Protect user-supplied runtime library functions in LTO

Add user-supplied C runtime and compiler-rt library functions to
llvm.compiler.used to protect them from premature optimization by
passes like -globalopt and -ipsccp.  Calls to (seemingly unused)
runtime library functions can be added by -instcombine and instruction
lowering.

Patch by Duncan Exon Smith, thanks!

Fixes <rdar://problem/14740087>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194514 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/LTO/LTOCodeGenerator.h

@@ -38,6 +38,7 @@
 #include "llvm-c/lto.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/Linker.h"
 #include "llvm/Target/TargetOptions.h"
 #include <string>
@@ -48,6 +49,7 @@ namespace llvm {
   class GlobalValue;
   class Mangler;
   class MemoryBuffer;
+  class TargetLibraryInfo;
   class TargetMachine;
   class raw_ostream;
 }
@@ -125,6 +127,7 @@ private:
                           std::string &errMsg);
   void applyScopeRestrictions();
   void applyRestriction(llvm::GlobalValue &GV,
+                        const llvm::ArrayRef<llvm::StringRef> &Libcalls,
                         std::vector<const char*> &MustPreserveList,
                         llvm::SmallPtrSet<llvm::GlobalValue*, 8> &AsmUsed,
                         llvm::Mangler &Mangler);

lib/LTO/LTOCodeGenerator.cpp

@@ -18,6 +18,7 @@
 #include "llvm/Analysis/Passes.h"
 #include "llvm/Analysis/Verifier.h"
 #include "llvm/Bitcode/ReaderWriter.h"
+#include "llvm/CodeGen/RuntimeLibcalls.h"
 #include "llvm/Config/config.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
@@ -40,6 +41,8 @@
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/ToolOutputFile.h"
 #include "llvm/Support/system_error.h"
+#include "llvm/Target/TargetLibraryInfo.h"
+#include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/Mangler.h"
@@ -312,6 +315,7 @@ bool LTOCodeGenerator::determineTarget(std::string &errMsg) {
 
 void LTOCodeGenerator::
 applyRestriction(GlobalValue &GV,
+                 const ArrayRef<StringRef> &Libcalls,
                  std::vector<const char*> &MustPreserveList,
                  SmallPtrSet<GlobalValue*, 8> &AsmUsed,
                  Mangler &Mangler) {
@@ -324,6 +328,15 @@ applyRestriction(GlobalValue &GV,
     MustPreserveList.push_back(GV.getName().data());
   if (AsmUndefinedRefs.count(Buffer))
     AsmUsed.insert(&GV);
+
+  // Conservatively append user-supplied runtime library functions to
+  // llvm.compiler.used.  These could be internalized and deleted by
+  // optimizations like -globalopt, causing problems when later optimizations
+  // add new library calls (e.g., llvm.memset => memset and printf => puts).
+  // Leave it to the linker to remove any dead code (e.g. with -dead_strip).
+  if (isa<Function>(GV) &&
+      std::binary_search(Libcalls.begin(), Libcalls.end(), GV.getName()))
+    AsmUsed.insert(&GV);
 }
 
 static void findUsedValues(GlobalVariable *LLVMUsed,
@@ -337,6 +350,33 @@ static void findUsedValues(GlobalVariable *LLVMUsed,
       UsedValues.insert(GV);
 }
 
+static void accumulateAndSortLibcalls(std::vector<StringRef> &Libcalls,
+                                      const TargetLibraryInfo& TLI,
+                                      const TargetLowering *Lowering)
+{
+  // TargetLibraryInfo has info on C runtime library calls on the current
+  // target.
+  for (unsigned I = 0, E = static_cast<unsigned>(LibFunc::NumLibFuncs);
+       I != E; ++I) {
+    LibFunc::Func F = static_cast<LibFunc::Func>(I);
+    if (TLI.has(F))
+      Libcalls.push_back(TLI.getName(F));
+  }
+
+  // TargetLowering has info on library calls that CodeGen expects to be
+  // available, both from the C runtime and compiler-rt.
+  if (Lowering)
+    for (unsigned I = 0, E = static_cast<unsigned>(RTLIB::UNKNOWN_LIBCALL);
+         I != E; ++I)
+      if (const char *Name
+          = Lowering->getLibcallName(static_cast<RTLIB::Libcall>(I)))
+        Libcalls.push_back(Name);
+
+  std::sort(Libcalls.begin(), Libcalls.end());
+  Libcalls.erase(std::unique(Libcalls.begin(), Libcalls.end()),
+                 Libcalls.end());
+}
+
 void LTOCodeGenerator::applyScopeRestrictions() {
   if (ScopeRestrictionsDone)
     return;
@@ -350,16 +390,19 @@ void LTOCodeGenerator::applyScopeRestrictions() {
   Mangler Mangler(TargetMach);
   std::vector<const char*> MustPreserveList;
   SmallPtrSet<GlobalValue*, 8> AsmUsed;
+  std::vector<StringRef> Libcalls;
+  TargetLibraryInfo TLI(Triple(TargetMach->getTargetTriple()));
+  accumulateAndSortLibcalls(Libcalls, TLI, TargetMach->getTargetLowering());
 
   for (Module::iterator f = mergedModule->begin(),
          e = mergedModule->end(); f != e; ++f)
-    applyRestriction(*f, MustPreserveList, AsmUsed, Mangler);
+    applyRestriction(*f, Libcalls, MustPreserveList, AsmUsed, Mangler);
   for (Module::global_iterator v = mergedModule->global_begin(),
          e = mergedModule->global_end(); v !=  e; ++v)
-    applyRestriction(*v, MustPreserveList, AsmUsed, Mangler);
+    applyRestriction(*v, Libcalls, MustPreserveList, AsmUsed, Mangler);
   for (Module::alias_iterator a = mergedModule->alias_begin(),
          e = mergedModule->alias_end(); a != e; ++a)
-    applyRestriction(*a, MustPreserveList, AsmUsed, Mangler);
+    applyRestriction(*a, Libcalls, MustPreserveList, AsmUsed, Mangler);
 
   GlobalVariable *LLVMCompilerUsed =
     mergedModule->getGlobalVariable("llvm.compiler.used");

test/LTO/runtime-library.ll

@@ -0,0 +1,27 @@
+; runtime library implementations should be added to llvm.compiler.used
+; RUN: llvm-as <%s >%t1
+; RUN: llvm-lto -o %t2 %t1
+; RUN: llvm-nm -no-sort %t2 | FileCheck %s -check-prefix=KEEP -check-prefix=LOSE
+
+target triple = "x86_64-apple-darwin9"
+
+; KEEP-LABEL: _puts
+define void @puts() {
+  ret void
+}
+
+; KEEP-LABEL: ___divti3
+define void @__divti3() {
+  ret void
+}
+
+; KEEP-LABEL: _memset
+define void @memset() {
+  ret void
+}
+
+; LOSE-NOT: _myprintf
+define void @myprintf() {
+  ret void
+}
+