https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91687
--- Comment #1 from barnaby.wilks at arm dot com ---
(In reply to barnaby.wilks from comment #0)
> Some architectures have instructions that allow the expressions of the form
> "(x * y) - z" to be done in one instruction (for example the FNMSUB
> instruction in AArch64).
>
> For example
>
> float f (float x, float y, float z) {
> return (y * x) - z;
> }
>
> Will generate
>
> f:
> fnmsub s0, s0, s1, s2
> ret
>
> This is done with -O2/-O3 with GCC but if -funsafe-math-optimizations are
> enabled
> it will convert expressions of the form
> (x * y) - x
> to
> (x - 1) * y
>
> which then means that the FNMSUB instruction is not generated.
>
> For example on AArch64 trunk (with -O2 -funsafe-math-optimizations)
>
> float f (float x, float y) {
> return (y * x) - y;
> }
>
> will generate
>
> f:
> fmov s2, 1.0e+0
> fsub s0, s0, s2
> fmul s0, s0, s1
> ret
>
> whereas if you just compile with (-funsafe-math-optimizations) then the
> correct
> code will be generated.
Ah, wrote the wrong thing :D
Here it should say
> whereas if you just compile with -O2 then the
> correct code will be generated.
The problem only occurs with -funsafe-math-optimizations.
> f:
> fnmsub s0, s1, s0, s1
> ret
>
> This also happens on x86-64.
>
> Godbolt demonstrating the problem.
> https://godbolt.org/z/HuU5LO
