[m-users.] How to write stack-friendly mercury-code?

[Dirk Ziegemeyer]

Hi Paul,

> Am 28.03.2016 um 12:51 schrieb Paul Bone <paul at bone.id.au>:
> 
> On Mon, Mar 28, 2016 at 11:51:52AM +0200, Dirk Ziegemeyer wrote:
>> Hi,
>> 
>> when I increase the data volume that is processed by a mercury application, the windows-version (asm_fast.gc/i686-w64-mingw32.static) stops at runtime with this error message:
>> 
>> *** Mercury runtime: caught segmentation violation ***
>> This may have been caused by a stack overflow, due to unbounded recursion.
>> exiting from signal handler
>> 
>> The Mac OS X version (hlc.gc/x86_64-apple-darwin14.5.0) still works.
> 
> Hi Dirk,  There's a couple of things you can do.

Thank you for explaining the options to improve stack usage and especially the examples of non-obvious non-tail-recursive code.

I’m running through these options to see what helps.

> + Enable --last-call-modulo-recursion optimisation (LCMC).
> 
>    This will make map tail recursive by performing the construction [Y | Ys]
>    before the recursive call, and filling in the value Ys in this term
>    within the recursive call.

I wasn’t successful yet with the best option of all: to make the code tail recursive. Call the compiler with command
„mmc --make --optimize-constructor-last-call“
didn’t solve it and the problematic part of the code was not apparent to me yet. Is this the option you mean with „--last-call-modulo-recursion“?

> + Use a segmented stack
> 
>    If you use --stack-segments (an stseg grade component, compatible with
>    low level C grades) then a segmented stack will be used, a segmented
>    stack grows and shrinks as necessary.  Unbounded recursion can now cause
>    your computer to thrash, rather than just crashing your program. This
>    is the easiest option, but it doesn't reduce your program's memory usage.
>    It's a good option if the limit is the size of the stack, and not how
>    much RAM you have.

I’ll try the stseg-grade next. This will take me some time because I first need to install this grade for the MingGW-Cross-Compiler.

> + Increase the size of the stack.
> 
>    In low-level graces, like asm_fast, put MERCURY_OPTIONS="--detstack-size
>    N" in your environment.  Check the Mercury user's guide.

A fruitless try was to put this in the last line of file Mercury.options:
MERCURY_OPTIONS = --detstack-size 8192

> I've been working on some changes to Mercury that will make it easier for
> developers to find which parts of their program are potentially causing
> crashes due to stack exhaustion.

Are your changes available in a recent rotd already? I have to make a new install of Mercury with stseg anyway.

Cheers.

Dirk