From 7cf5893dcc755a1bc706536088db3d34cfc8c46b Mon Sep 17 00:00:00 2001 From: Joshua Haberman Date: Mon, 25 Apr 2011 22:24:34 -0700 Subject: Revise/clarify comment about clear() implementation. --- src/upb_msg.h | 24 ++++++++++-------------- 1 file changed, 10 insertions(+), 14 deletions(-) (limited to 'src') diff --git a/src/upb_msg.h b/src/upb_msg.h index b172134..180f918 100644 --- a/src/upb_msg.h +++ b/src/upb_msg.h @@ -237,24 +237,20 @@ INLINE bool upb_msg_has(upb_msg *msg, upb_fielddef *f) { // We lazily clear objects if/when we reuse them. // 3. inside upb_msg_clear(), overwrite all values to be their default, // and recurse into submessages to set all their values to defaults also. -// 4. as a hybrid of (1) and (3), make each "set bit" tri-state, where it -// can have a value of "unset, but cached sub-message needs to be cleared." -// Like (2) we can cache sub-messages and lazily clear, but primitive values -// can always be returned straight from the message. +// 4. as a hybrid of (1) and (3), clear all set bits in upb_msg_clear() +// but also overwrite all primitive values to be their defaults. Only +// accessors for non-primitive values (submessage, strings, and arrays) +// need to check the has-bits in their accessors -- primitive values can +// always be returned straight from the msg. // // (1) is undesirable, because it prevents us from caching sub-objects. // (2) makes clear() cheaper, but makes get() branchier. -// (3) makes get() less branchy, but makes clear() have worse cache behavior. -// (4) makes get() differently branchy (only returns default from msgdef if -// NON-primitive value is unset), but uses more set bits. It's questionable -// whether it would be a performance improvement. +// (3) makes get() less branchy, but makes clear() traverse the message graph. +// (4) is probably the best bang for the buck. // -// For the moment we go with (2). Google's protobuf does (3), which is likely -// part of the reason we beat it in some benchmarks. -// -// If the branchiness of (2) is too great, this could be mitigated with cmov -// (both values and the conditional are cheap to calculate, much cheaper than -// the cost of a misprediction). +// For the moment upb does (2), but we should implement (4). Google's protobuf +// does (3), which is likely part of the reason that even our table-based +// decoder beats it in some benchmarks. // For submessages and strings, the returned value is not owned. upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f); -- cgit v1.2.3