Oliver,
Could you please clarify on the claim below in the comments to
bitcast_u2f: "[memcpy] is the only [method] supported by gcc" for
bitcasting uint to float.
On 01/10/2013 10:30 AM, Ian Romanick wrote:
> On 01/10/2013 12:10 AM, Chad Versace wrote:
>> That is, evaluate constant expressions of the following functions:
>> packSnorm2x16 unpackSnorm2x16
>> packUnorm2x16 unpackUnorm2x16
>> packHalf2x16 unpackHalf2x16
>
> I believe that there is already code somewhere in Mesa for doing float/half
> conversion. Could we just re-use that?
>
>> Signed-off-by: Chad Versace <chad.vers...@linux.intel.com>
>> ---
>> src/glsl/ir_constant_expression.cpp | 362
>> ++++++++++++++++++++++++++++++++++++
>> 1 file changed, 362 insertions(+)
I should have known that float/half conversions existed.
I just reviewed _mesa_half_to_float and _mesa_float_to_half, and they look like
suitable replacements except for one aspect: they use a union to cast between
uint and float, yet ir_constant_expression.cpp:bitcast_u2f contains this
comment:
/* This method is the only one supported by gcc. Unions in particular
* are iffy, and read-through-converted-pointer is killed by strict
* aliasing. OTOH, the compiler sees through the memcpy, so the
* resulting asm is reasonable.
*/
static float
bitcast_u2f(unsigned int u)
{
assert(sizeof(float) == sizeof(unsigned int));
float f;
memcpy(&f, &u, sizeof(f));
return f;
}
The claim that memcpy "is the only [method] supported by gcc" for float/uint
conversion is news to me. Does that seem reasonable to you? I've CC'd the
author of the comment for clarification.
If the claim holds up, then I can easily update _mesa_float_to_half and
_mesa_half_to_float to use these functions.
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