LTO: Mark undefined module asm symbols as used.

Marking them as used causes them to be considered visible outside of LTO. This
prevents the symbols from being internalized or discarded, either by GlobalDCE
or by summary-based dead stripping in ThinLTO.

This change makes it unnecessary to add these symbols to llvm.compiler.used
in the backend, as the symbols are kept alive by virtue of being external,
so remove the backend code that handles that.

Fixes PR32798.

Differential Revision: https://reviews.llvm.org/D32544

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301438 91177308-0d34-0410-b5e6-96231b3b80d8

lib/LTO/LTO.cpp

@@ -415,7 +415,8 @@ void LTO::addSymbolToGlobalRes(const InputFile::Symbol &Sym,
   // Flag as visible outside of ThinLTO if visible from a regular object or
   // if this is a reference in the regular LTO partition.
   GlobalRes.VisibleOutsideThinLTO |=
-      (Res.VisibleToRegularObj || (Partition == GlobalResolution::RegularLTO));
+      (Res.VisibleToRegularObj || Sym.isUsed() ||
+       Partition == GlobalResolution::RegularLTO);
 }
 
 static void writeToResolutionFile(raw_ostream &OS, InputFile *Input,

lib/LTO/LTOBackend.cpp

@@ -25,7 +25,6 @@
 #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/Object/ModuleSymbolTable.h"
 #include "llvm/Passes/PassBuilder.h"
@@ -353,19 +352,6 @@ finalizeOptimizationRemarks(std::unique_ptr<tool_output_file> DiagOutputFile) {
   DiagOutputFile->os().flush();
 }
 
-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;
-  ModuleSymbolTable::CollectAsmSymbols(
-      Mod,
-      [&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, AddStreamFn AddStream,
                    unsigned ParallelCodeGenParallelismLevel,
                    std::unique_ptr<Module> Mod,
@@ -377,8 +363,6 @@ Error lto::backend(Config &C, AddStreamFn AddStream,
   std::unique_ptr<TargetMachine> TM =
       createTargetMachine(C, Mod->getTargetTriple(), *TOrErr);
 
-  handleAsmUndefinedRefs(*Mod, *TM);
-
   // Setup optimization remarks.
   auto DiagFileOrErr = lto::setupOptimizationRemarks(
       Mod->getContext(), C.RemarksFilename, C.RemarksWithHotness);
@@ -416,8 +400,6 @@ Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
   std::unique_ptr<TargetMachine> TM =
       createTargetMachine(Conf, Mod.getTargetTriple(), *TOrErr);
 
-  handleAsmUndefinedRefs(Mod, *TM);
-
   if (Conf.CodeGenOnly) {
     codegen(Conf, TM.get(), AddStream, Task, Mod);
     return Error::success();

lib/Object/IRSymtab.cpp

@@ -163,6 +163,9 @@ Error Builder::addSymbol(const ModuleSymbolTable &Msymtab,
   Sym.ComdatIndex = -1;
   auto *GV = Msym.dyn_cast<GlobalValue *>();
   if (!GV) {
+    // Undefined module asm symbols act as GC roots and are implicitly used.
+    if (Flags & object::BasicSymbolRef::SF_Undefined)
+      Sym.Flags |= 1 << storage::Symbol::FB_used;
     setStr(Sym.IRName, "");
     return Error::success();
   }

test/LTO/Resolution/X86/Inputs/mod-asm-used.ll

@@ -0,0 +1,4 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@foo = global i32 1, align 4

test/LTO/Resolution/X86/mod-asm-used.ll

@@ -0,0 +1,10 @@
+; RUN: opt -module-summary -o %t.bc %s
+; RUN: opt -module-summary -o %t2.bc %S/Inputs/mod-asm-used.ll
+; RUN: llvm-lto2 run %t.bc -r %t.bc,foo,l %t2.bc -r %t2.bc,foo,pl -o %t3
+; RUN: llvm-nm %t3.1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: D foo
+module asm ".quad foo"

test/tools/gold/X86/asm_undefined2.ll

@@ -11,8 +11,8 @@
 ; RUN:     --plugin-opt=thinlto -o %t2 %t.o
 ; RUN: llvm-dis < %t.o.5.precodegen.bc | FileCheck %s
 
-; Check that foo is properly appended to llvm.compiler.used
-; CHECK: @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to i8*)], section "llvm.metadata"
+; Check that foo is not internalized
+; CHECK: define void @foo
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"