It only seems to happen with single precision, like:
#include <math.h>
float rsqrt1(float a) { return 1.0f/sqrtf(a); }
If the compiler doesn’t do what one expects, one probably expects wrong. (It
isn't favorable to do so.)
Marcus
-----Original Message-----
From: Friam <[email protected]> On Behalf Of u?l? ???
Sent: Thursday, January 7, 2021 4:09 PM
To: FriAM <[email protected]>
Subject: Re: [FRIAM] Q_rsqrt() vs 1/sqrt()
Great idea! Thanks. I'd tried gcc -S. Whew is my ASM rusty. But objdump -d
doesn't show any rsqrt instructions.
On 1/7/21 3:46 PM, Marcus Daniels wrote:
> There's a reciprocal sqrt instruction in SSE (x86) that your compiler may be
> pattern matching to, depending on your compiler options. You could
> disassemble to see what you get from the latter.
>
> -----Original Message-----
> From: Friam <[email protected]> On Behalf Of u?l? ???
> Sent: Thursday, January 7, 2021 3:24 PM
> To: FriAM <[email protected]>
> Subject: [FRIAM] Q_rsqrt() vs 1/sqrt()
>
> https://en.wikipedia.org/wiki/Fast_inverse_square_root
>
> So, why is Q_rsqrt() *slower* than 1/sqrt()?
>
> 1/sqrt() took 0.294771 s
> Q_rsqrt() took 0.51579 s
>
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