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

[Eli Friedman via cfe-commits]

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@@ -1209,8 +1301,21 @@ static void emitStoresForConstant(CodeGenModule &CGM, const VarDecl &D,
   // If the initializer is all or mostly the same, codegen with bzero / memset
   // then do a few stores afterward.
   if (shouldUseBZeroPlusStoresToInitialize(constant, ConstantSize)) {
-    auto *I = Builder.CreateMemSet(Loc, llvm::ConstantInt::get(CGM.Int8Ty, 0),
-                                   SizeVal, isVolatile);
+    size_t LeadingNonNullBytes =
+        CountLeadingNonNullBytes(CGM.getDataLayout(), constant);
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efriedma-quic wrote:

I don't like the way this duplicates the logic.  You aren't really trying to count bytes that are "non-null"; you're trying to find the offset of the first byte that isn't overwritten by emitStoresForInitAfterBZero.  The best place to compute that is emitStoresForInitAfterBZero itself.

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