[asan] Reduce binary size by using unnamed private aliases

[vitalybuka]

Summary

--asan-use-private-alias increases binary sizes by 10% or more.
Most of this space was long names of aliases and new symbols.
These symbols are not needed for the ODR check at all.

[eugenis]

I've added original contributors of the private-alias thing to double check that we are not breaking any assumptions by doing this.

[m.ostapenko]

I think this should be fine, aliases don't need names at all.

llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp

explicit AddressSanitizerModule(bool CompileKernel = false,
         // argument is designed as workaround. Therefore, disable both
         // ClWithComdat and ClUseGlobalsGC unless the frontend says it's ok to
         // do globals-gc.
         UseCtorComdat(UseGlobalsGC && ClWithComdat) {
           this->Recover = ClRecover.getNumOccurrences() > 0 ?
               ClRecover : Recover;
           this->CompileKernel = ClEnableKasan.getNumOccurrences() > 0 ?
               ClEnableKasan : CompileKernel;
-  }
+  }
 
   bool runOnModule(Module &M) override;
   StringRef getPassName() const override { return "AddressSanitizerModule"; }
 
 private:
   void initializeCallbacks(Module &M);
 
   bool InstrumentGlobals(IRBuilder<> &IRB, Module &M, bool *CtorComdat);
for (size_t i = 0; i < n; i++) {
     GlobalValue *InstrumentedGlobal = NewGlobal;
 
     bool CanUsePrivateAliases =
         TargetTriple.isOSBinFormatELF() || TargetTriple.isOSBinFormatMachO() ||
         TargetTriple.isOSBinFormatWasm();
     if (CanUsePrivateAliases && ClUsePrivateAliasForGlobals) {
       // Create local alias for NewGlobal to avoid crash on ODR between
       // instrumented and non-instrumented libraries.
-      auto *GA = GlobalAlias::create(GlobalValue::InternalLinkage,
-                                     NameForGlobal + M.getName(), NewGlobal);
+      auto *GA =
+          GlobalAlias::create(GlobalValue::PrivateLinkage, "", NewGlobal);
 
       // With local aliases, we need to provide another externally visible
       // symbol __odr_asan_XXX to detect ODR violation.
       auto *ODRIndicatorSym =
           new GlobalVariable(M, IRB.getInt8Ty(), false, Linkage,
                              Constant::getNullValue(IRB.getInt8Ty()),
                              kODRGenPrefix + NameForGlobal, nullptr,
                              NewGlobal->getThreadLocalMode());

llvm/trunk/test/Instrumentation/AddressSanitizer/local_alias.ll

 ; RUN: opt < %s -asan -asan-module -asan-use-private-alias=1 -S | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @a = internal global [2 x i32] zeroinitializer, align 4
 
 ; Check that we generate internal alias and odr indicator symbols for global to be protected.
 ; CHECK: @__odr_asan_gen_a = internal global i8 0, align 1
-; CHECK: @"a<stdin>" = internal alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @a
+; CHECK: @0 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @a
 
 ; Function Attrs: nounwind sanitize_address uwtable
 define i32 @foo(i32 %M) #0 {
 entry:
   %M.addr = alloca i32, align 4
   store i32 %M, i32* %M.addr, align 4
   store volatile i32 6, i32* getelementptr inbounds ([2 x i32], [2 x i32]* @a, i64 2, i64 0), align 4
   %0 = load i32, i32* %M.addr, align 4
   %idxprom = sext i32 %0 to i64
   %arrayidx = getelementptr inbounds [2 x i32], [2 x i32]* @a, i64 0, i64 %idxprom
   %1 = load volatile i32, i32* %arrayidx, align 4
   ret i32 %1
 }