Fwd: Re: Parentheses not honored when using FMA

[Tim Prince <tprince818 dot tp at gmail dot com>]

On 8/12/2015 7:03 AM, Marc Glisse wrote:
> 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.
No, this is a -std=gnu99 peculiarity which resembles K&R treatment.  I
know some people do mean K&R when they say C.
>
>> 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
>
Why not gcc -std=c99 .... ?  It appears to answer the original question. 

Speaking of compilers requiring options to achieve standard behavior:
icl -Qprotect-parens or icl -fp:strict  will suppress the fma on this
example.

Neither gcc nor icc are capable of always optimizing use (or not) of
fma.  We've done a lot of benchmarking of -mno-fma vs. (default)
-mfma.   icc is improving in this respect. 
g++ inner_product is slow with fma.  Altering the template seems
hopeless, as it doesn't optimize without -ffast-math (which discards
this aspect of -std=c99).

Why did gcc reject -fprotect-parens?  This is one of the reasons I find
I must split my gcc compiled source code into files which are to be
built with or without -ffast-math .

Going the other way, to cases where fma is preferable, I find it
annoying that gfortran translates
s(i)*aa(i)+aa(i)
the same as
(s(i)+1.)*aa(i)
and requires (what I don't consider obvious)
(s(i)*aa(i))+aa(i)
to implement by fma.  gcc doesn't try this non-optimization.  Fortran
rules do encourage the replacement
a*b + c*b => b*(a+c)  (avoiding a questionable use of fma)
but I dislike this variant of that optimization.

-- 
Tim Prince