GNU bug report logs - #15533
optimizing away noticeable effects

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Message #23 received at 15533 <at> debbugs.gnu.org (full text, mbox):

From: Ian Price <ianprice90 <at> googlemail.com>
To: Mark H Weaver <mhw <at> netris.org>
Cc: Andy Wingo <wingo <at> pobox.com>, 15533 <at> debbugs.gnu.org
Subject: Re: bug#15533: optimizing away noticeable effects
Date: Tue, 08 Oct 2013 18:13:05 +0100

Ian Price <ianprice90 <at> googlemail.com> writes:

> Does it make sense to add it to find-definition? or should we add it
> before the use in that case?

I've decided that it does, and I've made the following (tentative)
change on my own guile install.

         (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))))))
+               (if (var-set? (operand-var op))
+                   (values #f #f)
+                   
+                 ;; var-set? (operand-var ) => #f #f ?
+                 (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)))))))

It's a little invasive because of the 'if', but the meat of it is

+               (if (var-set? (operand-var op))
+                   (values #f #f)

The check for mutability needs to come before the let, since that's
where we do the lookup for a value, so it would be too late.

If Andy is happy with this change, I'll add a test, and push a commit,
but I'm going leave it to his discretion.

-- 
Ian Price -- shift-reset.com

"Programming is like pinball. The reward for doing it well is
the opportunity to do it again" - from "The Wizardy Compiled"

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