[Bug translator/11590] optimized pre/postfix operators on variable throw away initial variable assignment

[Bug translator/11590] optimized pre/postfix operators on variable throw away initial variable assignment

[davidjohnn51 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=11590

David John <davidjohnn51 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |davidjohnn51 at gmail dot com

--- Comment #3 from David John <davidjohnn51 at gmail dot com> ---
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[Bug translator/11590] New: optimized pre/postfix operators on variable throw away initial variable assignment

[mjw at redhat dot com]

The following testcase shows that the original assignment of the variable is
ignored:

probe begin {

        a = 5

        // printf("%d ", a)
        printf("%d ", (a++))
        printf("%d ", (++a))
        printf("%d ", (a--))
        printf("%d\n", (--a))
        exit()
}

$ stap test.stp
0 2 2 0

If we uncomment the actually first use the original assigned value of a things
look correct:

$ stap test.stp
5 5 7 7 5

This is an translation optimization issue, the original script also prints out
the correct value when unoptimized:

$ stap -u test.stp
5 7 7 5

-- 
           Summary: optimized pre/postfix operators on variable throw away
                    initial variable assignment
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: translator
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: mjw at redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=11590

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[Bug translator/11590] optimized pre/postfix operators on variable throw away initial variable assignment

[mjw at redhat dot com]

------- Additional Comments From mjw at redhat dot com  2010-05-12 09:44 -------
This seems to have been caused by commit 982b88bd which solved bug #6954.

commit 982b88bd950381434a8325e550eff9df59f59dcd
Author: Wenji Huang <wenji.huang@oracle.com>
Date:   Thu Jan 28 14:16:43 2010 +0800

    PR6954: make ++/-- operation trigger automatic global printing
    
    * staptree.cxx (varuse_collecting_visitor::visit_arrayindex):
      Regard operations as pure writes.
    * testsuite/systemtap.base/global_end.exp: Add test case.
    * testsuite/systemtap.base/global_end.stp: Ditto.

Note comment #3 of that bug:

"FWIW, I wonder why we didn't just go this route earlier.  I worry
that pretending that ++ only writes to its argument may mess up
some optimization, I can't think of one right now.  We'll keep
this special case in mind."

Reverting this commit fixes this bug (but obviously introduces bug #6954 again).

-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=11590

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[Bug translator/11590] optimized pre/postfix operators on variable throw away initial variable assignment

[mjw at redhat dot com]

------- Additional Comments From mjw at redhat dot com  2010-05-12 12:43 -------
commit d1cf0b627cfc9002b54c2ad40507de9ee6a2ce90
Author: Mark Wielaard <mjw@redhat.com>
Date:   Wed May 12 14:39:55 2010 +0200

    Add testcase for PR11590 optimized pre/postfix operators
    
    * testsuite/systemtap.base/prepost_optim.stp: New test.
    * testsuite/systemtap.base/prepost_optim.exp: Harness.

commit ad8c38ffc62200d6ba6ebc0b0339967cdd84669a
Author: Mark Wielaard <mjw@redhat.com>
Date:   Wed May 12 14:31:59 2010 +0200

    Add testcase for PR6954 ++ operations not triggering automatic global printing
    
    * testsuite/systemtap.base/global_end_var.stp: New test.
    * testsuite/systemtap.base/global_end.exp: Add xfail.

commit 980759f3da625c5697440efc1bfda046c247866e
Author: Mark Wielaard <mjw@redhat.com>
Date:   Wed May 12 14:16:26 2010 +0200

    PR11590 Revert "PR6954: make ++/-- operation trigger automatic global printing"
    
    This reverts commit 982b88bd950381434a8325e550eff9df59f59dcd.
    
    Pretending that pre/postfix ++/-- operations only writes to its argument
    messes up the optimization of a variable that is assigned and then
    only used through post/prefix ++/--operators.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://sourceware.org/bugzilla/show_bug.cgi?id=11590

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