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optimizer: don't move APPN_FLAG_OMITTABLE inside lambdas

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Some expressions are omittable only when the arguments have certain types.
In this case the application is marked with APPN_FLAG_OMITTABLE instead of relaying on the flags of the primitive.

The optimizer can't use this flag to move the expression inside a lamba or across a potential continuation capture, unlike other omittable expressions. They can be moved
only in more restricted conditions.

For example, in this program

  #lang racket/base

  (define n 10000)
  (define m 10000)
  (time
   (define xs (build-list n (lambda (x) 0)))
   (length xs)
   (define ws (list->vector xs)) ; <-- omittable
   (for ([i (in-range m)])
     (vector-ref ws 0)))     ; <-- ws is used once

If the optimizer moves the expression in the definition of ws inside the recursive
lambda that is created by the for, then the code is equivalent to:

  #lang racket/base

  (define n 10000)
  (define m 10000)
  (time
   (define xs (build-list n (lambda (x) 0)))
   (length xs)
   (for ([i (in-range m)])
     (vector-ref (list->vector xs) 0)))     ; <-- moved here

And the new code is O(n*m) instead of O(n+m). This example is a minimized version
of the function kde from the plot package, where n=m and the bug changed the run
time from linear to quadratic.
This commit is contained in:
Gustavo Massaccesi 2016-07-30 13:11:14 -03:00
parent 004fd02501
commit bfa269982f
2 changed files with 24 additions and 6 deletions
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12
pkgs/racket-test-core/tests/racket/optimize.rktl
View File

@ -2111,6 +2111,18 @@
'(lambda (z) (lambda ()
(lambda () z)))
#f)
;; Don't move omittable expressions that keep a reference:
(test-comp '(lambda (z) (let ([r (pair? z)])
(lambda () r)))
'(lambda (z) (lambda ()
(lambda () (pair? z))))
#f)
(test-comp '(lambda (z) (when (list? z)
(let ([r (list->vector z)])
(lambda () r))))
'(lambda (z) (when (list? z)
(lambda () (list->vector z))))
#f)
(test-comp '(if (let ([z (random)]) null) 1 2)

18
racket/src/racket/src/optimize.c
View File

@ -1616,8 +1616,10 @@ static int movable_expression(Scheme_Object *expr, Optimize_Info *info,
}
break;
case scheme_application_type:
if (SCHEME_APPN_FLAGS((Scheme_App_Rec *)expr) & APPN_FLAG_OMITTABLE)
can_move = 1;
if (!cross_lambda
&& !cross_k
&& (SCHEME_APPN_FLAGS((Scheme_App_Rec *)expr) & APPN_FLAG_OMITTABLE))
can_move = -1;
else
can_move = is_movable_prim(((Scheme_App_Rec *)expr)->args[0], ((Scheme_App_Rec *)expr)->num_args,
cross_lambda, cross_k, info);
@ -1633,8 +1635,10 @@ static int movable_expression(Scheme_Object *expr, Optimize_Info *info,
}
break;
case scheme_application2_type:
if (SCHEME_APPN_FLAGS((Scheme_App2_Rec *)expr) & APPN_FLAG_OMITTABLE)
can_move = 1;
if (!cross_lambda
&& !cross_k
&& (SCHEME_APPN_FLAGS((Scheme_App2_Rec *)expr) & APPN_FLAG_OMITTABLE))
can_move = -1;
else
can_move = is_movable_prim(((Scheme_App2_Rec *)expr)->rator, 1, cross_lambda, cross_k, info);
if (can_move) {
@ -1645,8 +1649,10 @@ static int movable_expression(Scheme_Object *expr, Optimize_Info *info,
}
break;
case scheme_application3_type:
if (SCHEME_APPN_FLAGS((Scheme_App3_Rec *)expr) & APPN_FLAG_OMITTABLE)
can_move = 1;
if (!cross_lambda
&& !cross_k
&& (SCHEME_APPN_FLAGS((Scheme_App3_Rec *)expr) & APPN_FLAG_OMITTABLE))
can_move = -1;
else
can_move = is_movable_prim(((Scheme_App3_Rec *)expr)->rator, 2, cross_lambda, cross_k, info);
if (can_move) {