Optimize KV display performance by reducing branch prediction misses

spdlog/src/formatter/full_formatter.rs

@@ -98,21 +98,7 @@ impl FullFormatter {
         dest.write_str("] ")?;
         dest.write_str(record.payload())?;
 
-        let kvs = record.key_values();
-        if !kvs.is_empty() {
-            dest.write_str(" { ")?;
-
-            let mut iter = kvs.peekable();
-            while let Some((key, value)) = iter.next() {
-                dest.write_str(key.as_str())?;
-                dest.write_str("=")?;
-                write!(dest, "{}", value)?;
-                if iter.peek().is_some() {
-                    dest.write_str(", ")?;
-                }
-            }
-            dest.write_str(" }")?;
-        }
+        record.key_values().write_to(dest, true)?;
 
         if self.with_eol {
             dest.write_str(__EOL)?;

spdlog/src/formatter/journald_formatter.rs

@@ -49,21 +49,7 @@ impl JournaldFormatter {
         dest.write_str("] ")?;
         dest.write_str(record.payload())?;
 
-        let kvs = record.key_values();
-        if !kvs.is_empty() {
-            dest.write_str(" { ")?;
-
-            let mut iter = kvs.peekable();
-            while let Some((key, value)) = iter.next() {
-                dest.write_str(key.as_str())?;
-                dest.write_str("=")?;
-                write!(dest, "{}", value)?;
-                if iter.peek().is_some() {
-                    dest.write_str(", ")?;
-                }
-            }
-            dest.write_str(" }")?;
-        }
+        record.key_values().write_to(dest, true)?;
 
         dest.write_str(__EOL)?;
 

spdlog/src/formatter/pattern_formatter/pattern/kv.rs

@@ -1,5 +1,3 @@
-use std::fmt::{self, Write};
-
 use crate::{
     formatter::pattern_formatter::{Pattern, PatternContext},
     Error, Record, StringBuf,
@@ -15,23 +13,9 @@ impl Pattern for KV {
         dest: &mut StringBuf,
         _ctx: &mut PatternContext,
     ) -> crate::Result<()> {
-        (|| -> Result<(), fmt::Error> {
-            let kvs = record.key_values();
-            if !kvs.is_empty() {
-                dest.write_str("{ ")?;
-                let mut iter = kvs.peekable();
-                while let Some((key, value)) = iter.next() {
-                    dest.write_str(key.as_str())?;
-                    dest.write_str("=")?;
-                    write!(dest, "{}", value)?;
-                    if iter.peek().is_some() {
-                        dest.write_str(", ")?;
-                    }
-                }
-                dest.write_str(" }")?;
-            }
-            Ok(())
-        })()
-        .map_err(Error::FormatRecord)
+        record
+            .key_values()
+            .write_to(dest, false)
+            .map_err(Error::FormatRecord)
     }
 }

spdlog/src/kv.rs

@@ -1,4 +1,4 @@
-use std::{borrow::Cow, marker::PhantomData};
+use std::{borrow::Cow, fmt, marker::PhantomData};
 
 use value_bag::{OwnedValueBag, ValueBag};
 
@@ -95,6 +95,37 @@ where
             phantom: PhantomData,
         }
     }
+
+    pub(crate) fn write_to(
+        mut self,
+        dest: &mut impl fmt::Write,
+        leading_space: bool,
+    ) -> fmt::Result {
+        let first = self.next();
+        if let Some((key, value)) = first {
+            if leading_space {
+                dest.write_str(" { ")?;
+            } else {
+                dest.write_str("{ ")?;
+            }
+
+            // Reduce branch prediction misses for performance
+            // So we manually process the first KV pair
+            dest.write_str(key.as_str())?;
+            dest.write_str("=")?;
+            write!(dest, "{}", value)?;
+
+            for (key, value) in self {
+                dest.write_str(", ")?;
+                dest.write_str(key.as_str())?;
+                dest.write_str("=")?;
+                write!(dest, "{}", value)?;
+            }
+
+            dest.write_str(" }")?;
+        }
+        Ok(())
+    }
 }
 
 impl<'a, I> Iterator for KeyValuesIter<'a, I>