;;; Tree-IL partial evaluator
-;; Copyright (C) 2011, 2012 Free Software Foundation, Inc.
+;; Copyright (C) 2011, 2012, 2013 Free Software Foundation, Inc.
;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
(define-module (language tree-il peval)
#:use-module (language tree-il)
#:use-module (language tree-il primitives)
+ #:use-module (language tree-il effects)
#:use-module (ice-9 vlist)
#:use-module (ice-9 match)
#:use-module (srfi srfi-1)
;; TODO: Record value size in operand structure?
;;
(define-record-type <operand>
- (%make-operand var sym visit source visit-count residualize?
- copyable? residual-value constant-value)
+ (%make-operand var sym visit source visit-count use-count
+ copyable? residual-value constant-value alias-value)
operand?
(var operand-var)
(sym operand-sym)
(visit %operand-visit)
(source operand-source)
(visit-count operand-visit-count set-operand-visit-count!)
- (residualize? operand-residualize? set-operand-residualize?!)
+ (use-count operand-use-count set-operand-use-count!)
(copyable? operand-copyable? set-operand-copyable?!)
(residual-value operand-residual-value %set-operand-residual-value!)
- (constant-value operand-constant-value set-operand-constant-value!))
-
-(define* (make-operand var sym #:optional source visit)
- ;; Bind SYM to VAR, with value SOURCE. Bound operands are considered
- ;; copyable until we prove otherwise. If we have a source expression,
- ;; truncate it to one value. Copy propagation does not work on
- ;; multiply-valued expressions.
+ (constant-value operand-constant-value set-operand-constant-value!)
+ (alias-value operand-alias-value set-operand-alias-value!))
+
+(define* (make-operand var sym #:optional source visit alias)
+ ;; Bind SYM to VAR, with value SOURCE. Unassigned bound operands are
+ ;; considered copyable until we prove otherwise. If we have a source
+ ;; expression, truncate it to one value. Copy propagation does not
+ ;; work on multiply-valued expressions.
(let ((source (and=> source truncate-values)))
- (%make-operand var sym visit source 0 #f (and source #t) #f #f)))
-
-(define (make-bound-operands vars syms sources visit)
- (map (lambda (x y z) (make-operand x y z visit)) vars syms sources))
+ (%make-operand var sym visit source 0 0
+ (and source (not (var-set? var))) #f #f
+ (and (not (var-set? var)) alias))))
+
+(define* (make-bound-operands vars syms sources visit #:optional aliases)
+ (if aliases
+ (map (lambda (name sym source alias)
+ (make-operand name sym source visit alias))
+ vars syms sources aliases)
+ (map (lambda (name sym source)
+ (make-operand name sym source visit #f))
+ vars syms sources)))
(define (make-unbound-operands vars syms)
(map make-operand vars syms))
(if (or counter (and (not effort-limit) (not size-limit)))
((%operand-visit op) (operand-source op) counter ctx)
(let/ec k
- (define (abort) (k #f))
+ (define (abort)
+ ;; If we abort when visiting the value in a
+ ;; fresh context, we won't succeed in any future
+ ;; attempt, so don't try to copy it again.
+ (set-operand-copyable?! op #f)
+ (k #f))
((%operand-visit op)
(operand-source op)
(make-top-counter effort-limit size-limit abort op)
(define (fresh-gensyms vars)
(map (lambda (var)
(let ((new (gensym (string-append (symbol->string (var-name var))
- "-"))))
+ " "))))
(set! store (vhash-consq new var store))
new))
vars))
+ (define (fresh-temporaries ls)
+ (map (lambda (elt)
+ (let ((new (gensym "tmp ")))
+ (record-new-temporary! 'tmp new 1)
+ new))
+ ls))
+
(define (assigned-lexical? sym)
(var-set? (lookup-var sym)))
(let ((x (vhash-assq new store)))
(if x (cdr x) new)))
+ (define (record-operand-use op)
+ (set-operand-use-count! op (1+ (operand-use-count op))))
+
+ (define (unrecord-operand-uses op n)
+ (let ((count (- (operand-use-count op) n)))
+ (when (zero? count)
+ (set-operand-residual-value! op #f))
+ (set-operand-use-count! op count)))
+
(define* (residualize-lexical op #:optional ctx val)
(log 'residualize op)
- (set-operand-residualize?! op #t)
- (if (eq? ctx 'value)
+ (record-operand-use op)
+ (if (memq ctx '(value values))
(set-operand-residual-value! op val))
(make-lexical-ref #f (var-name (operand-var op)) (operand-sym op)))
(else
(residualize-call))))
- (define (inline-values exp src names gensyms body)
+ (define (inline-values src exp nmin nmax consumer)
(let loop ((exp exp))
(match exp
;; Some expression types are always singly-valued.
($ <toplevel-set>) ; could return zero values in
($ <toplevel-define>) ; the future
($ <module-set>) ;
- ($ <dynset>)) ;
- (and (= (length names) 1)
- (make-let src names gensyms (list exp) body)))
- (($ <primcall> src (? singly-valued-primitive? name))
- (and (= (length names) 1)
- (make-let src names gensyms (list exp) body)))
+ ($ <dynset>) ;
+ ($ <primcall> src (? singly-valued-primitive?)))
+ (and (<= nmin 1) (or (not nmax) (>= nmax 1))
+ (make-call src (make-lambda #f '() consumer) (list exp))))
;; Statically-known number of values.
(($ <primcall> src 'values vals)
- (and (= (length names) (length vals))
- (make-let src names gensyms vals body)))
+ (and (<= nmin (length vals)) (or (not nmax) (>= nmax (length vals)))
+ (make-call src (make-lambda #f '() consumer) vals)))
;; Not going to copy code into both branches.
(($ <conditional>) #f)
(let ((tail (loop tail)))
(and tail (make-seq src head tail)))))))
+ (define compute-effects
+ (make-effects-analyzer assigned-lexical?))
+
(define (constant-expression? x)
;; Return true if X is constant, for the purposes of copying or
;; elision---i.e., if it is known to have no effects, does not
;; allocate storage for a mutable object, and does not access
;; mutable data (like `car' or toplevel references).
- (let loop ((x x))
- (match x
- (($ <void>) #t)
- (($ <const>) #t)
- (($ <lambda>) #t)
- (($ <lambda-case> _ req opt rest kw inits syms body alternate)
- (and (not (any assigned-lexical? syms))
- (every loop inits) (loop body)
- (or (not alternate) (loop alternate))))
- (($ <lexical-ref> _ _ gensym)
- (not (assigned-lexical? gensym)))
- (($ <primitive-ref>) #t)
- (($ <conditional> _ condition subsequent alternate)
- (and (loop condition) (loop subsequent) (loop alternate)))
- (($ <primcall> _ 'values exps)
- (and (not (null? exps))
- (every loop exps)))
- (($ <primcall> _ name args)
- (and (effect-free-primitive? name)
- (not (constructor-primitive? name))
- (types-check? name args)
- (if (accessor-primitive? name)
- (every const? args)
- (every loop args))))
- (($ <call> _ ($ <lambda> _ _ body) args)
- (and (loop body) (every loop args)))
- (($ <seq> _ head tail)
- (and (loop head) (loop tail)))
- (($ <let> _ _ syms vals body)
- (and (not (any assigned-lexical? syms))
- (every loop vals) (loop body)))
- (($ <letrec> _ _ _ syms vals body)
- (and (not (any assigned-lexical? syms))
- (every loop vals) (loop body)))
- (($ <fix> _ _ _ vals body)
- (and (every loop vals) (loop body)))
- (($ <let-values> _ exp body)
- (and (loop exp) (loop body)))
- (($ <prompt> _ tag body handler)
- (and (loop tag) (loop body) (loop handler)))
- (_ #f))))
+ (constant? (compute-effects x)))
(define (prune-bindings ops in-order? body counter ctx build-result)
;; This helper handles both `let' and `letrec'/`fix'. In the latter
;; marked as needing residualization. Here we hack around this
;; and treat all bindings as referenced if we are in operator
;; context.
- (or (eq? ctx 'operator) (operand-residualize? op)))
+ (or (eq? ctx 'operator)
+ (not (zero? (operand-use-count op)))))
;; values := (op ...)
;; effects := (op ...)
((vhash-assq var env) => cdr)
(else (error "unbound var" var))))
+ ;; Find a value referenced a specific number of times. This is a hack
+ ;; that's used for propagating fresh data structures like rest lists and
+ ;; prompt tags. Usually we wouldn't copy consed data, but we can do so in
+ ;; some special cases like `apply' or prompts if we can account
+ ;; for all of its uses.
+ ;;
+ ;; You don't want to use this in general because it introduces a slight
+ ;; nonlinearity by running peval again (though with a small effort and size
+ ;; counter).
+ ;;
+ (define (find-definition x n-aliases)
+ (cond
+ ((lexical-ref? x)
+ (cond
+ ((lookup (lexical-ref-gensym x))
+ => (lambda (op)
+ (let ((y (or (operand-residual-value op)
+ (visit-operand op counter 'value 10 10)
+ (operand-source op))))
+ (cond
+ ((and (lexical-ref? y)
+ (= (lexical-refcount (lexical-ref-gensym x)) 1))
+ ;; X is a simple alias for Y. Recurse, regardless of
+ ;; the number of aliases we were expecting.
+ (find-definition y n-aliases))
+ ((= (lexical-refcount (lexical-ref-gensym x)) n-aliases)
+ ;; We found a definition that is aliased the right
+ ;; number of times. We still recurse in case it is a
+ ;; lexical.
+ (values (find-definition y 1)
+ op))
+ (else
+ ;; We can't account for our aliases.
+ (values #f #f))))))
+ (else
+ ;; A formal parameter. Can't say anything about that.
+ (values #f #f))))
+ ((= n-aliases 1)
+ ;; Not a lexical: success, but only if we are looking for an
+ ;; unaliased value.
+ (values x #f))
+ (else (values #f #f))))
+
(define (visit exp ctx)
(loop exp env counter ctx))
((eq? ctx 'effect)
(log 'lexical-for-effect gensym)
(make-void #f))
+ ((operand-alias-value op)
+ ;; This is an unassigned operand that simply aliases some
+ ;; other operand. Recurse to avoid residualizing the leaf
+ ;; binding.
+ => for-tail)
((eq? ctx 'call)
;; Don't propagate copies if we are residualizing a call.
(log 'residualize-lexical-call gensym op)
exp
(make-seq src exp (make-void #f))))
(begin
- (set-operand-residualize?! op #t)
+ (record-operand-use op)
(make-lexical-set src name (operand-sym op) (for-value exp))))))
+ (($ <let> src
+ (names ... rest)
+ (gensyms ... rest-sym)
+ (vals ... ($ <primcall> _ 'list rest-args))
+ ($ <primcall> asrc (or 'apply '@apply)
+ (proc args ...
+ ($ <lexical-ref> _
+ (? (cut eq? <> rest))
+ (? (lambda (sym)
+ (and (eq? sym rest-sym)
+ (= (lexical-refcount sym) 1))))))))
+ (let* ((tmps (make-list (length rest-args) 'tmp))
+ (tmp-syms (fresh-temporaries tmps)))
+ (for-tail
+ (make-let src
+ (append names tmps)
+ (append gensyms tmp-syms)
+ (append vals rest-args)
+ (make-call
+ asrc
+ proc
+ (append args
+ (map (cut make-lexical-ref #f <> <>)
+ tmps tmp-syms)))))))
(($ <let> src names gensyms vals body)
+ (define (compute-alias exp)
+ ;; It's very common for macros to introduce something like:
+ ;;
+ ;; ((lambda (x y) ...) x-exp y-exp)
+ ;;
+ ;; In that case you might end up trying to inline something like:
+ ;;
+ ;; (let ((x x-exp) (y y-exp)) ...)
+ ;;
+ ;; But if x-exp is itself a lexical-ref that aliases some much
+ ;; larger expression, perhaps it will fail to inline due to
+ ;; size. However we don't want to introduce a useless alias
+ ;; (in this case, x). So if the RHS of a let expression is a
+ ;; lexical-ref, we record that expression. If we end up having
+ ;; to residualize X, then instead we residualize X-EXP, as long
+ ;; as it isn't assigned.
+ ;;
+ (match exp
+ (($ <lexical-ref> _ _ sym)
+ (let ((op (lookup sym)))
+ (and (not (var-set? (operand-var op)))
+ (or (operand-alias-value op)
+ exp))))
+ (_ #f)))
+
(let* ((vars (map lookup-var gensyms))
(new (fresh-gensyms vars))
(ops (make-bound-operands vars new vals
(lambda (exp counter ctx)
- (loop exp env counter ctx))))
+ (loop exp env counter ctx))
+ (map compute-alias vals)))
(env (fold extend-env env gensyms ops))
(body (loop body env counter ctx)))
(cond
(($ <letrec> src in-order? names gensyms vals body)
;; Note the difference from the `let' case: here we use letrec*
;; so that the `visit' procedure for the new operands closes over
- ;; an environment that includes the operands.
+ ;; an environment that includes the operands. Also we don't try
+ ;; to elide aliases, because we can't sensibly reduce something
+ ;; like (letrec ((a b) (b a)) a).
(letrec* ((visit (lambda (exp counter ctx)
(loop exp env* counter ctx)))
(vars (map lookup-var gensyms))
;; reconstruct the let-values, pevaling the consumer.
(let ((producer (for-values producer)))
(or (match consumer
- (($ <lambda-case> src req #f #f #f () gensyms body #f)
- (cond
- ((inline-values producer src req gensyms body)
- => for-tail)
- (else #f)))
+ (($ <lambda-case> src req opt rest #f inits gensyms body #f)
+ (let* ((nmin (length req))
+ (nmax (and (not rest) (+ nmin (if opt (length opt) 0)))))
+ (cond
+ ((inline-values lv-src producer nmin nmax consumer)
+ => for-tail)
+ (else #f))))
(_ #f))
(make-let-values lv-src producer (for-tail consumer)))))
(($ <dynwind> src winder pre body post unwinder)
((test) (make-const #f #t))
(else exp)))
(($ <conditional> src condition subsequent alternate)
- (let ((condition (for-test condition)))
- (if (const? condition)
- (if (const-exp condition)
- (for-tail subsequent)
- (for-tail alternate))
- (make-conditional src condition
- (for-tail subsequent)
- (for-tail alternate)))))
+ (define (call-with-failure-thunk exp proc)
+ (match exp
+ (($ <call> _ _ ()) (proc exp))
+ (($ <primcall> _ _ ()) (proc exp))
+ (($ <const>) (proc exp))
+ (($ <void>) (proc exp))
+ (($ <lexical-ref>) (proc exp))
+ (_
+ (let ((t (gensym "failure-")))
+ (record-new-temporary! 'failure t 2)
+ (make-let
+ src (list 'failure) (list t)
+ (list
+ (make-lambda
+ #f '()
+ (make-lambda-case #f '() #f #f #f '() '() exp #f)))
+ (proc (make-call #f (make-lexical-ref #f 'failure t)
+ '())))))))
+ (define (simplify-conditional c)
+ (match c
+ ;; Swap the arms of (if (not FOO) A B), to simplify.
+ (($ <conditional> src ($ <primcall> _ 'not (pred))
+ subsequent alternate)
+ (simplify-conditional
+ (make-conditional src pred alternate subsequent)))
+ ;; Special cases for common tests in the predicates of chains
+ ;; of if expressions.
+ (($ <conditional> src
+ ($ <conditional> src* outer-test inner-test ($ <const> _ #f))
+ inner-subsequent
+ alternate)
+ (let lp ((alternate alternate))
+ (match alternate
+ ;; Lift a common repeated test out of a chain of if
+ ;; expressions.
+ (($ <conditional> _ (? (cut tree-il=? outer-test <>))
+ other-subsequent alternate)
+ (make-conditional
+ src outer-test
+ (simplify-conditional
+ (make-conditional src* inner-test inner-subsequent
+ other-subsequent))
+ alternate))
+ ;; Likewise, but punching through any surrounding
+ ;; failure continuations.
+ (($ <let> let-src (name) (sym) ((and thunk ($ <lambda>))) body)
+ (make-let
+ let-src (list name) (list sym) (list thunk)
+ (lp body)))
+ ;; Otherwise, rotate AND tests to expose a simple
+ ;; condition in the front. Although this may result in
+ ;; lexically binding failure thunks, the thunks will be
+ ;; compiled to labels allocation, so there's no actual
+ ;; code growth.
+ (_
+ (call-with-failure-thunk
+ alternate
+ (lambda (failure)
+ (make-conditional
+ src outer-test
+ (simplify-conditional
+ (make-conditional src* inner-test inner-subsequent failure))
+ failure)))))))
+ (_ c)))
+ (match (for-test condition)
+ (($ <const> _ val)
+ (if val
+ (for-tail subsequent)
+ (for-tail alternate)))
+ (c
+ (simplify-conditional
+ (make-conditional src c (for-tail subsequent)
+ (for-tail alternate))))))
(($ <primcall> src '@call-with-values
(producer
($ <lambda> _ _
(for-tail (list->seq src (append (cdr vals) (list (car vals)))))
(make-primcall src 'values vals))))))
+ (($ <primcall> src (or 'apply '@apply) (proc args ... tail))
+ (let lp ((tail* (find-definition tail 1)) (speculative? #t))
+ (define (copyable? x)
+ ;; Inlining a result from find-definition effectively copies it,
+ ;; relying on the let-pruning to remove its original binding. We
+ ;; shouldn't copy non-constant expressions.
+ (or (not speculative?) (constant-expression? x)))
+ (match tail*
+ (($ <const> _ (args* ...))
+ (let ((args* (map (cut make-const #f <>) args*)))
+ (for-tail (make-call src proc (append args args*)))))
+ (($ <primcall> _ 'cons
+ ((and head (? copyable?)) (and tail (? copyable?))))
+ (for-tail (make-primcall src '@apply
+ (cons proc
+ (append args (list head tail))))))
+ (($ <primcall> _ 'list
+ (and args* ((? copyable?) ...)))
+ (for-tail (make-call src proc (append args args*))))
+ (tail*
+ (if speculative?
+ (lp (for-value tail) #f)
+ (let ((args (append (map for-value args) (list tail*))))
+ (make-primcall src '@apply
+ (cons (for-value proc) args))))))))
+
(($ <primcall> src (? constructor-primitive? name) args)
(cond
((and (memq ctx '(effect test))
(($ <call> src orig-proc orig-args)
;; todo: augment the global env with specialized functions
- (let ((proc (visit orig-proc 'operator)))
+ (let revisit-proc ((proc (visit orig-proc 'operator)))
(match proc
(($ <primitive-ref> _ name)
(for-tail (make-primcall src name orig-args)))
(($ <lambda> _ _
- ($ <lambda-case> _ req opt #f #f inits gensyms body #f))
- ;; Simple case: no rest, no keyword arguments.
+ ($ <lambda-case> _ req opt rest #f inits gensyms body #f))
+ ;; Simple case: no keyword arguments.
;; todo: handle the more complex cases
(let* ((nargs (length orig-args))
(nreq (length req))
(nopt (if opt (length opt) 0))
(key (source-expression proc)))
+ (define (inlined-call)
+ (make-let src
+ (append req
+ (or opt '())
+ (if rest (list rest) '()))
+ gensyms
+ (if (> nargs (+ nreq nopt))
+ (append (list-head orig-args (+ nreq nopt))
+ (list
+ (make-primcall
+ #f 'list
+ (drop orig-args (+ nreq nopt)))))
+ (append orig-args
+ (drop inits (- nargs nreq))
+ (if rest
+ (list (make-const #f '()))
+ '())))
+ body))
+
(cond
- ((or (< nargs nreq) (> nargs (+ nreq nopt)))
+ ((or (< nargs nreq) (and (not rest) (> nargs (+ nreq nopt))))
;; An error, or effecting arguments.
(make-call src (for-call orig-proc) (map for-value orig-args)))
((or (and=> (find-counter key counter) counter-recursive?)
(lp (counter-prev counter)))))))
(log 'inline-recurse key)
- (loop (make-let src (append req (or opt '()))
- gensyms
- (append orig-args
- (drop inits (- nargs nreq)))
- body)
- env counter ctx))
+ (loop (inlined-call) env counter ctx))
(else
;; An integration at the top-level, the first
;; recursion of a recursive procedure, or a nested
(make-top-counter effort-limit operand-size-limit
abort key))))
(define result
- (loop (make-let src (append req (or opt '()))
- gensyms
- (append orig-args
- (drop inits (- nargs nreq)))
- body)
- env new-counter ctx))
+ (loop (inlined-call) env new-counter ctx))
(if counter
;; The nested inlining attempt succeeded.
(log 'inline-end result exp)
result)))))
+ (($ <let> _ _ _ vals _)
+ ;; Attempt to inline `let' in the operator position.
+ ;;
+ ;; We have to re-visit the proc in value mode, since the
+ ;; `let' bindings might have been introduced or renamed,
+ ;; whereas the lambda (if any) in operator position has not
+ ;; been renamed.
+ (if (or (and-map constant-expression? vals)
+ (and-map constant-expression? orig-args))
+ ;; The arguments and the let-bound values commute.
+ (match (for-value orig-proc)
+ (($ <let> lsrc names syms vals body)
+ (log 'inline-let orig-proc)
+ (for-tail
+ (make-let lsrc names syms vals
+ (make-call src body orig-args))))
+ ;; It's possible for a `let' to go away after the
+ ;; visit due to the fact that visiting a procedure in
+ ;; value context will prune unused bindings, whereas
+ ;; visiting in operator mode can't because it doesn't
+ ;; traverse through lambdas. In that case re-visit
+ ;; the procedure.
+ (proc (revisit-proc proc)))
+ (make-call src (for-call orig-proc)
+ (map for-value orig-args))))
(_
(make-call src (for-call orig-proc) (map for-value orig-args))))))
(($ <lambda> src meta body)
((operator) exp)
(else (record-source-expression!
exp
- (make-lambda src meta (for-values body))))))
+ (make-lambda src meta (and body (for-values body)))))))
(($ <lambda-case> src req opt rest kw inits gensyms body alt)
(define (lift-applied-lambda body gensyms)
(and (not opt) rest (not kw)
(match body
(($ <primcall> _ '@apply
- (($ <lambda> _ _ lcase)
+ (($ <lambda> _ _ (and lcase ($ <lambda-case>)))
($ <lexical-ref> _ _ sym)
...))
(and (equal? sym gensyms)
head)
tail))))
(($ <prompt> src tag body handler)
- (define (singly-used-definition x)
+ (define (make-prompt-tag? x)
+ (match x
+ (($ <primcall> _ 'make-prompt-tag (or () ((? constant-expression?))))
+ #t)
+ (_ #f)))
+
+ (let ((tag (for-value tag))
+ (body (for-tail body)))
(cond
- ((and (lexical-ref? x)
- ;; Only fetch definitions with single uses.
- (= (lexical-refcount (lexical-ref-gensym x)) 1)
- (lookup (lexical-ref-gensym x)))
- => (lambda (x)
- (singly-used-definition (visit-operand x counter 'value 10 10))))
- (else x)))
- (match (singly-used-definition tag)
- (($ <primcall> _ 'make-prompt-tag (or () ((? constant-expression?))))
- ;; There is no way that an <abort> could know the tag
- ;; for this <prompt>, so we can elide the <prompt>
- ;; entirely.
- (for-tail body))
- (_
- (make-prompt src (for-value tag) (for-tail body)
- (for-value handler)))))
+ ((find-definition tag 1)
+ (lambda (val op)
+ (make-prompt-tag? val))
+ => (lambda (val op)
+ ;; There is no way that an <abort> could know the tag
+ ;; for this <prompt>, so we can elide the <prompt>
+ ;; entirely.
+ (unrecord-operand-uses op 1)
+ body))
+ ((find-definition tag 2)
+ (lambda (val op)
+ (and (make-prompt-tag? val)
+ (abort? body)
+ (tree-il=? (abort-tag body) tag)))
+ => (lambda (val op)
+ ;; (let ((t (make-prompt-tag)))
+ ;; (call-with-prompt t
+ ;; (lambda () (abort-to-prompt t val ...))
+ ;; (lambda (k arg ...) e ...)))
+ ;; => (let-values (((k arg ...) (values values val ...)))
+ ;; e ...)
+ (unrecord-operand-uses op 2)
+ (for-tail
+ (make-let-values
+ src
+ (make-primcall #f 'apply
+ `(,(make-primitive-ref #f 'values)
+ ,(make-primitive-ref #f 'values)
+ ,@(abort-args body)
+ ,(abort-tail body)))
+ (for-value handler)))))
+ (else
+ (make-prompt src tag body (for-value handler))))))
(($ <abort> src tag args tail)
(make-abort src (for-value tag) (map for-value args)
(for-value tail))))))
HCoop / bpt / guile.git / blobdiff