[LTO] Prevent asm references to be dropped from the output.

Differential Revision:  https://reviews.llvm.org/D24617

llvm-svn: 281741

llvm/lib/LTO/LTOBackend.cpp

@@ -25,6 +25,7 @@
 #include "llvm/IR/PassManager.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/LTO/LTO.h"
+#include "llvm/LTO/legacy/UpdateCompilerUsed.h"
 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Support/Error.h"
@@ -275,6 +276,19 @@ Expected<const Target *> initAndLookupTarget(Config &C, Module &Mod) {
 
 }
 
+static void handleAsmUndefinedRefs(Module &Mod, TargetMachine &TM) {
+  // Collect the list of undefined symbols used in asm and update
+  // llvm.compiler.used to prevent optimization to drop these from the output.
+  StringSet<> AsmUndefinedRefs;
+  object::IRObjectFile::CollectAsmUndefinedRefs(
+      Triple(Mod.getTargetTriple()), Mod.getModuleInlineAsm(),
+      [&AsmUndefinedRefs](StringRef Name, object::BasicSymbolRef::Flags Flags) {
+        if (Flags & object::BasicSymbolRef::SF_Undefined)
+          AsmUndefinedRefs.insert(Name);
+      });
+  updateCompilerUsed(Mod, TM, AsmUndefinedRefs);
+}
+
 Error lto::backend(Config &C, AddOutputFn AddOutput,
                    unsigned ParallelCodeGenParallelismLevel,
                    std::unique_ptr<Module> Mod) {
@@ -285,6 +299,8 @@ Error lto::backend(Config &C, AddOutputFn AddOutput,
   std::unique_ptr<TargetMachine> TM =
       createTargetMachine(C, Mod->getTargetTriple(), *TOrErr);
 
+  handleAsmUndefinedRefs(*Mod, *TM);
+
   if (!C.CodeGenOnly)
     if (!opt(C, TM.get(), 0, *Mod, /*IsThinLto=*/false))
       return Error();
@@ -310,6 +326,8 @@ Error lto::thinBackend(Config &Conf, unsigned Task, AddOutputFn AddOutput,
   std::unique_ptr<TargetMachine> TM =
       createTargetMachine(Conf, Mod.getTargetTriple(), *TOrErr);
 
+  handleAsmUndefinedRefs(Mod, *TM);
+
   if (Conf.CodeGenOnly) {
     codegen(Conf, TM.get(), AddOutput, Task, Mod);
     return Error();