Avoid checking twice if divisor is zero

[chambart]

The flambda branch before merging assumed that Pdivint and Pmodint where
already checked when entering Cmmgen. This was not the case anymore
after merging and this change was lost. This fix this overlook by adding
an annotation to the Pdivint and Pmodint primitive telling whether the
division by zero was already checked.

The reason to move the test generation to Closure_conversion in the
flambda branch was to allow the division primitive to be considered as
pure without needing to check for the effective value of the
divisor. This simplified Semantics_of_primitives a lot.

[chambart]

Also contains a fix for http://caml.inria.fr/mantis/view.php?id=7328

The justification for adding the fix here rather than in a separate patch is that it share a lot of this patch.

@damiendoligez would you still accept such a patch for 4.04 ?

[mshinwell]

I think we should merge this for 4.04 given half of it was actually a merge mistake in 4.03, IIUC.

[bobzhang]

according to the docs:

-unsafe
Turn bound checking off for array and string accesses (the v.(i) and s.[i] constructs). Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can happen if the program accesses an array or string outside of its bounds.

div by zero should always raise.
Btw, if you want to turn -unsafe on to avoid div by zero check, would you use inline record to make life easier for people who consume lambda? thanks

[chambart]

Well the doc was missing something then. It should probably be fixed.
By the way this only apply to native code.

[bobzhang]

here Lambda.primitive types are changed, it would be nice that we use inline records whenever we use tuple

[chambart]

@bobzhang I tried and it don't look too heavy, so here it is.

[damiendoligez]

@chambart OK for 4.04 but what does this have to do with the -unsafe flag?

If you think the doc is missing something, you need to update it in this PR...

[lpw25]

This looks correct to me.

@damiendoligez It doesn't have anything to do with the -unsafe flag, but it is adjacent to the code which disables raising an exception for division by zero when using -unsafe. I guess @bobzhang noticed it when looking at the patch and was surprised by it since it is not mentioned in the docs.

[damiendoligez]

@lpw25 Thanks for the clarification. I see the doc change is not so urgent and it belongs in another PR anyway.

This one seems ready to merge. Any objections?

[mshinwell]

Now this has been read we should be good to go.

[gasche]

@damiendoligez I cannot find a trace of this in 4.04, did we forget to cherry-pick it?

[damiendoligez]

cherry-picked to 4.04 (commit 0ea09d2)