[api] Pass pos by value through encode_varint_raw_loop

esphome/components/api/proto.h

@@ -298,15 +298,33 @@ constexpr uint32_t VARINT_MAX_2_BYTE = 1 << 14;  // 16384
 class ProtoEncode {
  public:
   /// Write a multi-byte varint directly through a pos pointer.
+  /// Unrolled based on the compile-time max varint length for T
+  /// (5 bytes for uint32_t, 10 bytes for uint64_t). The explicit unroll gives
+  /// the compiler straight-line code with early exits instead of a
+  /// data-dependent back-edge branch, which measurably speeds up BLE raw
+  /// advertisement MAC encoding (always 7-byte varints) among other hot paths.
+  ///
+  /// Takes/returns pos by value so this function can remain out-of-line without
+  /// forcing the caller to spill pos to memory on every byte write. Callers
+  /// (which are ALWAYS_INLINE) should assign the returned pos back to their
+  /// local. Code size win vs. inlining the whole unroll at every call site.
   template<typename T>
-  static inline void encode_varint_raw_loop(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM, T value) {
-    do {
+  static uint8_t *encode_varint_raw_loop(uint8_t *__restrict__ pos PROTO_ENCODE_DEBUG_PARAM, T value) {
+    constexpr int MAX_VARINT_BYTES = (sizeof(T) * 8 + 6) / 7;  // 5 for u32, 10 for u64
+#pragma GCC unroll 10
+    for (int i = 0; i < MAX_VARINT_BYTES - 1; i++) {
       PROTO_ENCODE_CHECK_BOUNDS(pos, 1);
-      *pos++ = static_cast<uint8_t>(value | 0x80);
+      *pos++ = static_cast<uint8_t>(value) | 0x80;
       value >>= 7;
-    } while (value > 0x7F);
+      if (value <= 0x7F) {
+        PROTO_ENCODE_CHECK_BOUNDS(pos, 1);
+        *pos++ = static_cast<uint8_t>(value);
+        return pos;
+      }
+    }
     PROTO_ENCODE_CHECK_BOUNDS(pos, 1);
     *pos++ = static_cast<uint8_t>(value);
+    return pos;
   }
   static inline void ESPHOME_ALWAYS_INLINE encode_varint_raw(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM,
                                                              uint32_t value) {
@@ -315,7 +333,7 @@ class ProtoEncode {
       *pos++ = static_cast<uint8_t>(value);
       return;
     }
-    encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
+    pos = encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
   }
   /// Encode a varint that is expected to be 1-2 bytes (e.g. zigzag RSSI, small lengths).
   static inline void ESPHOME_ALWAYS_INLINE encode_varint_raw_short(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM,
@@ -331,7 +349,7 @@ class ProtoEncode {
       *pos++ = static_cast<uint8_t>(value >> 7);
       return;
     }
-    encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
+    pos = encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
   }
   static inline void ESPHOME_ALWAYS_INLINE encode_varint_raw_64(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM,
                                                                 uint64_t value) {
@@ -340,7 +358,7 @@ class ProtoEncode {
       *pos++ = static_cast<uint8_t>(value);
       return;
     }
-    encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
+    pos = encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, value);
   }
   static inline void ESPHOME_ALWAYS_INLINE encode_field_raw(uint8_t *__restrict__ &pos PROTO_ENCODE_DEBUG_PARAM,
                                                             uint32_t field_id, uint32_t type) {
@@ -402,7 +420,7 @@ class ProtoEncode {
       PROTO_ENCODE_CHECK_BOUNDS(pos, 1 + len);
       *pos++ = static_cast<uint8_t>(len);
     } else {
-      encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, len);
+      pos = encode_varint_raw_loop(pos PROTO_ENCODE_DEBUG_ARG, len);
       PROTO_ENCODE_CHECK_BOUNDS(pos, len);
     }
     std::memcpy(pos, string, len);