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[api] Outline the fixed32 writers on ARM cores without unaligned access too

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Cortex-M0+ and ARM9 turn the four byte unaligned store into a memcpy call with a stack
temporary at every fixed32 field, and the outlined helper itself became a memcpy call
there, so the helper now spells out the byte stores. Xtensa and host objects are byte for
byte unchanged; on the RP2040 bench config the api object loses 28 bytes and the fixed32
memcpy calls.
This commit is contained in:
J. Nick Koston
2026-09-07 15:24:30 +02:00
parent ea71a24a9b
commit 7c774699d7
1 changed files with 18 additions and 6 deletions
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esphome/components/api/proto.h
+18 -6
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@@ -287,13 +287,16 @@ class ProtoWriteBuffer {
uint8_t *pos_;
};
// ESP32 builds pass -fno-builtin-memcpy, so the inline write was already a memcpy call there and
// one shared copy is both smaller and faster. Elsewhere the write inlines to a few stores, which is
// still cheaper than a call (measured on ESP8266), so keep it inline.
#ifdef USE_ESP32
// A four byte unaligned store is a memcpy call on ESP-IDF (-fno-builtin-memcpy) and on ARM cores without
// unaligned access (Cortex-M0+, ARM9), so those targets share one outlined byte store helper per fixed32
// field. Elsewhere the write inlines to a single store, or on ESP8266 to a few stores that measured
// faster than a call, so it stays inline.
#if defined(USE_ESP32) || (defined(__arm__) && !defined(__ARM_FEATURE_UNALIGNED))
#define PROTO_OUTLINE_FOR_SIZE __attribute__((noinline))
#define PROTO_FIXED32_BYTE_STORES true
#else
#define PROTO_OUTLINE_FOR_SIZE inline
#define PROTO_FIXED32_BYTE_STORES false
#endif
// Varint encoding thresholds — used by both proto_encode_* free functions and ProtoSize.
@@ -417,8 +420,16 @@ class ProtoEncode {
}
/// Unaligned little-endian store; __builtin_memcpy stays inline even under -fno-builtin-memcpy.
static inline void ESPHOME_ALWAYS_INLINE write_fixed32_le(uint8_t *__restrict__ pos, uint32_t value) {
const uint32_t le = convert_little_endian(value);
__builtin_memcpy(pos, &le, 4);
if constexpr (PROTO_FIXED32_BYTE_STORES) {
// Spelled out so the outlined helper does not itself become a memcpy call
pos[0] = static_cast<uint8_t>(value);
pos[1] = static_cast<uint8_t>(value >> 8);
pos[2] = static_cast<uint8_t>(value >> 16);
pos[3] = static_cast<uint8_t>(value >> 24);
} else {
const uint32_t le = convert_little_endian(value);
__builtin_memcpy(pos, &le, 4);
}
}
/// Write a precomputed tag byte + 32-bit value. Outlined on embedded: one copy beats inline stores per field.
[[nodiscard]] static PROTO_OUTLINE_FOR_SIZE uint8_t *write_tag_and_fixed32(
@@ -585,6 +596,7 @@ class ProtoEncode {
}
};
#undef PROTO_OUTLINE_FOR_SIZE
#undef PROTO_FIXED32_BYTE_STORES
#ifdef HAS_PROTO_MESSAGE_DUMP
/**