Allow Module name to be used to generate a unique Module ID

[tmsriram]

Summary

Modify getUniqueModuleID to use Module name for better uniqueness guarantees.

Module ID helps when there are no globals exported or when multiple definitions/ of the same globals are allowed in different modules (with "-z muldefs"). While it is highly likely that Module identifier is unique, it is not guaranteed.

[pcc]

If you don't need a guarantee of uniqueness wouldn't it be better to just take a hash of Module::getSourceFileName() instead of looking at the names of the globals?

[tmsriram]

In D73310#1838950, @pcc wrote:

If you don't need a guarantee of uniqueness wouldn't it be better to just take a hash of Module::getSourceFileName() instead of looking at the names of the globals?

So you are suggesting that when "UseModuleId" is true, we just return the hash of getModuleIdentifier()? That works for us and I can modify this function accordingly.

[pcc]

I am suggesting that you wouldn't change this function at all but would hash getSourceFileName yourself. (You shouldn't use getModuleIdentifier for anything that affects the contents of the output file because it isn't serialized.)

[tmsriram]

In D73310#1839155, @pcc wrote:

I am suggesting that you wouldn't change this function at all but would hash getSourceFileName yourself. (You shouldn't use getModuleIdentifier for anything that affects the contents of the output file because it isn't serialized.)

I modified https://reviews.llvm.org/D73307 to do exactly this. I will delete this patch if the new change is alright.

[tmsriram]

D73307 directly computes the hash so this is no longer necessary.

llvm/include/llvm/Transforms/Utils/ModuleUtils.h

 /// members.
 ///
 /// This identifier is normally guaranteed to be unique, or the program would
 /// fail to link due to multiply defined symbols.
 ///
 /// If the module has no strong external symbols (such a module may still have a
 /// semantic effect if it performs global initialization), we cannot produce a
 /// unique identifier for this module, so we return the empty string.
-std::string getUniqueModuleId(Module *M);
+///
+/// If UseModuleId is true, we include the ModuleIdentifier string.  This is
+/// however not guaranteed to be unique.
+std::string getUniqueModuleId(Module *M, bool UseModuleId = false);
 
 class CallInst;
 namespace VFABI {
 /// Overwrite the Vector Function ABI variants attribute with the names provide
 /// in \p VariantMappings.
 void setVectorVariantNames(CallInst *CI,
                            const SmallVector<std::string, 8> &VariantMappings);
 } // End VFABI namespace
 } // End llvm namespace
 
 #endif //  LLVM_TRANSFORMS_UTILS_MODULEUTILS_H

llvm/lib/Transforms/Utils/ModuleUtils.cpp

void llvm::filterDeadComdatFunctions(
 
   // Remove the entries that were not covering.
   erase_if(DeadComdatFunctions, [&](GlobalValue *GV) {
     return ComdatEntriesCovered.find(GV->getComdat()) ==
            ComdatEntriesCovered.end();
   });
 }
 
-std::string llvm::getUniqueModuleId(Module *M) {
+std::string llvm::getUniqueModuleId(Module *M, bool UseModuleId) {
   MD5 Md5;
   bool ExportsSymbols = false;
   auto AddGlobal = [&](GlobalValue &GV) {
     if (GV.isDeclaration() || GV.getName().startswith("llvm.") ||
         !GV.hasExternalLinkage() || GV.hasComdat())
       return;
     ExportsSymbols = true;
     Md5.update(GV.getName());
     Md5.update(ArrayRef<uint8_t>{0});
   };
 
   for (auto &F : *M)
     AddGlobal(F);
   for (auto &GV : M->globals())
     AddGlobal(GV);
   for (auto &GA : M->aliases())
     AddGlobal(GA);
   for (auto &IF : M->ifuncs())
     AddGlobal(IF);
 
+  // When UseModuleId is true, we include the module identifier also.  Module
+  // ID helps when there are no globals exported or when multiple definitions
+  // of the same globals are allowed in different modules (with "-z muldefs").
+  // While it is highly likely that Module identifier is unique, it is not
+  // guaranteed.
+  if (UseModuleId && !M->getModuleIdentifier().empty()) {
+    Md5.update(M->getModuleIdentifier());
+    Md5.update(ArrayRef<uint8_t>{0});
+  } else {
     if (!ExportsSymbols)
       return "";
+  }
 
   MD5::MD5Result R;
   Md5.final(R);
 
   SmallString<32> Str;
   MD5::stringifyResult(R, Str);
   return ("$" + Str).str();
 }