RUE-1466: omit redundant live-out range scan

crates/rue-codegen/src/liveness.rs

@@ -457,7 +457,6 @@ where
         &inst_uses,
         &inst_defs,
         live_in,
-        live_out,
         has_back_edge,
     );
 
@@ -702,8 +701,10 @@ fn materialize_live_out(
 ///   element values before storing them); the bitset scan is then O(N²) in the
 ///   number of live bits, whereas this path stays linear (RUE-302).
 /// * **Has back-edges (loops):** loop-carried values are live past their textual
-///   last use, so we extend the same dense range table from the exact
-///   `live_in`/`live_out` scan as well as definitions and uses.
+///   last use, so we extend the same dense range table from the exact `live_in`
+///   scan as well as definitions and uses. Scanning `live_out` too would be
+///   redundant: `live_in = uses ∪ (live_out - defs)`, so every live-out value
+///   is already either live-in or defined at that instruction.
 ///
 /// # A range is a textual interval, not liveness
 ///
@@ -736,7 +737,6 @@ fn build_live_ranges(
     inst_uses: &[VRegList],
     inst_defs: &[VRegList],
     live_in: &[FixedBitSet],
-    live_out: &[FixedBitSet],
     has_back_edge: bool,
 ) -> IndexMap<VReg, Option<LiveRange>> {
     let mut ranges: IndexMap<VReg, Option<LiveRange>> =
@@ -778,9 +778,6 @@ fn build_live_ranges(
         for vreg_idx in live_in[idx].ones() {
             extend(VReg::new(vreg_idx as u32), idx);
         }
-        for vreg_idx in live_out[idx].ones() {
-            extend(VReg::new(vreg_idx as u32), idx);
-        }
     }
 
     ranges
@@ -1115,7 +1112,9 @@ mod tests {
         );
 
         // The back-edge keeps v0 live through instruction 3 even though its
-        // last textual use is instruction 2.
+        // last textual use is instruction 2. This is the cyclic extent that
+        // must survive when range construction omits the redundant live-out
+        // scan.
         assert_eq!(info.range(VReg::new(0)), Some(&LiveRange::new(0, 3)));
     }
 

docs/notes/post-adr-0063-cold-compiler-architecture-audit.md

@@ -1802,6 +1802,21 @@ allocation-accounted pairs are neutral at +101--320 calls (under 0.003%) and
 -3.3--118.6 KB requested bytes. Every compiler-work counter, source/output
 metric, and executable byte remains identical.
 
+RUE-1466 removes the redundant `live_out` set-bit walk from cyclic MIR
+live-range construction shared by both backends. The transfer equation
+`live_in = uses ∪ (live_out - defs)` proves that every live-out register is
+already either live-in or defined at the same instruction, and those two inputs
+were already extending the range. Retained per-instruction liveness and debug
+views keep their live-out table; only the duplicate range pass disappears.
+
+Across 16 balanced fixed one-worker cold Lattice pairs, retired instructions
+improve by 0.1051% (0.0868% MAD). End-to-end clock is neutral at +0.7974%
+(2.6807% MAD), cycles at +0.4182% (1.5224% MAD), peak RSS at -0.0454%
+(0.3189% MAD), and peak footprint at -0.0619% (0.5094% MAD). Four
+allocation-accounted pairs are neutral, as expected for a set-bit iteration
+reduction. Every compiler-work counter, source/output metric, and executable
+byte remains identical.
+
 ## Next actions and decision boundary
 
 Authorized low-risk work: