GCC CM4 FPU - sqrtf

[Jaroslav Buchta]

Zajimave, muzete poslat options ktere pouziva prekladac a linker?

Dne 11.02.2017 v 20:51 Miroslav Mraz napsal(a):
> Asi to záleží na verzi překladače. Mě to do paměti neukládá (-Os). Gcc 
> version 6.2.1 20161205.
>
> 20000446:    eddd 5a00     vldr    s11, [sp]
> 2000044a:    ed9d 6a00     vldr    s12, [sp]
> 2000044e:    eddd 7a01     vldr    s15, [sp, #4]
> 20000452:    ed9d 5a01     vldr    s10, [sp, #4]
> 20000456:    eddd 6a02     vldr    s13, [sp, #8]
> 2000045a:    ed9d 7a02     vldr    s14, [sp, #8]
> ------- tohle jsou inline nepodstatné věci z kontextu programu
> 2000045e:    4a0c          ldr    r2, [pc, #48]    ; (20000490 
> 20000460:    480c ldr    r0, [pc, #48]    ; (20000494 20000462: 
> 490d          ldr    r1, [pc, #52]    ; (20000498
> ------- konec nepodstatných věcí
>   float amp = (ph1*ph1 + ph2*ph2 + ph3*ph3) * 2.0;
> 20000464:    ee67 7a85     vmul.f32    s15, s15, s10
> 20000468:    eee5 7a86     vfma.f32    s15, s11, s12
> 2000046c:    eee6 7a87     vfma.f32    s15, s13, s14
> 20000470:    ee77 7aa7     vadd.f32    s15, s15, s15
>   asm ("vsqrt.f32 %0, %1" : "=w" (y) : "w" (x));
> 20000474:    eef1 7ae7     vsqrt.f32    s15, s15
>   y = _sqrt (amp);
> ------- tady na konci to uloží, ale něco s tím udělat musí, že
> 20000478:    edcd 7a03     vstr    s15, [sp, #12]
>   (void) y;
>
> Zdroj jsem ještě pro jistotu zkomplikoval
>
> static inline float _sqrt (float x) {
>   float y;
>   asm ("vsqrt.f32 %0, %1" : "=w" (y) : "w" (x));
>   return y;
> }
> static void test_func (void) {
>   volatile float ph1=0.0f, ph2=1.0f, ph3=2.0f, y;
>   float amp = (ph1*ph1 + ph2*ph2 + ph3*ph3) * 2.0;
>   y = _sqrt (amp);
>   (void) y;
> }
>
> Mrazík
>
> Dne 11.2.2017 v 19:46 Jaroslav Buchta napsal(a):
>> Jeste jedna podotazka, stve me, ze prekladac kazdy mezivysledek strka do
>> pameti, napr. takto
>>
>> (786)          amp = (ph1*ph1 + ph2*ph2 + ph3*ph3) * 2.0; 
>> //-fno-math-errno
>> 080017E8    vldr    s14, [r7, #56]    ; 0x38
>> 080017EC    vldr    s15, [r7, #56]    ; 0x38
>> 080017F0    vmul.f32    s14, s14, s15
>> 080017F4    vldr    s13, [r7, #52]    ; 0x34
>> 080017F8    vldr    s15, [r7, #52]    ; 0x34
>> 080017FC    vmul.f32    s15, s13, s15
>> 08001800    vadd.f32    s14, s14, s15
>> 08001804    vldr    s13, [r7, #48]    ; 0x30
>> 08001808    vldr    s15, [r7, #48]    ; 0x30
>> 0800180C    vmul.f32    s15, s13, s15
>> 08001810    vadd.f32    s15, s14, s15
>> 08001814    vadd.f32    s15, s15, s15
>> *08001818    vstr    s15, [r7, #44]    ; 0x2c**
>> **(787)          asm volatile  ("vsqrt.f32 %0, %1" : "=w" (amp) : "w"
>> (amp) );**
>> **0800181C    vldr    s15, [r7, #44]    ; 0x2c**
>> *08001820    vsqrt.f32    s15, s15
>> 08001824    vstr    s15, [r7, #44]    ; 0x2c
>>
>> program vypada takto:
>>          float amp;
>> ...
>>         amp = (ph1*ph1 + ph2*ph2 + ph3*ph3) * 2.0;
>>         asm volatile  ("vsqrt.f32 %0, %1" : "=w" (amp) : "w" (amp) );
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