Re: Problem with MLton

On Thu, Feb 16, 2012 at 10:38 PM, Matthew Fluet <[email protected]> wrote:
> On Thu, Feb 16, 2012 at 7:41 AM, Lars Magnusson <[email protected]> wrote:
>> We use MLton to build our system for automatic programming called
>> ADATE, which has its own internal compiler capable of compiling a
>> subset of SML. In the last year I have been working on upgrading this
>> internal compiler to generate x86-64 code and to include a simple
>> garbage collector in the runtime environment. For the last month I've
>> been using MLton as a reference compiler for testing our internal
>> compiler on a great number of different programs developed by the
>> ADATE system i.e. I compile the generated programs with both compilers
>> and compare the results.
>>
>> I've found an instance that gives a mismatch, and I've spent the last
>> week investigating why my compiler fails to properly compile the
>> program. I wasn't able to find the problem by investigating the
>> generated code, so I thought I'd try executing the program on paper by
>> hand to figure out where my code goes wrong. To my surprise my manual
>> execution produced the exact same output as my compiler, so I did it
>> again, and again, and all three times with the same result. I then
>> decided to test the program on other existing ML compilers. I've tried
>> the program on both SML/NJ and Poly/ML, and they both produce the same
>> output as mine.
>>
>> I'm currently running MLton 20100608, but we have also tested the
>> program on an older release (2006) with the same problems.
>
> ...
>
>> I'm hesitant about drawing this conclusion, but based on the
>> observations I am forced to think that there's a bug in MLton.
>
> I agree that the evidence strongly suggests a bug in MLton.  I was
> able to reproduce the "1 2 3 4 5 6 7" with MLton and the "1 2 3 5 6 7"
> with SML/NJ, PolyML, and HaMLet.  The "f" and "gA1F7A0" are still
> quite recognizable in the later intermediate representations, so I'm
> hopeful that it won't take too much work to identify the offending
> optimization pass.

Seems to be a bug with the SSA redundant pass, which attempts to find
variables (e.g., arguments) that are redundant due to the presence of
another variable that always has the same value.  Compiling with
"-drop-pass redundant" yields the "1 2 3 5 6 7" output.

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