[Vm-dev] The byte order fallacy

[Ben Coman]

cool. thx Nicolas.
cheers -ben

On Tue, Jun 13, 2017 at 3:38 PM, Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> wrote:

>
> Hi Ben,
> last year I added SQ_SWAP_4_BYTES and SQ_SWAP_8_BYTES macros in
> platforms/Cross/vm/sqMemoryAccess.h
> These macros use the compiler intrinsics if possible.
> https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/
> c0da5abcc7a538efd3ab6dcc4bb2d2ef3ac5c65a
> So we should not define any new one and rather replace the suboptimal ones
> with the ones above.
>
> i = *((int*)data);
> is doubly bad because of possible alignment problem as you told,but also
> because it introduces pointer aliasing, what the C compiler does not like,
> and the standards neither (C89 C99 ...).
>
> However, our int are 32 bits long in all supported platforms. Would it not
> be the case, the VM would break in many places.
> For example, you can see the cost in term of number of commits for the
> fact that sizeof long < sizeof void * in LLP64 both in VMMaker and
> Opensmalltalk-vm source.
>
> 2017-06-13 6:13 GMT+02:00 Ben Coman <btc at openinworld.com>:
>
>>
>> I bumped into an interesting article [1] by Rob Pike that...
>> i = *((int*)data);
>> #ifdef BIG_ENDIAN
>> /* swap the bytes */
>> i = ((i&0xFF)<<24) | (((i>>8)&0xFF)<<16) | (((i>>16)&0xFF)<<8) |
>> (((i>>24)&0xFF)<<0);
>> #endif
>>
>> is the wrong way to handle endianness since...
>>   "It assumes integers are addressable at any byte offset; on some
>> machines that's not true."
>>   "It depends on integers being 32 bits long, or requires more #ifdefs
>> to pick a 32-bit integer type."
>>
>> I guess some people here have byte-ordering war wounds so I'd value your
>> opinion on whether I should file that as good advice.
>> I notice we have a couple of instances of this...
>>
>> $  find platforms -name "*.c" -exec grep -Hi "<<" {} \; | grep ">>" |
>> grep 24 | grep 8
>> ./win32/plugins/FontPlugin/sqWin32FontPlugin.c:  #define BYTE_SWAP(w)
>> ((w << 24) | ((w & 0xFF00) << 8) | ((w >> 8) & 0xFF00) | (w >> 24))
>> ./win32/vm/sqWin32Window.c:   #define BYTE_SWAP(w) w = (w<<24) |
>> ((w&0xFF00)<<8) | ((w>>8)&0xFF00) | (w>>24)
>>
>> cheers -ben
>>
>> [1] https://commandcenter.blogspot.com.au/2012/04/byte-order-fallacy.html
>>
>>
>>
>
>
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