[Bug fortran/44156] dot_product / matmul and signed zeros

["kargl at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from kargl at gcc dot gnu dot org  2010-05-17 23:18 -------
(In reply to comment #5)
> (In reply to comment #4)
> > We have complete control of whether to print the negative sign with
> > -fno-sign-zero.  I am tempted to say this is a no-never-mind situation.
> 
> Using the testcase shown in comment #3:
> 
> $> gfortran-svn pr44156-c3.f90 -fno-sign-zero && ./a.out
>    0.0000000       0.0000000       0.0000000    
> $> gfortran-svn pr44156-c3.f90 -fsign-zero && ./a.out
>    0.0000000       0.0000000      -0.0000000
> 
> The flag does work for the third value, i.e. the line
> 
>    real, parameter :: e = a(1) * b(1)
> 
> but has no say for the others.
> 
> 
> Out of curiosity, what do other compilers do? I, right now, don't have any to
> test with.
> 

I already explained what dot_product is doing in comment #2.
dot_product(a,b) is equivalent to

   s = 0
   do n = 1, m
      s = s + a(n) * b(n)
   end do
   (return s)

For Thomas' code, you end up with 0 + (-0), which is 0 (which
is what IEEE 754 explicitly says in Sec 8.3).  If someone wants
to get -0, then s/he needs to rewrite the in-lining of dot_product
to do

  s = a(1) * b(1)
  do n = 2, m
     s = s + a(n) * b(n)
  end do
  (return n)

Here, the loop won't be executed (and hopefully, the middle-end
reaps the dead code).  In the new looping structure, one might
ask whether it inhibits optimization and/or vectorization.


-- 

kargl at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sgk at troutmask dot apl dot
                   |                            |washington dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44156