[Bug middle-end/20434] New: pessimization of complex expression

[Bug middle-end/20434] New: pessimization of complex expression

[Thomas dot Koenig at online dot de]

$ cat complex-parts.c
#include <math.h>
#include <complex.h>

int main()
{
    float cr,ci;
    float complex c;
    foo(&cr,&ci);
    c = cr+I*ci;
    return creal(c)+cimag(c)<0;
}
$ gcc -S -O2 -fdump-tree-optimized complex-parts.c
$ tail -10 complex-parts.c.t66.optimized
  float D.2359;
  float cr.18;

<bb 0>:
  foo (&cr, &ci);
  return (double) (cr + 0.0) + (double) (ci + 0.0) < 0.0;

}


$ gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.1/configure --prefix=/home/ig25 --enable-languages=c,f95
Thread model: posix
gcc version 4.1.0 20050311 (experimental)

-- 
           Summary: pessimization of complex expression
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: Thomas dot Koenig at online dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[Thomas dot Koenig at online dot de]

------- Additional Comments From Thomas dot Koenig at online dot de  2005-03-11 21:59 -------
There are two strange things here:

- Why the + 0. ?

- Why the casts to double?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

Re: [Bug middle-end/20434] pessimization of complex expression

[Andrew Pinski]

On Mar 11, 2005, at 4:59 PM, Thomas dot Koenig at online dot de wrote:

>
> ------- Additional Comments From Thomas dot Koenig at online dot de  
> 2005-03-11 21:59 -------
> There are two strange things here:
>
> - Why the + 0. ?
>
> - Why the casts to double?
Because that is required by the C standard.

-- Pinski

[Bug middle-end/20434] pessimization of complex expression

[pinskia at physics dot uc dot edu]

------- Additional Comments From pinskia at physics dot uc dot edu  2005-03-11 22:41 -------
Subject: Re:  pessimization of complex expression


On Mar 11, 2005, at 4:59 PM, Thomas dot Koenig at online dot de wrote:

>
> ------- Additional Comments From Thomas dot Koenig at online dot de  
> 2005-03-11 21:59 -------
> There are two strange things here:
>
> - Why the + 0. ?
>
> - Why the casts to double?
Because that is required by the C standard.

-- Pinski



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[Thomas dot Koenig at online dot de]

------- Additional Comments From Thomas dot Koenig at online dot de  2005-03-11 22:49 -------
> > - Why the casts to double?
> Because that is required by the C standard.

Isn't that covered by the as-if rule?  I'm fairly
sure the cast to double won't change the result of
the "<" operator :-)

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-11 22:55 -------
(In reply to comment #3)
> > > - Why the casts to double?
> > Because that is required by the C standard.
> 
> Isn't that covered by the as-if rule?  I'm fairly
> sure the cast to double won't change the result of
> the "<" operator :-)

No but the addition is done in double.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[Thomas dot Koenig at online dot de]

------- Additional Comments From Thomas dot Koenig at online dot de  2005-03-12 09:45 -------
(In reply to comment #4)
> (In reply to comment #3)
> > > > - Why the casts to double?
> > > Because that is required by the C standard.
> > 
> > Isn't that covered by the as-if rule?  I'm fairly
> > sure the cast to double won't change the result of
> > the "<" operator :-)

> No but the addition is done in double.

Again, the as-if rule:  If a and b are floats, the expression
a+b < 0 should be the same for any a and b regardless wether they
are done in float or double.

The same is true, for float a,b and c, of

c = a+b;

It is _not_ true for exmperssions like c = a+b-a, of course.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-12 18:07 -------
(In reply to comment #5)
> Again, the as-if rule:  If a and b are floats, the expression
> a+b < 0 should be the same for any a and b regardless wether they
> are done in float or double.

Well the real reason is creal/cimag returns double and not float.
Use crealf/cimagf instead.

Well yes this is a missed optimization,  which should be filed separately.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[Thomas dot Koenig at online dot de]

------- Additional Comments From Thomas dot Koenig at online dot de  2005-03-12 23:13 -------
(In reply to comment #6)

> Well the real reason is creal/cimag returns double and not float.
> Use crealf/cimagf instead.

You're right, of course.  Doing that gets me

<bb 0>:
  foo (&cr, &ci);
  return cr + 0.0 + ci + 0.0 < 0.0;
 
OK, the direct cast has gone away.  I wonder if this still converts
to double, though...

> Well yes this is a missed optimization,  which should be filed separately.

I think using crealf/cimagf pretty much solves this.  If you lie to the
compiler... :-)



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[tkoenig at gcc dot gnu dot org]

------- Additional Comments From tkoenig at gcc dot gnu dot org  2005-07-07 09:14 -------
This appears to be fixed now in mainline:

$ cat complex-parts.c
#include <math.h>
#include <complex.h>

int main()
{
    float cr,ci;
    float complex c;
    foo(&cr,&ci);
    c = cr+I*ci;
    return crealf(c)+cimagf(c)<0;
}
$ gcc -S -O3 -fdump-tree-optimized complex-parts.c
$ tail -10 complex-parts.c.t82.optimized
  <unnamed type> D.2371;
  float cr.0;

<bb 0>:
  foo (&cr, &ci);
  return cr + ci < 0.0;

}


$ gcc -v
Using built-in specs.
Target: ia64-unknown-linux-gnu
Configured with: ../gcc-4.1-20050702/configure --prefix=/home/zfkts --enable-
languages=c,f95
Thread model: posix
gcc version 4.1.0 20050702 (experimental)


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.1.0
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434

[Bug middle-end/20434] pessimization of complex expression

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20434