Re: Constant divisions not converted to multiplications

On Thu, May 21, 2026 at 6:04 AM Undisclosed via Gcc <[email protected]> wrote:
>
> Hi, I am using gcc 14.2 on windows (mingw64).
> I always gave for granted that any compiler would automatically turn constant divisions into multiplications. Thus, I never cared of writing i.e
> float x = a * 0.1f
> but I always wrote
> float x = a / 10.f
> trusting the compiler.
> Now I decided to inspect the asm, to be 100% sure, cos I was detecting abnormal cpu consumption in some time-critical code, and I had a very bad surprise !!!
>
> Here is how:
>
> float Func(float x)
> {
>   return x / 10.f;
> }
>
> gets resolved:
>
> divss .LC12(%rip), %xmm0 !!!
>
> This with:
>
> -m64 -march=x86-64-v3 -O3
>
> Does it make any sense ? Why doesn't it convert the division by 10 to a multiplication by 0.1 ?? Has one to enable some specific option for that ?

0.1 is not exactly representable in floating point types. This is why
it is not converted by default.
If you had used -ffast-math it will be converted but I am not sure you
want that option in general.
Note 0.5 is exactly representable in floating point types so GCC
already converts `/2.0` into `*0.5`. This is true of many power of 2s.

basically if `1/value` is exactly representable in floating point GCC
will convert it into a multiply. IIRC this is only true for powers of
2.
Because of the way floating point is represented. as 1.xxxE-N .

Thanks,
Andrea

>
> Thx