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libexpr: Reduce the size of Value down to 16 bytes (on 64 bit systems) #13407

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Merged
merged 6 commits into from
Jul 2, 2025
Merged

libexpr: Reduce the size of Value down to 16 bytes (on 64 bit systems) #13407

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ea32580
libexpr: Format value.hh
xokdvium Jun 28, 2025
1a033ee
libexpr: Use single `tSmallList` `Value` discriminator for small lists
xokdvium Jun 28, 2025
810455f
libexpr: Simplify Value::is* methods by introducing isa function temp…
xokdvium Jun 28, 2025
c39cc00
libexpr: Factor out `Payload` union to a default implementation of `V…
xokdvium Jun 28, 2025
e73fcf7
libexpr: Use proxy ListView for all Value list accesses
xokdvium Jul 2, 2025
5a20a48
libexpr: Reduce the size of Value down to 16 bytes
xokdvium Jun 30, 2025
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1 change: 0 additions & 1 deletion maintainers/flake-module.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -256,7 +256,6 @@
''^src/libexpr/include/nix/expr/value-to-json\.hh$''
''^src/libexpr/value-to-xml\.cc$''
''^src/libexpr/include/nix/expr/value-to-xml\.hh$''
''^src/libexpr/include/nix/expr/value\.hh$''
''^src/libexpr/value/context\.cc$''
''^src/libexpr/include/nix/expr/value/context\.hh$''
''^src/libfetchers/attrs\.cc$''
Expand Down
2 changes: 1 addition & 1 deletion src/libexpr-c/nix_api_value.cc
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Expand Up @@ -324,7 +324,7 @@ nix_value * nix_get_list_byidx(nix_c_context * context, const nix_value * value,
try {
auto & v = check_value_in(value);
assert(v.type() == nix::nList);
auto * p = v.listElems()[ix];
auto * p = v.listView()[ix];
nix_gc_incref(nullptr, p);
if (p != nullptr)
state->state.forceValue(*p, nix::noPos);
Expand Down
101 changes: 54 additions & 47 deletions src/libexpr-tests/primops.cc
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Expand Up @@ -150,8 +150,8 @@ namespace nix {
TEST_F(PrimOpTest, attrValues) {
auto v = eval("builtins.attrValues { x = \"foo\"; a = 1; }");
ASSERT_THAT(v, IsListOfSize(2));
ASSERT_THAT(*v.listElems()[0], IsIntEq(1));
ASSERT_THAT(*v.listElems()[1], IsStringEq("foo"));
ASSERT_THAT(*v.listView()[0], IsIntEq(1));
ASSERT_THAT(*v.listView()[1], IsStringEq("foo"));
}

TEST_F(PrimOpTest, getAttr) {
Expand Down Expand Up @@ -250,8 +250,8 @@ namespace nix {
TEST_F(PrimOpTest, catAttrs) {
auto v = eval("builtins.catAttrs \"a\" [{a = 1;} {b = 0;} {a = 2;}]");
ASSERT_THAT(v, IsListOfSize(2));
ASSERT_THAT(*v.listElems()[0], IsIntEq(1));
ASSERT_THAT(*v.listElems()[1], IsIntEq(2));
ASSERT_THAT(*v.listView()[0], IsIntEq(1));
ASSERT_THAT(*v.listView()[1], IsIntEq(2));
}

TEST_F(PrimOpTest, functionArgs) {
Expand Down Expand Up @@ -320,7 +320,8 @@ namespace nix {
TEST_F(PrimOpTest, tail) {
auto v = eval("builtins.tail [ 3 2 1 0 ]");
ASSERT_THAT(v, IsListOfSize(3));
for (const auto [n, elem] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [n, elem] : enumerate(listView))
ASSERT_THAT(*elem, IsIntEq(2 - static_cast<int>(n)));
}

Expand All @@ -331,17 +332,17 @@ namespace nix {
TEST_F(PrimOpTest, map) {
auto v = eval("map (x: \"foo\" + x) [ \"bar\" \"bla\" \"abc\" ]");
ASSERT_THAT(v, IsListOfSize(3));
auto elem = v.listElems()[0];
auto elem = v.listView()[0];
ASSERT_THAT(*elem, IsThunk());
state.forceValue(*elem, noPos);
ASSERT_THAT(*elem, IsStringEq("foobar"));

elem = v.listElems()[1];
elem = v.listView()[1];
ASSERT_THAT(*elem, IsThunk());
state.forceValue(*elem, noPos);
ASSERT_THAT(*elem, IsStringEq("foobla"));

elem = v.listElems()[2];
elem = v.listView()[2];
ASSERT_THAT(*elem, IsThunk());
state.forceValue(*elem, noPos);
ASSERT_THAT(*elem, IsStringEq("fooabc"));
Expand All @@ -350,7 +351,7 @@ namespace nix {
TEST_F(PrimOpTest, filter) {
auto v = eval("builtins.filter (x: x == 2) [ 3 2 3 2 3 2 ]");
ASSERT_THAT(v, IsListOfSize(3));
for (const auto elem : v.listItems())
for (const auto elem : v.listView())
ASSERT_THAT(*elem, IsIntEq(2));
}

Expand All @@ -367,7 +368,8 @@ namespace nix {
TEST_F(PrimOpTest, concatLists) {
auto v = eval("builtins.concatLists [[1 2] [3 4]]");
ASSERT_THAT(v, IsListOfSize(4));
for (const auto [i, elem] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [i, elem] : enumerate(listView))
ASSERT_THAT(*elem, IsIntEq(static_cast<int>(i)+1));
}

Expand Down Expand Up @@ -405,7 +407,8 @@ namespace nix {
auto v = eval("builtins.genList (x: x + 1) 3");
ASSERT_EQ(v.type(), nList);
ASSERT_EQ(v.listSize(), 3u);
for (const auto [i, elem] : enumerate(v.listItems())) {
auto listView = v.listView();
for (const auto [i, elem] : enumerate(listView)) {
ASSERT_THAT(*elem, IsThunk());
state.forceValue(*elem, noPos);
ASSERT_THAT(*elem, IsIntEq(static_cast<int>(i)+1));
Expand All @@ -418,7 +421,8 @@ namespace nix {
ASSERT_EQ(v.listSize(), 6u);

const std::vector<int> numbers = { 42, 77, 147, 249, 483, 526 };
for (const auto [n, elem] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [n, elem] : enumerate(listView))
ASSERT_THAT(*elem, IsIntEq(numbers[n]));
}

Expand All @@ -429,17 +433,17 @@ namespace nix {
auto right = v.attrs()->get(createSymbol("right"));
ASSERT_NE(right, nullptr);
ASSERT_THAT(*right->value, IsListOfSize(2));
ASSERT_THAT(*right->value->listElems()[0], IsIntEq(23));
ASSERT_THAT(*right->value->listElems()[1], IsIntEq(42));
ASSERT_THAT(*right->value->listView()[0], IsIntEq(23));
ASSERT_THAT(*right->value->listView()[1], IsIntEq(42));

auto wrong = v.attrs()->get(createSymbol("wrong"));
ASSERT_NE(wrong, nullptr);
ASSERT_EQ(wrong->value->type(), nList);
ASSERT_EQ(wrong->value->listSize(), 3u);
ASSERT_THAT(*wrong->value, IsListOfSize(3));
ASSERT_THAT(*wrong->value->listElems()[0], IsIntEq(1));
ASSERT_THAT(*wrong->value->listElems()[1], IsIntEq(9));
ASSERT_THAT(*wrong->value->listElems()[2], IsIntEq(3));
ASSERT_THAT(*wrong->value->listView()[0], IsIntEq(1));
ASSERT_THAT(*wrong->value->listView()[1], IsIntEq(9));
ASSERT_THAT(*wrong->value->listView()[2], IsIntEq(3));
}

TEST_F(PrimOpTest, concatMap) {
Expand All @@ -448,7 +452,8 @@ namespace nix {
ASSERT_EQ(v.listSize(), 6u);

const std::vector<int> numbers = { 1, 2, 0, 3, 4, 0 };
for (const auto [n, elem] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [n, elem] : enumerate(listView))
ASSERT_THAT(*elem, IsIntEq(numbers[n]));
}

Expand Down Expand Up @@ -682,7 +687,8 @@ namespace nix {
ASSERT_THAT(v, IsListOfSize(4));

const std::vector<std::string_view> strings = { "1", "2", "3", "git" };
for (const auto [n, p] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [n, p] : enumerate(listView))
ASSERT_THAT(*p, IsStringEq(strings[n]));
}

Expand Down Expand Up @@ -772,67 +778,67 @@ namespace nix {
auto v = eval("builtins.split \"(a)b\" \"abc\"");
ASSERT_THAT(v, IsListOfSize(3));

ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
ASSERT_THAT(*v.listView()[0], IsStringEq(""));

ASSERT_THAT(*v.listElems()[1], IsListOfSize(1));
ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
ASSERT_THAT(*v.listView()[1], IsListOfSize(1));
ASSERT_THAT(*v.listView()[1]->listView()[0], IsStringEq("a"));

ASSERT_THAT(*v.listElems()[2], IsStringEq("c"));
ASSERT_THAT(*v.listView()[2], IsStringEq("c"));
}

TEST_F(PrimOpTest, split2) {
// v is expected to be a list [ "" [ "a" ] "b" [ "c"] "" ]
auto v = eval("builtins.split \"([ac])\" \"abc\"");
ASSERT_THAT(v, IsListOfSize(5));

ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
ASSERT_THAT(*v.listView()[0], IsStringEq(""));

ASSERT_THAT(*v.listElems()[1], IsListOfSize(1));
ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
ASSERT_THAT(*v.listView()[1], IsListOfSize(1));
ASSERT_THAT(*v.listView()[1]->listView()[0], IsStringEq("a"));

ASSERT_THAT(*v.listElems()[2], IsStringEq("b"));
ASSERT_THAT(*v.listView()[2], IsStringEq("b"));

ASSERT_THAT(*v.listElems()[3], IsListOfSize(1));
ASSERT_THAT(*v.listElems()[3]->listElems()[0], IsStringEq("c"));
ASSERT_THAT(*v.listView()[3], IsListOfSize(1));
ASSERT_THAT(*v.listView()[3]->listView()[0], IsStringEq("c"));

ASSERT_THAT(*v.listElems()[4], IsStringEq(""));
ASSERT_THAT(*v.listView()[4], IsStringEq(""));
}

TEST_F(PrimOpTest, split3) {
auto v = eval("builtins.split \"(a)|(c)\" \"abc\"");
ASSERT_THAT(v, IsListOfSize(5));

// First list element
ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
ASSERT_THAT(*v.listView()[0], IsStringEq(""));

// 2nd list element is a list [ "" null ]
ASSERT_THAT(*v.listElems()[1], IsListOfSize(2));
ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
ASSERT_THAT(*v.listElems()[1]->listElems()[1], IsNull());
ASSERT_THAT(*v.listView()[1], IsListOfSize(2));
ASSERT_THAT(*v.listView()[1]->listView()[0], IsStringEq("a"));
ASSERT_THAT(*v.listView()[1]->listView()[1], IsNull());

// 3rd element
ASSERT_THAT(*v.listElems()[2], IsStringEq("b"));
ASSERT_THAT(*v.listView()[2], IsStringEq("b"));

// 4th element is a list: [ null "c" ]
ASSERT_THAT(*v.listElems()[3], IsListOfSize(2));
ASSERT_THAT(*v.listElems()[3]->listElems()[0], IsNull());
ASSERT_THAT(*v.listElems()[3]->listElems()[1], IsStringEq("c"));
ASSERT_THAT(*v.listView()[3], IsListOfSize(2));
ASSERT_THAT(*v.listView()[3]->listView()[0], IsNull());
ASSERT_THAT(*v.listView()[3]->listView()[1], IsStringEq("c"));

// 5th element is the empty string
ASSERT_THAT(*v.listElems()[4], IsStringEq(""));
ASSERT_THAT(*v.listView()[4], IsStringEq(""));
}

TEST_F(PrimOpTest, split4) {
auto v = eval("builtins.split \"([[:upper:]]+)\" \" FOO \"");
ASSERT_THAT(v, IsListOfSize(3));
auto first = v.listElems()[0];
auto second = v.listElems()[1];
auto third = v.listElems()[2];
auto first = v.listView()[0];
auto second = v.listView()[1];
auto third = v.listView()[2];

ASSERT_THAT(*first, IsStringEq(" "));

ASSERT_THAT(*second, IsListOfSize(1));
ASSERT_THAT(*second->listElems()[0], IsStringEq("FOO"));
ASSERT_THAT(*second->listView()[0], IsStringEq("FOO"));

ASSERT_THAT(*third, IsStringEq(" "));
}
Expand All @@ -850,14 +856,14 @@ namespace nix {
TEST_F(PrimOpTest, match3) {
auto v = eval("builtins.match \"a(b)(c)\" \"abc\"");
ASSERT_THAT(v, IsListOfSize(2));
ASSERT_THAT(*v.listElems()[0], IsStringEq("b"));
ASSERT_THAT(*v.listElems()[1], IsStringEq("c"));
ASSERT_THAT(*v.listView()[0], IsStringEq("b"));
ASSERT_THAT(*v.listView()[1], IsStringEq("c"));
}

TEST_F(PrimOpTest, match4) {
auto v = eval("builtins.match \"[[:space:]]+([[:upper:]]+)[[:space:]]+\" \" FOO \"");
ASSERT_THAT(v, IsListOfSize(1));
ASSERT_THAT(*v.listElems()[0], IsStringEq("FOO"));
ASSERT_THAT(*v.listView()[0], IsStringEq("FOO"));
}

TEST_F(PrimOpTest, match5) {
Expand All @@ -874,7 +880,8 @@ namespace nix {

// ensure that the list is sorted
const std::vector<std::string_view> expected { "a", "x", "y", "z" };
for (const auto [n, elem] : enumerate(v.listItems()))
auto listView = v.listView();
for (const auto [n, elem] : enumerate(listView))
ASSERT_THAT(*elem, IsStringEq(expected[n]));
}

Expand Down
2 changes: 1 addition & 1 deletion src/libexpr/attr-path.cc
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Expand Up @@ -95,7 +95,7 @@ std::pair<Value *, PosIdx> findAlongAttrPath(EvalState & state, const std::strin
if (*attrIndex >= v->listSize())
throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", *attrIndex, attrPath);

v = v->listElems()[*attrIndex];
v = v->listView()[*attrIndex];
pos = noPos;
}

Expand Down
2 changes: 1 addition & 1 deletion src/libexpr/eval-cache.cc
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Expand Up @@ -721,7 +721,7 @@ std::vector<std::string> AttrCursor::getListOfStrings()

std::vector<std::string> res;

for (auto & elem : v.listItems())
for (auto elem : v.listView())
res.push_back(std::string(root->state.forceStringNoCtx(*elem, noPos, "while evaluating an attribute for caching")));

if (root->db)
Expand Down
8 changes: 8 additions & 0 deletions src/libexpr/eval-gc.cc
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Expand Up @@ -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"

Expand Down Expand Up @@ -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)
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I wouldn't introduce std::uintptr_t here because sizeof(std::uintptr_t) == sizeof(void *) isn't necessarily true.

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here because sizeof(std::uintptr_t) == sizeof(void *) isn't necessarily true.

Yeah, that's true. Though I can't think of a platform where that is actually the case.

GC_register_displacement(i);

GC_set_oom_fn(oomHandler);

/* Set the initial heap size to something fairly big (25% of
Expand Down
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