From df5efaa0d05b3583eb6ca3b5f67df83827889e42 Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Wed, 7 Oct 2026 11:18:47 -0700
Subject: Make RA deterministic

---
 .../java/com/seaofnodes/simple/codegen/RegAlloc.java     | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

(limited to 'chapter20/src/main')

diff --git a/chapter20/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java b/chapter20/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
index 420f1007..af28cbb3 100644
--- a/chapter20/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
+++ b/chapter20/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
@@ -290,15 +290,17 @@ public class RegAlloc {
                         (use instanceof MachNode mach && mach.twoAddress()!=0 && use.in(mach.twoAddress())==def) )
                     insertBefore( use, use._inputs.find(def), "use/self/use",round,lrg );
             }
-            // Split after the Phi which extends the LRG.  Split also before
-            // Phi slot 1 (and not all inputs), because Phis extend the live range.
-            // TODO: split before all inputs (except the last; at least 1 split here must be extra)
+            // Split after the Phi and before its entry.  Also isolate inputs
+            // shared with another Phi: otherwise the shared input keeps the
+            // conflicting live ranges joined, even after splitting the entry.
             if( def instanceof PhiNode phi && !(def instanceof ParmNode) ) {
                 SplitNode split = makeSplit("def/self",round,lrg);
                 insertAfterAndReplace(split,def,false);
                 if( split.nOuts()==0 )
                     split.killOrdered();
-                insertBefore(phi,1,"use/self/phi",round,lrg);
+                for( int i=1; i<phi.nIns(); i++ )
+                    if( i==1 || sharedPhiInput(phi,i) )
+                        insertBefore(phi,i,"use/self/phi",round,lrg);
             }
             // Split before two-address ops which extend the live range
             if( def instanceof MachNode mach && mach.twoAddress()!= 0 )
@@ -308,6 +310,12 @@ public class RegAlloc {
     }
 
 
+    private static boolean sharedPhiInput(PhiNode phi, int i) {
+        for( Node use : phi.in(i)._outputs )
+            if( use instanceof PhiNode && use!=phi ) return true;
+        return false;
+    }
+
     // Generic: split around the outermost loop with non-split def/uses.  This
     // covers both self-conflicts (once we split deep enough) and register
     // pressure issues.
-- 
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