Fix codegen of consteval functions returning an empty class, and related issues (#93115)

Fix codegen of consteval functions returning an empty class, and related
issues

If a class is empty, don't store it to memory: the store might overwrite
useful data. Similarly, if a class has tail padding that might overlap
other fields, don't store the tail padding to memory.

The problem here turned out a bit more general than I initially thought:
basically all uses of EmitAggregateStore were broken. Call lowering had
a method that did mostly the right thing, though: CreateCoercedStore.
Adapt CreateCoercedStore so it always does the conservatively right
thing, and use it for both calls and ConstantExpr.

Also, along the way, fix the "overlap" bit in AggValueSlot: the bit was
set incorrectly for empty classes in some cases.

Fixes #93040.

1 files changed, 14 insertions, 9 deletions

diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index c3c10e7..d9f44f4 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -131,15 +131,12 @@ public:
     EnsureDest(E->getType());
 
     if (llvm::Value *Result = ConstantEmitter(CGF).tryEmitConstantExpr(E)) {
-      Address StoreDest = Dest.getAddress();
-      // The emitted value is guaranteed to have the same size as the
-      // destination but can have a different type. Just do a bitcast in this
-      // case to avoid incorrect GEPs.
-      if (Result->getType() != StoreDest.getType())
-        StoreDest = StoreDest.withElementType(Result->getType());
-
-      CGF.EmitAggregateStore(Result, StoreDest,
-                             E->getType().isVolatileQualified());
+      CGF.CreateCoercedStore(
+          Result, Dest.getAddress(),
+          llvm::TypeSize::getFixed(
+              Dest.getPreferredSize(CGF.getContext(), E->getType())
+                  .getQuantity()),
+          E->getType().isVolatileQualified());
       return;
     }
     return Visit(E->getSubExpr());
@@ -2050,6 +2047,10 @@ CodeGenFunction::getOverlapForFieldInit(const FieldDecl *FD) {
   if (!FD->hasAttr<NoUniqueAddressAttr>() || !FD->getType()->isRecordType())
     return AggValueSlot::DoesNotOverlap;
 
+  // Empty fields can overlap earlier fields.
+  if (FD->getType()->getAsCXXRecordDecl()->isEmpty())
+    return AggValueSlot::MayOverlap;
+
   // If the field lies entirely within the enclosing class's nvsize, its tail
   // padding cannot overlap any already-initialized object. (The only subobjects
   // with greater addresses that might already be initialized are vbases.)
@@ -2072,6 +2073,10 @@ AggValueSlot::Overlap_t CodeGenFunction::getOverlapForBaseInit(
   if (IsVirtual)
     return AggValueSlot::MayOverlap;
 
+  // Empty bases can overlap earlier bases.
+  if (BaseRD->isEmpty())
+    return AggValueSlot::MayOverlap;
+
   // If the base class is laid out entirely within the nvsize of the derived
   // class, its tail padding cannot yet be initialized, so we can issue
   // stores at the full width of the base class.