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List: cfe-commits
Subject: [clang] [CodeGen] Revamp counted_by calculations (PR #70606)
From: Yeoul Na via cfe-commits <cfe-commits () lists ! llvm ! org>
Date: 2023-10-31 23:09:03
Message-ID: 6541890f.170a0220.4197f.0709 () mx ! google ! com
[Download RAW message or body]
================
@@ -859,53 +859,60 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned \
Type, }
if (IsDynamic) {
- LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
- getLangOpts().getStrictFlexArraysLevel();
- const Expr *Base = E->IgnoreParenImpCasts();
-
- if (FieldDecl *FD = FindCountedByField(Base, StrictFlexArraysLevel)) {
- const auto *ME = dyn_cast<MemberExpr>(Base);
- llvm::Value *ObjectSize = nullptr;
-
- if (!ME) {
- const auto *DRE = dyn_cast<DeclRefExpr>(Base);
- ValueDecl *VD = nullptr;
-
- ObjectSize = ConstantInt::get(
- ResType,
- getContext().getTypeSize(DRE->getType()->getPointeeType()) / 8,
- true);
-
- if (auto *RD = DRE->getType()->getPointeeType()->getAsRecordDecl())
- VD = RD->getLastField();
-
- Expr *ICE = ImplicitCastExpr::Create(
- getContext(), DRE->getType(), CK_LValueToRValue,
- const_cast<Expr *>(cast<Expr>(DRE)), nullptr, VK_PRValue,
- FPOptionsOverride());
- ME = MemberExpr::CreateImplicit(getContext(), ICE, true, VD,
- VD->getType(), VK_LValue, OK_Ordinary);
- }
-
- // At this point, we know that \p ME is a flexible array member.
- const auto *ArrayTy = getContext().getAsArrayType(ME->getType());
+ // The code generated here calculates the size of a struct with a flexible
+ // array member that uses the counted_by attribute. There are two instances
+ // we handle:
+ //
+ // struct s {
+ // unsigned long flags;
+ // int count;
+ // int array[] __attribute__((counted_by(count)));
+ // }
+ //
+ // 1) bdos of the flexible array itself:
+ //
+ // __builtin_dynamic_object_size(p->array, 1) ==
+ // p->count * sizeof(*p->array)
+ //
+ // 2) bdos of the whole struct, including the flexible array:
+ //
+ // __builtin_dynamic_object_size(p, 1) ==
+ // sizeof(*p) + p->count * sizeof(*p->array)
----------------
rapidsna wrote:
Okay, I think we are talking past to each other a little bit. That comment I was \
responding to this:
> Why wouldn't it include the FAM part if the full struct pointer is specified to \
> __bdos?
I'm saying "the full struct size" isn't exactly `struct + fam` because fam doesn't \
always exactly start from sizeof(struct) when there is a padding in the struct due to \
the alignment.
As we all see in the previous examples, the full struct size is `offsetof(struct s, \
fam) + sizeof(*p->array) * p->count` and align the result to `alignof(struct s)`.
https://github.com/llvm/llvm-project/pull/70606
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