[Bug tree-optimization/24333] New: missed div optimizations?

[Bug tree-optimization/24333] New: missed div optimizations?

[pluto at agmk dot net]

unsigned foo(const unsigned x) {  return (x / x); }
double bar(const double x) { return (x / x); }

$ gcc -Wall -O2 -mregparm=3 -fomit-frame-pointer tmp.c -S

foo:    xorl    %edx, %edx
        divl    %eax
        ret

bar:    fldl    4(%esp)
        fdiv    %st(0), %st
        ret

I think optimizer should return 1 for both cases.


-- 
           Summary: missed div optimizations?
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net
 GCC build triplet: i686-pld-linux
  GCC host triplet: i686-pld-linux
GCC target triplet: i686-pld-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations?

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-10-12 15:42 -------
(In reply to comment #0)

This one, we have to deal with x = 0, what should we do, trap or not to trap.
> unsigned foo(const unsigned x) {  return (x / x); }


For the following one:
> double bar(const double x) { return (x / x); }

Only with -ffast-math.


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations?

[pluto at agmk dot net]

------- Comment #2 from pluto at agmk dot net  2005-10-12 15:57 -------
basing on the http://en.wikipedia.org/wiki/L'hospital we get:

lim  (x / x) = lim (x' / x') = lim (1 / 1) = 1
x->0           x->0            x->0

should we trap/check anything?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations?

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2005-10-12 16:04 -------
(In reply to comment #2)
> basing on the http://en.wikipedia.org/wiki/L'hospital we get:
> 
> lim  (x / x) = lim (x' / x') = lim (1 / 1) = 1
> x->0           x->0            x->0

for float/float, we should get NANs with -ffast-math which assumes
-ffinite-math, for long/long and unsigned/unsigned, we have this thing called
undefinedness. BUT for java, 0/0 is defined to throw IIRC so we can only do
that for signed/signed and with -fno-wrapv.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2005-12-30 18:33 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2005-12-30 18:33:47
               date|                            |
            Summary|missed div optimizations?   |missed div optimizations


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2006-04-08 01:41 -------
x/x is done for fp now.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations

[pluto at agmk dot net]

------- Comment #6 from pluto at agmk dot net  2007-03-19 09:15 -------
`int f( int x ) { return x / x; }' still not optimized on 4.2/4.3


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

[Bug tree-optimization/24333] missed div optimizations

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

Gabriel Ravier <gabravier at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gabravier at gmail dot com

--- Comment #8 from Gabriel Ravier <gabravier at gmail dot com> ---
Seems to be fixed since GCC 7

[Bug tree-optimization/24333] missed div optimizations

[pluto at agmk dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24333

--- Comment #7 from Pawel Sikora <pluto at agmk dot net> 2011-06-22 21:56:27 UTC ---
4.6 still affected:

f:      movl    %edi, %edx
        movl    %edi, %eax
        sarl    $31, %edx
        idivl   %edi
        ret