[m-users.] Float array initialisation

[fabrice nicol]

I have some trouble interpreting the float-to-word/word-to-float 
conversion issue in the particular case of array initialisation.

This is performed by 'ML_init_array(MR_ArrayPtr array, MR_Integer size, 
MR_Word item)'  (DEV branch, array.m:1007), using an MR_Word initialiser 
'item' in a simple loop:

         for (i = 0; i < size; i++) {
             array->elements[i] = item;
         }

Now, the manual says that some implementations have sizeof(MR_Float) > 
sizeof(MR_Word), which is why lists of floats should be converted using 
'MR_word_to_float', as explained in the (snapshot) manual (p. 109):

     "The following fragment of C code illustrates the correct way to 
extract the head of a
     Mercury list of floats.
     MR_Float f;
     f = MR_word_to_float(MR_list_head(list));
     Omitting the call to MR_word_to_float in the above example would 
yield incorrect
     results for implementations where ‘sizeof(MR_Float)’ is greater 
than ‘sizeof(MR_Word)’."

A cursory look at runtime/mercury_float.h points to it being necessary 
when MR_BOXED_FLOAT is defined.

So one would expect that the initialiser 'item', if of float type (i.e. 
in an 'array(float)'), would be checked as not exceeding the upper bound 
to MR_Word, when entered in init_2/3 (array.m:1494), at least if 
MR_BOXED_FLOAT is defined elsewhere. If this is the case, my guess is 
that the float initialiser item will not be passed unharmed to the above 
loop (behaviour may possibly be undefined).

The code in array.m has been too stable and polished for over 20 years 
now for it to be a real bug, so it must be that I did not catch something.

Thanks for giving me a hint.

F. N.


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