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Issue #2148 - Shrink Vector from (usually) four pointers in size to three when no inline storage is used.

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See Bug 1338374
This commit is contained in:
FranklinDM 2023-03-09 15:10:02 +08:00 • committed by roytam1
commit 481069044d
2 changed files with 168 additions and 82 deletions
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43
mfbt/tests/TestVector.cpp
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@ -396,6 +396,49 @@ mozilla::detail::VectorTesting::testInsert()
MOZ_RELEASE_ASSERT(S::destructCount == 1);
}
// Declare but leave (permanently) incomplete.
struct Incomplete;
// We could even *construct* a Vector<Incomplete, 0> if we wanted. But we can't
// destruct it, so it's not worth the trouble.
static_assert(sizeof(Vector<Incomplete, 0>) > 0,
"Vector of an incomplete type will compile");
// Vector with no inline storage should occupy the absolute minimum space in
// non-debug builds. (Debug adds a laundry list of other constraints, none
// directly relevant to shipping builds, that aren't worth precisely modeling.)
#ifndef DEBUG
template<typename T>
struct NoInlineStorageLayout
{
T* mBegin;
size_t mLength;
struct CRAndStorage {
size_t mCapacity;
} mTail;
};
// Only one of these should be necessary, but test a few of them for good
// measure.
static_assert(sizeof(Vector<int, 0>) == sizeof(NoInlineStorageLayout<int>),
"Vector of int without inline storage shouldn't occupy dead "
"space for that absence of storage");
static_assert(sizeof(Vector<bool, 0>) == sizeof(NoInlineStorageLayout<bool>),
"Vector of bool without inline storage shouldn't occupy dead "
"space for that absence of storage");
static_assert(sizeof(Vector<S, 0>) == sizeof(NoInlineStorageLayout<S>),
"Vector of S without inline storage shouldn't occupy dead "
"space for that absence of storage");
static_assert(sizeof(Vector<Incomplete, 0>) == sizeof(NoInlineStorageLayout<Incomplete>),
"Vector of an incomplete class without inline storage shouldn't "
"occupy dead space for that absence of storage");
#endif // DEBUG
int
main()
{