I thought to might go about was something like summarily including libcalls into the symbol table without importing them. Still seems worth its own change either way.

https://github.com/llvm/llvm-project/pull/164916