Parentheses not honored when using FMA

[Marcin Krotkiewski <marcin.krotkiewski@xxxxxxxxx>]

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 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.

This is the assembly generated by 'gcc-5.1.0 -O3 -mfma test.c -c -S'

        .cfi_startproc
        vmovsd  8(%rdi), %xmm0
        vmovsd  8(%rsi), %xmm2
        vmovsd  (%rdi), %xmm1
        vsubsd  %xmm0, %xmm2, %xmm3
        vmovsd  (%rdx), %xmm2
        vsubsd  %xmm1, %xmm2, %xmm2
        vmulsd  %xmm2, %xmm3, %xmm2
        vmovsd  (%rsi), %xmm3
        vsubsd  %xmm1, %xmm3, %xmm3
        vmovsd  8(%rdx), %xmm1
        vsubsd  %xmm0, %xmm1, %xmm1
        vfmsub132sd     %xmm1, %xmm2, %xmm3
        vmovapd %xmm3, %xmm0
        ret
        .cfi_endproc

I believe that in order to honor the parentheses gcc should NOT generate 
the FMA, but stick to the programmed operation order. If I wanted a FMA, 
I would not use parentheses around the multiplications. Current behavior 
of gcc results in inconsistent behavior in my code due to floating point 
arithmetic errors.

As a side note, the Intel compiler does exactly the same thing.

Can this be considered a bug, or is this the expected behavior? If this 
is ok, how can I assure in a portable way the effect that I need?

Thanks!

Marcin Krotkiewski