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Add single-byte fast path to ProtoVarInt::parse

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Single-byte varints (0-127) are the most common case in protobuf
messages (booleans, small enums, field tags). Skip the loop entirely
for these values by checking the first byte before entering the
multi-byte parsing loop.
This commit is contained in:
J. Nick Koston
2026-02-18 08:44:13 -06:00
parent bc2dbd3cf5
commit 99a77e9549
1 changed files with 15 additions and 7 deletions
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22
esphome/components/api/proto.h
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@@ -108,18 +108,26 @@ class ProtoVarInt {
#ifdef ESPHOME_DEBUG_API
assert(consumed != nullptr);
#endif
// 32-bit phase: shifts 0, 7, 14, 21 are native on 32-bit platforms.
// Without USE_API_VARINT64: also cover byte 4 (shift 28) — the uint32_t
// shift truncates upper bits but those are always zero for valid uint32 values.
// With USE_API_VARINT64: stop at byte 3 so parse_wide handles byte 4+
// with full 64-bit arithmetic (avoids truncating values > UINT32_MAX).
uint32_t result32 = 0;
if (len == 0)
return {};
// Fast path: single-byte varints (0-127) are the most common case
// (booleans, small enums, field tags). Avoid loop overhead entirely.
if ((buffer[0] & 0x80) == 0) {
*consumed = 1;
return ProtoVarInt(buffer[0]);
}
// 32-bit phase: process remaining bytes with native 32-bit shifts.
// Without USE_API_VARINT64: cover bytes 1-4 (shifts 7, 14, 21, 28) — the uint32_t
// shift at byte 4 truncates upper bits but those are always zero for valid uint32 values.
// With USE_API_VARINT64: cover bytes 1-3 (shifts 7, 14, 21) so parse_wide handles
// byte 4+ with full 64-bit arithmetic (avoids truncating values > UINT32_MAX).
uint32_t result32 = buffer[0] & 0x7F;
#ifdef USE_API_VARINT64
uint32_t limit = std::min(len, uint32_t(4));
#else
uint32_t limit = std::min(len, uint32_t(5));
#endif
for (uint32_t i = 0; i < limit; i++) {
for (uint32_t i = 1; i < limit; i++) {
uint8_t val = buffer[i];
result32 |= uint32_t(val & 0x7F) << (i * 7);
if ((val & 0x80) == 0) {