[clang] [clang] Only set the trailing bytes to zero when filling a partially … (PR #79502)

[Eli Friedman via cfe-commits]

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@@ -945,48 +950,77 @@ static bool canEmitInitWithFewStoresAfterBZero(llvm::Constant *Init,
 
 /// For inits that canEmitInitWithFewStoresAfterBZero returned true for, emit
 /// the scalar stores that would be required.
-static void emitStoresForInitAfterBZero(CodeGenModule &CGM,
-                                        llvm::Constant *Init, Address Loc,
-                                        bool isVolatile, CGBuilderTy &Builder,
-                                        bool IsAutoInit) {
+static size_t emitStoresForInitAfterBZero(CodeGenModule &CGM,
+                                          llvm::Constant *Init, Address Loc,
+                                          bool isVolatile, CGBuilderTy &Builder,
+                                          bool IsAutoInit) {
   assert(!Init->isNullValue() && !isa<llvm::UndefValue>(Init) &&
          "called emitStoresForInitAfterBZero for zero or undef value.");
 
+  auto const &DL = CGM.getDataLayout();
+
   if (isa<llvm::ConstantInt>(Init) || isa<llvm::ConstantFP>(Init) ||
       isa<llvm::ConstantVector>(Init) || isa<llvm::BlockAddress>(Init) ||
       isa<llvm::ConstantExpr>(Init)) {
     auto *I = Builder.CreateStore(Init, Loc, isVolatile);
     if (IsAutoInit)
       I->addAnnotationMetadata("auto-init");
-    return;
+    return DL.getTypeAllocSize(Init->getType());
   }
 
   if (llvm::ConstantDataSequential *CDS =
           dyn_cast<llvm::ConstantDataSequential>(Init)) {
+    bool CountNonNullBytes = true;
+    size_t LeadingNonNullElementsCount = 0;
     for (unsigned i = 0, e = CDS->getNumElements(); i != e; ++i) {
       llvm::Constant *Elt = CDS->getElementAsConstant(i);
 
       // If necessary, get a pointer to the element and emit it.
-      if (!Elt->isNullValue() && !isa<llvm::UndefValue>(Elt))
+      if (!isNullOrUndef(Elt)) {
         emitStoresForInitAfterBZero(
             CGM, Elt, Builder.CreateConstInBoundsGEP2_32(Loc, 0, i), isVolatile,
             Builder, IsAutoInit);
+        LeadingNonNullElementsCount += CountNonNullBytes;
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efriedma-quic wrote:

This is too clever; just write `if (CountNonNullBytes) ++LeadingNonNullElementsCount;`

https://github.com/llvm/llvm-project/pull/79502