From 8701c671c8e6d6eedc01436ea38bde01fba634f0 Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Sun, 30 Mar 2025 10:54:39 -0700
Subject: Fix BB layout bug

Fix split of popular value bug.
---
 .../java/com/seaofnodes/simple/codegen/Encoding.java | 20 +++++++++++++++-----
 .../java/com/seaofnodes/simple/codegen/RegAlloc.java |  1 +
 .../java/com/seaofnodes/simple/progs/HellowWorld.smp | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+), 5 deletions(-)
 create mode 100644 chapter21/src/test/java/com/seaofnodes/simple/progs/HellowWorld.smp

(limited to 'chapter21/src')

diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/Encoding.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/Encoding.java
index f2a2449e..fa703edc 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/Encoding.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/Encoding.java
@@ -186,9 +186,14 @@ public class Encoding {
             // If the *next* BB has already been visited, we may need an
             // unconditional jump here
             if( visit.get(next._nid) && !(next instanceof StopNode) ) {
-                // If all the blocks, in RPO order, to our target
-                // are empty, we will fall in and not need a jump.
-                if( next instanceof LoopNode || !isEmptyBackwardsScan(rpo,next) ) {
+                boolean needJump = next instanceof LoopNode
+                    // If backwards to a loop, and the block has statements,
+                    // will need a jump.  Empty blocks can just backwards branch.
+                    ? (bb.nOuts()>1)
+                    // Forwards jump.  If all the blocks, in RPO order, to our
+                    // target are empty, we will fall in and not need a jump.
+                    : !isEmptyBackwardsScan(rpo,next);
+                if( needJump ) {
                     CFGNode jmp = _code._mach.jump();
                     jmp.setDefX(0,bb);
                     next.setDef(next._inputs.find(bb),jmp);
@@ -230,8 +235,13 @@ public class Encoding {
             // when the False RPO visit returns, the IF is immediately next.
             // When the RPO is reversed, the fall-through path will always be
             // following the IF.
-            _rpo_cfg(t,visit,rpo);
-            _rpo_cfg(f,visit,rpo);
+            if( t.nOuts()==1 ) {
+                _rpo_cfg(f,visit,rpo);
+                _rpo_cfg(t,visit,rpo);
+            } else {
+                _rpo_cfg(t,visit,rpo);
+                _rpo_cfg(f,visit,rpo);
+            }
         }
         rpo.add(bb);
     }
diff --git a/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java b/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
index 36de2cdb..9c0f5f51 100644
--- a/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
+++ b/chapter21/src/main/java/com/seaofnodes/simple/codegen/RegAlloc.java
@@ -284,6 +284,7 @@ public class RegAlloc {
         for( RegMask rmask : rclass ) {
             Node split = makeSplit(def,"popular",round,lrg);
             split.insertAfter( def );
+            if( split.nIns()>1 ) split.setDef(1,def);
             // all uses by class to split
             for( int j=0; j < def._outputs._len; j++ ) {
                 Node use = def._outputs.at(j);
diff --git a/chapter21/src/test/java/com/seaofnodes/simple/progs/HellowWorld.smp b/chapter21/src/test/java/com/seaofnodes/simple/progs/HellowWorld.smp
new file mode 100644
index 00000000..172dffa0
--- /dev/null
+++ b/chapter21/src/test/java/com/seaofnodes/simple/progs/HellowWorld.smp
@@ -0,0 +1,18 @@
+
+{ i32 byte[] i64 -> i64 } write = extern;
+byte[] buf = new byte[100];
+buf[ 0] = 'H';
+buf[ 1] = 'e';
+buf[ 2] = 'l';
+buf[ 3] = 'l';
+buf[ 4] = 'o';
+buf[ 5] = ',';
+buf[ 6] = 'W';
+buf[ 7] = 'o';
+buf[ 8] = 'r';
+buf[ 9] = 'l';
+buf[10] = 'd';
+buf[11] = '!';
+buf[12] = '\n';
+
+return write(0,buf,12);
-- 
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