add exercise 5.37

Author: thinker1990

chapter05/5_37.scm

@@ -0,0 +1,17 @@
+;; remove the condition, when (preserving regs, seq1 seq2), always (save first-reg),  
+;; then (restore first-reg) 
+(define (preserving regs seq1 seq2) 
+  (if (null? regs) 
+    (append-instruction-sequences seq1 seq2) 
+    (let ((first-reg (car regs))) 
+      (preserving (cdr regs) 
+        (make-instruction-sequence 
+          (list-union (list first-reg) (registers-needed seq1)) 
+          (list-difference 
+            (registers-modified seq1) 
+            (list first-reg)) 
+          (append 
+            `((save ,first-reg)) 
+            (statements seq1) 
+            `((restore ,first-reg)))) 
+        seq2))))