[Interest] armv7a-hard-float in Qt android apps

[Elvis Stansvik]

2016-04-30 11:51 GMT+02:00 Ola Røer Thorsen <ola at silentwings.no>:
>
> 2016-04-30 10:46 GMT+02:00 Jean-Michaël Celerier
> <jeanmichael.celerier at gmail.com>:
>>
>> https://godbolt.org/g/DqBlFG
>>
>> With gcc -O1
>>
>> float f(float x)
>> {
>>   return 2. * x;
>> }
>>
>> becomes
>>
>> f(float):
>>         addss   %xmm0, %xmm0
>>         ret
>>
>>
>> and
>>
>> float g (float x)
>> {
>>   return 2.f * x;
>> }
>>
>> becomes
>>
>> g(float):
>>         addss   %xmm0, %xmm0
>>         ret
>>
>
> This is on x86. I was talking about ARM with single-precision FPUs and the
> general advice not to use double precision constants if possible there.

Also, Jean-Michaël's example is a little unfortunate since the
compiler is smart enough to convert the multiplication by 2 to an
addss instruction. Changing the constant to 2.1 is more interesting
(https://godbolt.org/g/Cgw2cR):

f(float):
        cvtss2sd        %xmm0, %xmm0
        mulsd   .LC0(%rip), %xmm0
        cvtsd2ss        %xmm0, %xmm0
        ret
g(float):
        mulss   .LC1(%rip), %xmm0
        ret
.LC0:
        .long   3435973837
        .long   1073794252
.LC1:
        .long   1074161254

So conversion to double-precision and then back to single-precision in
the code for f.

For reference, here's the result with ARM GCC 4.5.3:
https://godbolt.org/g/r7Kf1H

Elvis

>
> This is what GCC itself says in the documentation for -Wdouble-promotion:
>
> -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)Give a
> warning when a value of type float is implicitly promoted to double. CPUs
> with a 32-bit “single-precision” floating-point unit implement float in
> hardware, but emulate double in software. On such a machine, doing
> computations using double values is much more expensive because of the
> overhead required for software emulation.
>
> It is easy to accidentally do computations with double because
> floating-point literals are implicitly of type double. For example, in:
>
>           float area(float radius)
>           {
>              return 3.14159 * radius * radius;
>           }
>
> the compiler performs the entire computation with double because the
> floating-point literal is a double.
>
>
>
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