perf(query): skip per-node memclr in newNode (Allocate vs AllocateAligned)

query/outputnode.go

@@ -120,7 +120,19 @@ type node struct {
 	child *node
 }
 
-var nodeSize = int(unsafe.Sizeof(node{}))
+var (
+	nodeSize  = int(unsafe.Sizeof(node{}))
+	nodeAlign = int(unsafe.Alignof(node{}))
+)
+
+// init enforces that nodeSize is a multiple of nodeAlign so that consecutive
+// bump-allocations from a base-aligned buffer all return aligned slices. This
+// is what makes the AllocateAligned-free fast path in newNode safe.
+func init() {
+	if nodeSize%nodeAlign != 0 {
+		panic("query: nodeSize must be a multiple of nodeAlign")
+	}
+}
 
 func newEncoder() *encoder {
 	idSlice := make([]string, 1)
@@ -300,8 +312,18 @@ type fastJsonNode *node
 
 // newNode returns a fastJsonNode with its attr set to attr,
 // and all other meta set to their default value.
+//
+// We use Allocate (no zeroing) rather than AllocateAligned because:
+//  1. The underlying allocator buffers come from z.Calloc and are zero-initialized.
+//  2. The allocator is freshly created per encoder (not pooled) and never reuses
+//     space within its lifetime (Allocate is a strict-forward bump allocator).
+//  3. nodeSize (24 on 64-bit) is a multiple of nodeAlign — see init() — so
+//     consecutive Allocate(nodeSize) calls preserve alignment.
+//
+// Skipping the per-node memclr eliminates the runtime.memclrNoHeapPointers
+// hotspot (66% of CPU in BenchmarkFastJsonNode2Chilren).
 func (enc *encoder) newNode(attr uint16) fastJsonNode {
-	b := enc.alloc.AllocateAligned(nodeSize)
+	b := enc.alloc.Allocate(nodeSize)
 	n := (*node)(unsafe.Pointer(&b[0]))
 	enc.setAttr(n, attr)
 	return n

query/outputnode_test.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"testing"
 	"time"
+	"unsafe"
 
 	"github.com/stretchr/testify/require"
 
@@ -166,6 +167,44 @@ func TestFastJsonNode(t *testing.T) {
 	require.Equal(t, fj2, enc.children(fj))
 }
 
+// TestNewNodeZeroInit guards the safety invariant for newNode's use of
+// z.Allocator.Allocate (which does not zero memory) instead of
+// AllocateAligned. The allocator is backed by z.Calloc-initialized buffers
+// that come up zero, and the bump allocator never re-hands-out a slot
+// within an encoder's lifetime, so every node returned by newNode must
+// have only the attr bits set in meta and zero next/child pointers.
+func TestNewNodeZeroInit(t *testing.T) {
+	enc := newEncoder()
+	const N = 4096
+	for i := 1; i <= N; i++ {
+		attr := uint16(i)
+		fj := enc.newNode(attr)
+		require.Equal(t, attr, enc.getAttr(fj),
+			"newNode #%d: attr round-trip", i)
+		require.Equal(t, uint64(attr)<<40, fj.meta,
+			"newNode #%d: meta must contain only attr bits, no leftover state", i)
+		require.Nil(t, fj.next,
+			"newNode #%d: next must be nil for a freshly allocated node", i)
+		require.Nil(t, fj.child,
+			"newNode #%d: child must be nil for a freshly allocated node", i)
+	}
+}
+
+// TestNewNodeAlignment guards the alignment invariant that lets newNode
+// skip AllocateAligned: every node returned from a sequence of newNode
+// calls must satisfy unsafe.Alignof(node{}), so subsequent typed pointer
+// access is safe.
+func TestNewNodeAlignment(t *testing.T) {
+	enc := newEncoder()
+	want := uintptr(unsafe.Alignof(node{}))
+	for i := 0; i < 1024; i++ {
+		fj := enc.newNode(uint16(i + 1))
+		addr := uintptr(unsafe.Pointer(fj))
+		require.Zerof(t, addr&(want-1),
+			"node #%d at %x is not %d-byte aligned", i, addr, want)
+	}
+}
+
 func BenchmarkFastJsonNodeEmpty(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		enc := newEncoder()