[Mesa-dev] [PATCH 2/7] gallium/auxiliary: Add u_bitcast.h header.

[Gustaw Smolarczyk]

Is that really the case? I always thought that using union is one of
the best ways to play it safe while not disabling strict aliasing
rules.

Here is an answer on stack overflow that uses this reasoning:
http://stackoverflow.com/a/99010

Here is another site with an extract from C11 standard:
http://dbp-consulting.com/tutorials/StrictAliasing.html

Look especially at footnote 85 of 6.5.2.3.

Regards,
Gustaw Smolarczyk

2016-07-29 15:22 GMT+02:00 Roland Scheidegger <sroland at vmware.com>:
> Am 29.07.2016 um 06:55 schrieb Connor Abbott:
>> On Thu, Jul 28, 2016 at 7:58 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>>> Am 29.07.2016 um 00:35 schrieb Matt Turner:
>>>> ---
>>>>  src/gallium/auxiliary/Makefile.sources |  1 +
>>>>  src/gallium/auxiliary/util/u_bitcast.h | 57 ++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 58 insertions(+)
>>>>  create mode 100644 src/gallium/auxiliary/util/u_bitcast.h
>>>>
>>>> diff --git a/src/gallium/auxiliary/Makefile.sources b/src/gallium/auxiliary/Makefile.sources
>>>> index e0311bf..26f7eee 100644
>>>> --- a/src/gallium/auxiliary/Makefile.sources
>>>> +++ b/src/gallium/auxiliary/Makefile.sources
>>>> @@ -175,6 +175,7 @@ C_SOURCES := \
>>>>       translate/translate_generic.c \
>>>>       translate/translate_sse.c \
>>>>       util/dbghelp.h \
>>>> +     util/u_bitcast.h \
>>>>       util/u_bitmask.c \
>>>>       util/u_bitmask.h \
>>>>       util/u_blend.h \
>>>> diff --git a/src/gallium/auxiliary/util/u_bitcast.h b/src/gallium/auxiliary/util/u_bitcast.h
>>>> new file mode 100644
>>>> index 0000000..b1f9938
>>>> --- /dev/null
>>>> +++ b/src/gallium/auxiliary/util/u_bitcast.h
>>>> @@ -0,0 +1,57 @@
>>>> +/**************************************************************************
>>>> + *
>>>> + * Copyright © 2016 Intel Corporation
>>>> + * All Rights Reserved.
>>>> + *
>>>> + * Permission is hereby granted, free of charge, to any person obtaining a
>>>> + * copy of this software and associated documentation files (the
>>>> + * "Software"), to deal in the Software without restriction, including
>>>> + * without limitation the rights to use, copy, modify, merge, publish,
>>>> + * distribute, sub license, and/or sell copies of the Software, and to
>>>> + * permit persons to whom the Software is furnished to do so, subject to
>>>> + * the following conditions:
>>>> + *
>>>> + * The above copyright notice and this permission notice (including the
>>>> + * next paragraph) shall be included in all copies or substantial portions
>>>> + * of the Software.
>>>> + *
>>>> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
>>>> + * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
>>>> + * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT.
>>>> + * IN NO EVENT SHALL THE AUTHORS AND/OR ITS SUPPLIERS BE LIABLE FOR
>>>> + * ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
>>>> + * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
>>>> + * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
>>>> + *
>>>> + **************************************************************************/
>>>> +
>>>> +#ifndef U_BITCAST_H_
>>>> +#define U_BITCAST_H_
>>>> +
>>>> +#include <string.h>
>>>> +
>>>> +#ifdef __cplusplus
>>>> +extern "C" {
>>>> +#endif
>>>> +
>>>> +static inline unsigned
>>>> +u_bitcast_f2u(float f)
>>>> +{
>>>> +   unsigned u;
>>>> +   memcpy(&u, &f, sizeof(u));
>>>> +   return u;
>>>> +}
>>>> +
>>>> +static inline float
>>>> +u_bitcast_u2f(unsigned u)
>>>> +{
>>>> +   float f;
>>>> +   memcpy(&f, &u, sizeof(f));
>>>> +   return f;
>>>> +}
>>>> +
>>>> +#ifdef __cplusplus
>>>> +}
>>>> +#endif
>>>> +
>>>> +#endif /* U_BITCAST_H_ */
>>>>
>>>
>>> I think traditionally we've used the union fi from u_math.h to do this.
>>> That said, I don't have anything against using an inline memcpy helper
>>> function instead - both should hopefully get optimized away...
>>
>> Unfortunately, that won't work with the strict aliasing rules either
>> since it's undefined behavior to reinterpret a union by dereferencing
>> more than member. That is, once you assign to one field of a union,
>> you can read from/write to that field only. You can, however,
>> reinterpret anything as a char * and dereference, hence why memcopy
>> etc. aren't undefined.
>>
>
> That might be technically true, but I'm pretty sure it's not in
> practice. Compilers don't warn about it, and are well prepared to not
> misompile it, since it's very commonly used for avoiding aliasing problems.
>
> Roland
>
>
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