[PATCH] D150892: [clang][ExprConstant] fix __builtin_object_size for flexible array members

[Nick Desaulniers via Phabricator via cfe-commits]

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As reported by @kees, GCC treats __builtin_object_size of structures
containing flexible array members (aka arrays with incomplete type) not
just as the sizeof the underlying type, but additionally the size of the
members in a designated initializer list.

Fixes: https://github.com/llvm/llvm-project/issues/62789


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150892

Files:
  clang/lib/AST/ExprConstant.cpp
  clang/test/CodeGen/object-size.c


Index: clang/test/CodeGen/object-size.c
===================================================================
--- clang/test/CodeGen/object-size.c
+++ clang/test/CodeGen/object-size.c
@@ -531,9 +531,7 @@
   .snd = { 0, 1, 2, },
 };
 unsigned long test32(void) {
-  // FIXME: GCC returns the sizeof the base type, plus the number of bytes from
-  // members of the initializer of the flexible array: 19.
-  // CHECK: ret i64 16
+  // CHECK: ret i64 19
   return __builtin_object_size(&D, 1);
 }
 
Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -11732,7 +11732,14 @@
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
     if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
       return false;
-    return HandleSizeof(Info, ExprLoc, Ty, Result);
+    bool Ret = HandleSizeof(Info, ExprLoc, Ty, Result);
+    if (Ty->isStructureType() &&
+        Ty->getAsStructureType()->getDecl()->hasFlexibleArrayMember()) {
+      const auto *VD = dyn_cast_or_null<VarDecl>(
+          LVal.getLValueBase().dyn_cast<const ValueDecl *>());
+      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+    }
+    return Ret;
   };
 
   // We want to evaluate the size of the entire object. This is a valid fallback


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