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

llvm-svn: 301438

diff --git a/lld/test/ELF/lto/asmundef.ll b/lld/test/ELF/lto/asmundef.ll
index 1c87cd0..d03a5e3 100644 (file)
--- a/lld/test/ELF/lto/asmundef.ll
+++ b/lld/test/ELF/lto/asmundef.ll
@@ -20,6 +20,5 @@ define void @_start() {
   ret void
 }
 
-; CHECK: @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to i8*)], section "llvm.metadata"
-; CHECK: define internal void @foo
+; CHECK: define void @foo
 

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index 9782c89..1bc0d73 100644 (file)
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/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,

diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index 4bd251f..30447c5 100644 (file)
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/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();

diff --git a/llvm/lib/Object/IRSymtab.cpp b/llvm/lib/Object/IRSymtab.cpp
index 367b1e8..5f08378 100644 (file)
--- a/llvm/lib/Object/IRSymtab.cpp
+++ b/llvm/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();
   }

diff --git a/llvm/test/LTO/Resolution/X86/Inputs/mod-asm-used.ll b/llvm/test/LTO/Resolution/X86/Inputs/mod-asm-used.ll
new file mode 100644 (file)
index 0000000..3b1988b
--- /dev/null
+++ b/llvm/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

diff --git a/llvm/test/LTO/Resolution/X86/mod-asm-used.ll b/llvm/test/LTO/Resolution/X86/mod-asm-used.ll
new file mode 100644 (file)
index 0000000..01befca
--- /dev/null
+++ b/llvm/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"

diff --git a/llvm/test/tools/gold/X86/asm_undefined2.ll b/llvm/test/tools/gold/X86/asm_undefined2.ll
index 6cb7a1c..a170f45 100644 (file)
--- a/llvm/test/tools/gold/X86/asm_undefined2.ll
+++ b/llvm/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"