[Ffmpeg-devel] gcc4 support & MMX fixups (from Debian)

Måns Rullgård mru
Thu Feb 2 12:12:09 CET 2006

Pawel Sikora said:
> On Thu, 2 Feb 2006, Michael Niedermayer wrote:
>> Hi
>> On Thu, Feb 02, 2006 at 09:41:37AM +0100, Pawe? Sikora wrote:
>>>>> I suggest we should use a size_t type for params like *_stride
>>>>> to avoid (on x86-64) unneeded 32<->64 conversions.
>>>> no, size_t is unsigned, stride must be signed!
>>> Right, I think off_t from sys/types.h is a good candidate.
>>> It's signed, on x86-32 has 32 bits, on x86-64 has 64 bits.
>> NO!
>> -- Data Type: off_t
>>     This is an arithmetic data type used to represent file sizes.  In
>>     the GNU system, this is equivalent to `fpos_t' or `long int'.
>>     If the source is compiled with `_FILE_OFFSET_BITS == 64' this type
>>     is transparently replaced by `off64_t'.
> Ok, last attempt  - ptrdiff_t.
> C and C++ define a special type for pointer arithmetic, namely ptrdiff_t,
> which is a typedef of a platform-specific signed integral type. You can
> use a variable of type ptrdiff_t to store the result of subtracting and
> adding pointers.

The int_fastXX_t types were invented for exactly this type of situations.
Specifically, int_fast32_t is 32 bits on 32-bit x86 and 64 bits on x86_64.

Using ptrdiff_t for anything other than things involving pointers is bad.
There could theoretically exist a machine with 64-bit longs and 32-bit
pointers.  On one of these, ptrdiff_t could legally be 32 bits, and result
in the same suboptimal code we wanted to avoid.

M?ns Rullg?rd
mru at inprovide.com

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