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
