CodeGen: Do not memset if following stores will overwrite entire variable

[vitalybuka]

Summary

In some cases llvm::getMostFrequentByte can't analyze a
constant so we write zeros and then override the rest with stores.
These zeros are not needed and we can keep only stores.

clang/lib/CodeGen/CGDecl.cpp

if (0) {
     }
   }
 
   // TODO: Tweak these constants or use "density".
   uint64_t SizeLimit = 32;
   if (GlobalSize <= SizeLimit)
     return nullptr;
 
-  if (!MFB.Value) {
-    // We don't know most frequent byte, we may still completely initialize it
-    // with small number of stores.
-    // TODO: Use llvm::UndefValue::get. If we get here and later we decide
-    // that number of stores is small enough to use memset+store then store are
-    // going to replace entire bzeroed range. With UndefValue we can skip that
-    // bogus memset. I will do that as a separate patch.
-    MFB.Value = llvm::ConstantInt::get(CGM.Int8Ty, 0);
-  }
+  if (!MFB.Value)
+    MFB.Value = llvm::UndefValue::get(CGM.Int8Ty);
 
   unsigned StoreBudget = 6;
   return canEmitStoresForInitAfterMemset(CGM, Init, MFB.Value, StoreBudget)
              ? MFB.Value
              : nullptr;
 }
 
 /// Decide whether we want to split a constant structure or array store into a

clang/test/CodeGen/init-memset.c

void test_most_a() {
   // CHECK: call void @llvm.memset.{{.*}}
   // CHECK: store i8 122
   use(a);
 }
 
 void test_pointers() {
   // CHECK-LABEL: define void @test_pointers()
   void *a[] = {&use, &use, &use, &use, &use, &use};
-  // CHECK: call void @llvm.memset.{{.*}}
+  // CHECK-NOT: @llvm.memset.
   // CHECK: store i8*
   // CHECK: store i8*
   // CHECK: store i8*
   // CHECK: store i8*
   // CHECK: store i8*
   // CHECK: store i8*
   use(a);
 }