http://d.puremagic.com/issues/show_bug.cgi?id=9387

           Summary: Compiler switch -O changes behavior of correct code
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Mac OS X
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: stephan.schiffels@mac.com


--- Comment #0 from Stephan <stephan.schiffels@mac.com> 2013-01-24 07:56:09 PST ---
Created an attachment (id=1182)
Source file with program that

The attached program implements a part of Brent's minimization algorithm for one-dimensionsal functions. The code is from Numerical Recipes 3rd edition.

I use dmd 2.061/

When I run the program with "rdmd brent_test.d" it runs fine and gives the correct result.

When I run it with optimization, i.e. with "rdmd -O brent_test.d", it behaves differently. It enters some infinite loop and eventually throws the expected exception for too many iterations.

You can see that I placed a writefln() into line 45, which outputs the value of variable a. When you move this writefln statement just one line below, i.e. below the if-statement, the code runs fine, even with optimization.

I colleague of mine suggested that there might be a bug related to a large number of local variables. Maybe some limiting number of registers causes the machine to cache things into memory and pulling them back in a wrong way or something.

Appreciate help!

Stephan

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--- Comment #1 from Stephan <stephan.schiffels@mac.com> 2013-01-24 08:06:00 PST ---
During debugging, I actually looked at the value of every single local variable, and you can actually see how the value of some variables (for example "a") changes from one iteration to the next, without any assignment.

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--- Comment #2 from Stephan <stephan.schiffels@mac.com> 2013-01-24 10:00:41 PST ---
I just checked: The bug definitely was introduced with version 2.061! With dmd version 2.060, everything works fine, with and without the "-O" switch.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla@digitalmars.com
           Severity|major                       |regression


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--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> 2013-01-25 00:30:21 PST ---
I can't reproduce this with the latest dmd. I'll upload a new beta tomorrow you can try.

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--- Comment #4 from Stephan <stephan.schiffels@mac.com> 2013-01-25 05:01:07 PST ---
(In reply to comment #3)
> I can't reproduce this with the latest dmd. I'll upload a new beta tomorrow you can try.

What actually seems to be corrupted are the precompiled executables on the zip-file on the web. We checked this for the osx and the linux version. Both of these precompiled versions produce this bug.

When we compile dmd from source, even for version 2.061 from the web, this bug does not occur.

Stephan

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--- Comment #5 from Stephan <stephan.schiffels@mac.com> 2013-01-25 06:53:50 PST ---
(In reply to comment #3)
> I can't reproduce this with the latest dmd. I'll upload a new beta tomorrow you can try.

Sorry to jump back and forth here. I have to again correct my previous
statement: With the latest version of dmd/druntime/phobos (2.062 from git),
this bug does occur!
But only when you compile and run separately. When you use dmd -run, both
versions with and without -O work fine. This is quite weird.

So:
dmd -O brent_test.d
./brent_test

should produce a different outcome than

dmd brent_test.d
./brent_test

I will try use bisect to find out when this bug was introduced.

Stephan

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--- Comment #6 from Walter Bright <bugzilla@digitalmars.com> 2013-01-25 10:37:02 PST ---
When I compile and run separately, it works fine.

You should also clarify whether you are using -m64 or not.

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--- Comment #7 from Stephan <stephan.schiffels@mac.com> 2013-01-25 10:43:26 PST ---
Right, I use the 64bit model.
And I tested this on OSX and on linux, with same outcomes on both platforms.
It's frustating that you can't reproduce. Thanks for responding quickly on this
anyway.
I will see what I can find out with bisect.

Stephan

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--- Comment #8 from Don <clugdbug@yahoo.com.au> 2013-01-28 01:01:42 PST ---
BTW you might be interested in std.numeric.findRoot, which is the
root-finding-by-bracketing algorithm (in contrast to "Brent's algorithm" which
is minima-finding-by-bracketing). In terms of number of calls, I believe it
beats all published algorithms (in some cases, by an order of magnitude). I
should really publish it. I did some work on the minima problem as well, and
put it into Tango, but it isn't in Phobos.
The code is very old now, dating from a time where there were many compiler
limitations, and it could use a review.

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