Re: Constant divisions not converted to multiplications

On Fri, 22 May 2026, 02:20 Andrew Pinski via Gcc, <[email protected]> wrote:

> 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.
>

Consider:

(0.1f / 10.0f) == (0.1f * 0.1f)

This is false, they are not equal.



>