+#!+compare-and-swap-vops
+(define-vop (word-index-cas)
+ (:args (object :scs (descriptor-reg))
+ (index :scs (any-reg zero immediate))
+ (old-value :scs (any-reg descriptor-reg))
+ (new-value :scs (any-reg descriptor-reg)))
+ (:arg-types * tagged-num * *)
+ (:temporary (:sc non-descriptor-reg) temp)
+ (:results (result :scs (any-reg descriptor-reg) :from :load))
+ (:result-types *)
+ (:variant-vars offset lowtag)
+ (:policy :fast-safe)
+ (:generator 5
+ (sc-case index
+ ((immediate zero)
+ (let ((offset (- (+ (if (sc-is index zero)
+ 0
+ (ash (tn-value index) word-shift))
+ (ash offset word-shift))
+ lowtag)))
+ (inst lr temp offset)))
+ (t
+ ;; KLUDGE: This relies on N-FIXNUM-TAG-BITS being the same as
+ ;; WORD-SHIFT. I know better than to do this. --AB, 2010-Jun-16
+ (inst addi temp index
+ (- (ash offset word-shift) lowtag))))
+
+ (inst sync)
+ LOOP
+ (inst lwarx result temp object)
+ (inst cmpw result old-value)
+ (inst bne EXIT)
+ (inst stwcx. new-value temp object)
+ (inst bne LOOP)
+ EXIT
+ (inst isync)))