From 5c45f07f1bb6a46611938f12d411f7017b64c7ed Mon Sep 17 00:00:00 2001
From: Christopher Haster <geky@geky.net>
Date: Thu, 7 Mar 2024 00:52:07 -0600
Subject: [PATCH] Added LFSR_TAG_DIVERGEDDONE instead of reusing LFSR_TAG_RM in
 appendattr

I think this is a bit more readable.

Curiously, the bit flip and bit change resulted in a surprising code
cost, even though it removes a couple statements. I guess because the
sign bit is that much cheaper to predicate on?

           code          stack
  before: 33980           2880
  after:  34024 (+0.1%)   2880 (+0.0%)
---
 lfs.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/lfs.c b/lfs.c
index 121fad87..809792fe 100644
--- a/lfs.c
+++ b/lfs.c
@@ -786,6 +786,7 @@ enum lfsr_tag {
     LFSR_TAG_DIVERGED       = 0x4000,
     LFSR_TAG_DIVERGEDUPPER  = 0x2000,
     LFSR_TAG_DIVERGEDLOWER  = 0x0000,
+    LFSR_TAG_DIVERGEDDONE   = 0x1000,
 };
 
 // some other tag encodings with their own subfields
@@ -875,6 +876,10 @@ static inline bool lfsr_tag_isdivergedlower(lfsr_tag_t tag) {
     return !(tag & LFSR_TAG_DIVERGEDUPPER);
 }
 
+static inline bool lfsr_tag_isdivergeddone(lfsr_tag_t tag) {
+    return tag & LFSR_TAG_DIVERGEDDONE;
+}
+
 // alt operations
 static inline bool lfsr_tag_isblack(lfsr_tag_t tag) {
     return !(tag & LFSR_TAG_R);
@@ -2732,9 +2737,6 @@ static int lfsr_rbyd_appendattr(lfs_t *lfs, lfsr_rbyd_t *rbyd,
             other_tag_ = tag_;
         }
     }
-    // mark as rmed until found
-    tag_ |= LFSR_TAG_RM;
-    other_tag_ |= LFSR_TAG_RM;
 
     // keep track of bounds as we descend down the tree
     //
@@ -3021,15 +3023,15 @@ static int lfsr_rbyd_appendattr(lfs_t *lfs, lfsr_rbyd_t *rbyd,
 
         // found end of tree?
         } else {
-            // update the found tag/rid
-            //
-            // note we:
-            // - clear valid bit, marking the tag as found
-            // - preserve diverged state
-            tag_ = lfsr_tag_mode(tag_ & ~LFSR_TAG_RM) | alt;
+            // update the found tag/rid, marking as done while preserving
+            // any diverged state
+            tag_ = LFSR_TAG_DIVERGEDDONE
+                    | lfsr_tag_mode(tag_)
+                    | alt;
 
             // done?
-            if (!lfsr_tag_hasdiverged(tag_) || !lfsr_tag_isrm(other_tag_)) {
+            if (!lfsr_tag_hasdiverged(tag_)
+                    || lfsr_tag_isdivergeddone(other_tag_)) {
                 break;
             }
         }
@@ -3051,8 +3053,9 @@ static int lfsr_rbyd_appendattr(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     LFS_ASSERT(lfsr_tag_isblack(p_alts[0]));
 
     // if we diverged, merge the bounds
-    LFS_ASSERT(!lfsr_tag_isrm(tag_));
-    LFS_ASSERT(!lfsr_tag_hasdiverged(tag_) || !lfsr_tag_isrm(other_tag_));
+    LFS_ASSERT(lfsr_tag_isdivergeddone(tag_));
+    LFS_ASSERT(!lfsr_tag_hasdiverged(tag_)
+            || lfsr_tag_isdivergeddone(other_tag_));
     if (lfsr_tag_hasdiverged(tag_)) {
         if (lfsr_tag_isdivergedlower(tag_)) {
             // finished on lower path