[m-dev.] MSVC and --no-smart-indexing

[Julien Fischer]

Hi Paul,

On Sun, 8 Feb 2015, Paul Bone wrote:

> When we disable smart indexing Mercury can generate long chains of
> if-then-else code.  In hlc grades it looks like this:
>
>    if (...) {
>        ...
>    } else {
>        if (...) {
>            ...
>        }
>        else {
>            ...
>        }
>    }
>
> This can cause MSVC 2010 to overflow it's stack, for example when compiling
> the very big switch in options.m's long_options predicate.  This is a
> problem because now we cannot install from the source distribution (which is
> compiled with --no-smart-indexing) with MSVC 2010.  (I don't know if this
> affects other versions of MSVC).
>
> We disabled smart indexing for the source distribution after a problem was
> discovered the with the computed goto code on low level backends:
>
>    http://www.mercurylang.org/list-archives/users/2010-June/005155.html
>    https://www.mercurylang.org/bugs/view.php?id=115
>
> At the time we suspected that some code generation doesn't understand that
> the target only has 32 bits per word while the host has 64 bits per word.
> We could either fix this directly or re-enable smart indexing but cause
> --cross-compiling to disable computed gotos.
>
> After searching ml_switch_gen.m and other files it doesn't appear if
> computed gotos are used at all in the high level backends.  This seems
> strange so maybe I've made a mistake, but if I haven't then we could safely
> re-enable smart indexing, which would avoid my problem with MSVC.  Ideally
> of course we could generate correctly cross-compiled computed goto code.  Am
> I correct?

About computed gotos?  No, the problem was that the target of the
computed goto was invalid because of the bug.  The fact that it was a
computed goto was incidental.

> Can anyone think of any other problems with re-enabling smart
> indexing for the source distribution?

Other than that the whole system needs to be thoroughly tested with
source distributions built with --smart-indexing enabled, no.

Cheers,
Julien.