Re: Bytevector VM ops

[Ludovic Courtès]

Good morning,

Andy Wingo <address@hidden> writes:

> On Tue 30 Jun 2009 00:23, address@hidden (Ludovic Courtès) writes:
>
>> "Andy Wingo" <address@hidden> writes:
>>
>>> +#define BV_FIXABLE_INT_REF(stem, fn_stem, type, size)                   \
>>> +{                                                                       \
>>> +  long i;                                                               \
>>> +  ARGS2 (bv, idx);                                                      \
>>> +  VM_VALIDATE_BYTEVECTOR (bv);                                          \
>>> +  if (SCM_LIKELY (SCM_I_INUMP (idx)                                     \
>>> +                  && ((i = SCM_I_INUM (idx)) >= 0)                        \
>>> +                  && (i < SCM_BYTEVECTOR_LENGTH (bv))                   \
>>> +                  && (i % size == 0)))                                  \
>>> +    RETURN (SCM_I_MAKINUM (*(scm_t_##type*)                             \
>>> +                           (SCM_BYTEVECTOR_CONTENTS (bv) + i)));        \
>>
>> Did you test this on SPARC or some such?  I'm 90% sure
>> `(bv-u32-ref bv 1)' would lead to SIGBUS there, due to the unaligned access.
>> This is why `INTEGER_REF ()' in `bytevectors.c' uses memcpy(3).
>
> Wouldn't the i % size == 0 case catch that? (This is used in native-ref
> instructions) 

Oh yes, probably, I had overlooked this.

>> Given that there's some duplication with `bytevectors.c', maybe we could
>> share some of the accessor macros between both files?
>
> Perhaps! The one difference is that we can fast-path only the normal
> cases here, calling out to those functions to handle stranger things
> (like unaligned access).

Right.  So maybe the macros are different enough that we'd be better off
keeping things as they are.

Thanks,
Ludo'.