From c70c642ddb0cfa158d5f9148076dca109fec9165 Mon Sep 17 00:00:00 2001
From: Michael McLoughlin <mmcloughlin@gmail.com>
Date: Thu, 13 Dec 2018 00:34:44 -0800
Subject: [PATCH] attempt to make register allocation deterministic

---
 examples/sum/sum.s | 16 ++++++++--------
 pass/alloc.go      |  2 +-
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/examples/sum/sum.s b/examples/sum/sum.s
index 82945d1..fab47b1 100644
--- a/examples/sum/sum.s
+++ b/examples/sum/sum.s
@@ -3,16 +3,16 @@
 
 // func Sum(xs []uint64) uint64
 TEXT ·Sum(SB),0,$0-32
-	MOVQ	xs_base(FP), DX
-	MOVQ	xs_len+8(FP), AX
-	XORQ	CX, CX
+	MOVQ	xs_base(FP), AX
+	MOVQ	xs_len+8(FP), CX
+	XORQ	DX, DX
 loop:
-	CMPQ	AX, $0x0
+	CMPQ	CX, $0x0
 	JE	done
-	ADDQ	(DX), CX
-	ADDQ	$0x8, DX
-	DECQ	AX
+	ADDQ	(AX), DX
+	ADDQ	$0x8, AX
+	DECQ	CX
 	JMP	loop
 done:
-	MOVQ	CX, ret+24(FP)
+	MOVQ	DX, ret+24(FP)
 	RET	
diff --git a/pass/alloc.go b/pass/alloc.go
index 8bef077..ce07890 100644
--- a/pass/alloc.go
+++ b/pass/alloc.go
@@ -111,7 +111,7 @@ func (a *Allocator) mostrestricted() reg.Virtual {
 	n := int(math.MaxInt32)
 	var v reg.Virtual
 	for r, p := range a.possible {
-		if len(p) < n {
+		if len(p) < n || (len(p) == n && v != nil && r.VirtualID() < v.VirtualID()) {
 			n = len(p)
 			v = r
 		}