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

[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.

>
> Roland
>
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