libexpr: Reduce the size of Value down to 16 bytes

This shaves off a very significand amount of memory used
for evaluation as well as reduces the GC-managed heap.

Previously the union discriminator (InternalType) was
stored as a separate field in the Value, which takes up
whole 8 bytes due to padding needed for member alignment.
This effectively wasted 7 whole bytes of memory. Instead
of doing that InternalType is instead packed into pointer
alignment niches. As it turns out, there's more than enough
unused bits there for the bit packing to be effective.

See the doxygen comment in the ValueStorage specialization
for more details.

This does not add any performance overhead, even though
we now consistently assert the InternalType in all getters.

This can also be made atomic with a double width compare-and-swap
instruction on x86_64 (CMPXCHG16B instruction) for parallel evaluation.

src/libexpr/eval-gc.cc

@@ -4,6 +4,7 @@
 #include "nix/util/config-global.hh"
 #include "nix/util/serialise.hh"
 #include "nix/expr/eval-gc.hh"
+#include "nix/expr/value.hh"
 
 #include "expr-config-private.hh"
 
@@ -52,6 +53,13 @@ static inline void initGCReal()
 
     GC_INIT();
 
+    /* Register valid displacements in case we are using alignment niches
+       for storing the type information. This way tagged pointers are considered
+       to be valid, even when they are not aligned. */
+    if constexpr (detail::useBitPackedValueStorage<sizeof(void *)>)
+        for (std::size_t i = 1; i < sizeof(std::uintptr_t); ++i)
+            GC_register_displacement(i);
+
     GC_set_oom_fn(oomHandler);
 
     /* Set the initial heap size to something fairly big (25% of

src/libexpr/include/nix/expr/value.hh

@@ -19,23 +19,35 @@ namespace nix {
 struct Value;
 class BindingsBuilder;
 
+/**
+ * Internal type discriminator, which is more detailed than `ValueType`, as
+ * it specifies the exact representation used (for types that have multiple
+ * possible representations).
+ *
+ * @warning The ordering is very significant. See ValueStorage::getInternalType() for details
+ * about how this is mapped into the alignment bits to save significant memory.
+ * This also restricts the number of internal types represented with distinct memory layouts.
+ */
 typedef enum {
     tUninitialized = 0,
+    /* layout: Single/zero field payload */
     tInt = 1,
     tBool,
+    tNull,
+    tFloat,
+    tExternal,
+    tPrimOp,
+    tAttrs,
+    /* layout: Pair of pointers payload */
+    tListSmall,
+    tPrimOpApp,
+    tApp,
+    tThunk,
+    tLambda,
+    /* layout: Single untaggable field */
+    tListN,
     tString,
     tPath,
-    tNull,
-    tAttrs,
-    tListSmall,
-    tListN,
-    tThunk,
-    tApp,
-    tLambda,
-    tPrimOp,
-    tPrimOpApp,
-    tExternal,
-    tFloat
 } InternalType;
 
 /**
@@ -307,7 +319,7 @@ inline constexpr InternalType payloadTypeToInternalType = PayloadTypeToInternalT
  * All specializations of this type need to implement getStorage, setStorage and
  * getInternalType methods.
  */
-template<std::size_t ptrSize>
+template<std::size_t ptrSize, typename Enable = void>
 class ValueStorage : public detail::ValueBase
 {
 protected:
@@ -351,6 +363,287 @@ protected:
     }
 };
 
+namespace detail {
+
+/* Whether to use a specialization of ValueStorage that does bitpacking into
+   alignment niches. */
+template<std::size_t ptrSize>
+inline constexpr bool useBitPackedValueStorage = (ptrSize == 8) && (__STDCPP_DEFAULT_NEW_ALIGNMENT__ >= 8);
+
+} // namespace detail
+
+/**
+ * Value storage that is optimized for 64 bit systems.
+ * Packs discriminator bits into the pointer alignment niches.
+ */
+template<std::size_t ptrSize>
+class ValueStorage<ptrSize, std::enable_if_t<detail::useBitPackedValueStorage<ptrSize>>> : public detail::ValueBase
+{
+    /* Needs a dependent type name in order for member functions (and
+     * potentially ill-formed bit casts) to be SFINAE'd out.
+     *
+     * Otherwise some member functions could possibly be instantiated for 32 bit
+     * systems and fail due to an unsatisfied constraint.
+     */
+    template<std::size_t size>
+    struct PackedPointerTypeStruct
+    {
+        using type = std::uint64_t;
+    };
+
+    using PackedPointer = typename PackedPointerTypeStruct<ptrSize>::type;
+    using Payload = std::array<PackedPointer, 2>;
+    Payload payload = {};
+
+    static constexpr int discriminatorBits = 3;
+    static constexpr PackedPointer discriminatorMask = (PackedPointer(1) << discriminatorBits) - 1;
+
+    /**
+     * The value is stored as a pair of 8-byte double words. All pointers are assumed
+     * to be 8-byte aligned. This gives us at most 6 bits of discriminator bits
+     * of free storage. In some cases when one double word can't be tagged the whole
+     * discriminator is stored in the first double word.
+     *
+     * The layout of discriminator bits is determined by the 3 bits of PrimaryDiscriminator,
+     * which are always stored in the lower 3 bits of the first dword of the payload.
+     * The memory layout has 3 types depending on the PrimaryDiscriminator value.
+     *
+     * PrimaryDiscriminator::pdSingleDWord - Only the second dword carries the data.
+     * That leaves the first 8 bytes free for storing the InternalType in the upper
+     * bits.
+     *
+     * PrimaryDiscriminator::pdListN - pdPath - Only has 3 available padding bits
+     * because:
+     * - tListN needs a size, whose lower bits we can't borrow.
+     * - tString and tPath have C-string fields, which don't necessarily need to
+     * be aligned.
+     *
+     * In this case we reserve their discriminators directly in the PrimaryDiscriminator
+     * bits stored in payload[0].
+     *
+     * PrimaryDiscriminator::pdPairOfPointers - Payloads that consist of a pair of pointers.
+     * In this case the 3 lower bits of payload[1] can be tagged.
+     *
+     * The primary discriminator with value 0 is reserved for uninitialized Values,
+     * which are useful for diagnostics in C bindings.
+     */
+    enum PrimaryDiscriminator : int {
+        pdUninitialized = 0,
+        pdSingleDWord, //< layout: Single/zero field payload
+        /* The order of these enumations must be the same as in InternalType. */
+        pdListN, //< layout: Single untaggable field.
+        pdString,
+        pdPath,
+        pdPairOfPointers, //< layout: Pair of pointers payload
+    };
+
+    template<typename T>
+        requires std::is_pointer_v<T>
+    static T untagPointer(PackedPointer val) noexcept
+    {
+        return std::bit_cast<T>(val & ~discriminatorMask);
+    }
+
+    PrimaryDiscriminator getPrimaryDiscriminator() const noexcept
+    {
+        return static_cast<PrimaryDiscriminator>(payload[0] & discriminatorMask);
+    }
+
+    static void assertAligned(PackedPointer val) noexcept
+    {
+        assert((val & discriminatorMask) == 0 && "Pointer is not 8 bytes aligned");
+    }
+
+    template<InternalType type>
+    void setSingleDWordPayload(PackedPointer untaggedVal) noexcept
+    {
+        /* There's plenty of free upper bits in the first dword, which is
+           used only for the discriminator. */
+        payload[0] = static_cast<int>(pdSingleDWord) | (static_cast<int>(type) << discriminatorBits);
+        payload[1] = untaggedVal;
+    }
+
+    template<PrimaryDiscriminator discriminator, typename T, typename U>
+    void setUntaggablePayload(T * firstPtrField, U untaggableField) noexcept
+    {
+        static_assert(discriminator >= pdListN && discriminator <= pdPath);
+        auto firstFieldPayload = std::bit_cast<PackedPointer>(firstPtrField);
+        assertAligned(firstFieldPayload);
+        payload[0] = static_cast<int>(discriminator) | firstFieldPayload;
+        payload[1] = std::bit_cast<PackedPointer>(untaggableField);
+    }
+
+    template<InternalType type, typename T, typename U>
+    void setPairOfPointersPayload(T * firstPtrField, U * secondPtrField) noexcept
+    {
+        static_assert(type >= tListSmall && type <= tLambda);
+        {
+            auto firstFieldPayload = std::bit_cast<PackedPointer>(firstPtrField);
+            assertAligned(firstFieldPayload);
+            payload[0] = static_cast<int>(pdPairOfPointers) | firstFieldPayload;
+        }
+        {
+            auto secondFieldPayload = std::bit_cast<PackedPointer>(secondPtrField);
+            assertAligned(secondFieldPayload);
+            payload[1] = (type - tListSmall) | secondFieldPayload;
+        }
+    }
+
+    template<typename T, typename U>
+        requires std::is_pointer_v<T> && std::is_pointer_v<U>
+    void getPairOfPointersPayload(T & firstPtrField, U & secondPtrField) const noexcept
+    {
+        firstPtrField = untagPointer<T>(payload[0]);
+        secondPtrField = untagPointer<U>(payload[1]);
+    }
+
+protected:
+    /** Get internal type currently occupying the storage. */
+    InternalType getInternalType() const noexcept
+    {
+        switch (auto pd = getPrimaryDiscriminator()) {
+        case pdUninitialized:
+            /* Discriminator value of zero is used to distinguish uninitialized values. */
+            return tUninitialized;
+        case pdSingleDWord:
+            /* Payloads that only use up a single double word store the InternalType
+               in the upper bits of the first double word. */
+            return InternalType(payload[0] >> discriminatorBits);
+        /* The order must match that of the enumerations defined in InternalType. */
+        case pdListN:
+        case pdString:
+        case pdPath:
+            return static_cast<InternalType>(tListN + (pd - pdListN));
+        case pdPairOfPointers:
+            return static_cast<InternalType>(tListSmall + (payload[1] & discriminatorMask));
+        [[unlikely]] default:
+            unreachable();
+        }
+    }
+
+#define NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(TYPE, MEMBER_A, MEMBER_B)                                   \
+                                                                                                       \
+    void getStorage(TYPE & val) const noexcept                                                         \
+    {                                                                                                  \
+        getPairOfPointersPayload(val MEMBER_A, val MEMBER_B);                                          \
+    }                                                                                                  \
+                                                                                                       \
+    void setStorage(TYPE val) noexcept                                                                 \
+    {                                                                                                  \
+        setPairOfPointersPayload<detail::payloadTypeToInternalType<TYPE>>(val MEMBER_A, val MEMBER_B); \
+    }
+
+    NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(SmallList, [0], [1])
+    NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(PrimOpApplicationThunk, .left, .right)
+    NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(FunctionApplicationThunk, .left, .right)
+    NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(ClosureThunk, .env, .expr)
+    NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS(Lambda, .env, .fun)
+
+#undef NIX_VALUE_STORAGE_DEF_PAIR_OF_PTRS
+
+    void getStorage(NixInt & integer) const noexcept
+    {
+        /* PackedPointerType -> int64_t here is well-formed, since the standard requires
+           this conversion to follow 2's complement rules. This is just a no-op. */
+        integer = NixInt(payload[1]);
+    }
+
+    void getStorage(bool & boolean) const noexcept
+    {
+        boolean = payload[1];
+    }
+
+    void getStorage(Null & null) const noexcept {}
+
+    void getStorage(NixFloat & fpoint) const noexcept
+    {
+        fpoint = std::bit_cast<NixFloat>(payload[1]);
+    }
+
+    void getStorage(ExternalValueBase *& external) const noexcept
+    {
+        external = std::bit_cast<ExternalValueBase *>(payload[1]);
+    }
+
+    void getStorage(PrimOp *& primOp) const noexcept
+    {
+        primOp = std::bit_cast<PrimOp *>(payload[1]);
+    }
+
+    void getStorage(Bindings *& attrs) const noexcept
+    {
+        attrs = std::bit_cast<Bindings *>(payload[1]);
+    }
+
+    void getStorage(List & list) const noexcept
+    {
+        list.elems = untagPointer<decltype(list.elems)>(payload[0]);
+        list.size = payload[1];
+    }
+
+    void getStorage(StringWithContext & string) const noexcept
+    {
+        string.context = untagPointer<decltype(string.context)>(payload[0]);
+        string.c_str = std::bit_cast<const char *>(payload[1]);
+    }
+
+    void getStorage(Path & path) const noexcept
+    {
+        path.accessor = untagPointer<decltype(path.accessor)>(payload[0]);
+        path.path = std::bit_cast<const char *>(payload[1]);
+    }
+
+    void setStorage(NixInt integer) noexcept
+    {
+        setSingleDWordPayload<tInt>(integer.value);
+    }
+
+    void setStorage(bool boolean) noexcept
+    {
+        setSingleDWordPayload<tBool>(boolean);
+    }
+
+    void setStorage(Null path) noexcept
+    {
+        setSingleDWordPayload<tNull>(0);
+    }
+
+    void setStorage(NixFloat fpoint) noexcept
+    {
+        setSingleDWordPayload<tFloat>(std::bit_cast<PackedPointer>(fpoint));
+    }
+
+    void setStorage(ExternalValueBase * external) noexcept
+    {
+        setSingleDWordPayload<tExternal>(std::bit_cast<PackedPointer>(external));
+    }
+
+    void setStorage(PrimOp * primOp) noexcept
+    {
+        setSingleDWordPayload<tPrimOp>(std::bit_cast<PackedPointer>(primOp));
+    }
+
+    void setStorage(Bindings * bindings) noexcept
+    {
+        setSingleDWordPayload<tAttrs>(std::bit_cast<PackedPointer>(bindings));
+    }
+
+    void setStorage(List list) noexcept
+    {
+        setUntaggablePayload<pdListN>(list.elems, list.size);
+    }
+
+    void setStorage(StringWithContext string) noexcept
+    {
+        setUntaggablePayload<pdString>(string.context, string.c_str);
+    }
+
+    void setStorage(Path path) noexcept
+    {
+        setUntaggablePayload<pdPath>(path.accessor, path.path);
+    }
+};
+
 /**
  * View into a list of Value * that is itself immutable.
  *