Re: Parentheses not honored when using FMA

[Marc Glisse <marc dot glisse at inria dot fr>]

On Wed, 12 Aug 2015, Marcin Krotkiewski wrote:

> Hello, all,
>
> I have doubts about asm generated for the following code that performs 
> orientation test for a point and a segment:
>
> double orient_test_2d(const double m[2], const double a[2], const double 
> b[2])
> {
>  double am1 = a[0]-m[0];
>  double bm1 = b[1]-m[1];
>  double am2 = a[1]-m[1];
>  double bm2 = b[0]-m[0];
>
>  return ((am1)*(bm1)) - ((am2)*(bm2));
> }
>
> In the return statement the operands are all in parentheses. gcc optimizes

Parentheses don't have the meaning you believe they have in C. All those 
in your return statement are useless.

> the statement and introduces a FMA instruction. I think this is wrong because 
> FMA causes the subtraction and multiplication to be effectively executed at 
> the same time, while the source specifies that the multiplications should be 
> performed before the subtraction.

-ffp-contract=off

--
Marc Glisse