From 7d8f420f9d25c48306e8f337e44fc60eec7f46d5 Mon Sep 17 00:00:00 2001
From: Ignacio Van Droogenbroeck <ignacio@vandroogenbroeck.net>
Date: Mon, 2 Mar 2026 19:40:31 -0600
Subject: [PATCH] perf(decode): inline hasNilCode for byte-slice reader path

When decoding from a byte slice (Unmarshal path), peek directly at
the underlying data instead of going through two interface method
calls (ReadByte + UnreadByte). This saves ~3-7 ns per struct pointer
field decode.
---
 decode.go | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/decode.go b/decode.go
index 8040370..13ee2e5 100644
--- a/decode.go
+++ b/decode.go
@@ -624,6 +624,11 @@ func (d *Decoder) ReadFull(buf []byte) error {
 }
 
 func (d *Decoder) hasNilCode() bool {
+	// Fast path: when decoding from a byte slice, peek directly
+	// to avoid two interface method calls (ReadByte + UnreadByte).
+	if d.s == &d.bsr {
+		return d.bsr.pos < len(d.bsr.data) && d.bsr.data[d.bsr.pos] == msgpcode.Nil
+	}
 	code, err := d.PeekCode()
 	return err == nil && code == msgpcode.Nil
 }