[Bug rtl-optimization/15492] New: floating-point arguments are loaded too early to x87 stack

[Bug rtl-optimization/15492] New: floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

This testcase shows another problem with x87 stack:

double test (double a, double b) {
        return a*a +  b*b;
}

In this case, floating point arguments are loaded too early to x87 stack,
with 'gcc -O2', resulting asm is:
test:
        pushl   %ebp
        movl    %esp, %ebp
        fldl    8(%ebp)
        fldl    16(%ebp)
        fxch    %st(1)
        fmul    %st(0), %st
        fxch    %st(1)
        popl    %ebp
        fmul    %st(0), %st
        faddp   %st, %st(1)
        ret
 
The situation is a little better with '-O2 -fomit-frame-pointer':
test:
        fldl    4(%esp)
        fldl    12(%esp)
        fmul    %st(0), %st
        fxch    %st(1)
        fmul    %st(0), %st
        faddp   %st, %st(1)
        ret

But optimal code would look something like this (second arg should be loaded to
stack in some kind of 'just in time' fashion.
test:
        fldl    4(%esp)
        fmul    %st(0), %st
        fldl    12(%esp)
        fmul    %st(0), %st
        faddp   %st, %st(1)
        ret

-- 
           Summary: floating-point arguments are loaded too early to x87
                    stack
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: uros at kss-loka dot si
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug rtl-optimization/15492] floating-point arguments are loaded too early to x87 stack

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-05-17 13:18 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |pessimizes-code
   Last reconfirmed|0000-00-00 00:00:00         |2004-05-17 13:18:50
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-08-17 08:00 -------
When testcase is compiled without optimizations (with gcc -fomit-frame-pointer),
following code is produced:

test:
        fldl    4(%esp)
        fmull   4(%esp)
        fldl    12(%esp)
        fmull   12(%esp)
        faddp   %st, %st(1)
        ret




-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-08-17 08:58 -------
A couple of other testcases (please note the number of fxch isns!):

double test1 (double a, int x, double b, int y, double c)
{
        return sin (c) + tan (b) * sqrt (a) + x * fabs (b) + y;
}

with 'gcc -ffast-math -fomit-frame-pointer':
test1:
        fldl    28(%esp)
        fsin
        fldl    16(%esp)
        fptan
        fstp    %st(0)
        fldl    4(%esp)
        fsqrt
        fmulp   %st, %st(1)
        faddp   %st, %st(1)
        fildl   12(%esp)
        fldl    16(%esp)
        fabs
        fmulp   %st, %st(1)
        faddp   %st, %st(1)
        fildl   24(%esp)
        faddp   %st, %st(1)
        ret

with 'gcc -O2 -ffast-math -fomit-frame-pointer':
test1:
        fldl    16(%esp)
        fldl    4(%esp)
        fld     %st(1)
        fptan
        fstp    %st(0)
        fldl    28(%esp)
        fxch    %st(3)
        fabs
        fxch    %st(2)
        fsqrt
        fxch    %st(3)
        fsin
        fxch    %st(1)
        fmulp   %st, %st(3)
        faddp   %st, %st(2)
        fildl   12(%esp)
        fmulp   %st, %st(1)
        faddp   %st, %st(1)
        fildl   24(%esp)
        faddp   %st, %st(1)
        ret

The shortest testcase could be:
double test1 (double y, double x)
{
        return atan2(x, y);
}

with 'gcc -ffast-math -fomit-frame-pointer':
test1:
        fldl    12(%esp)
        fldl    4(%esp)
        fpatan
        ret


with 'gcc -O2 -ffast-math -fomit-frame-pointer':
test1:
        fldl    4(%esp)
        fldl    12(%esp)
        fxch    %st(1)
        fpatan
        ret


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-08-19 08:07 -------
According to "How to optimize for the Pentium family of microprocessors" by
Agner Fog, "fld r/m32/m64" consumes one clock cycle on P1, PMMX, PPRO, P2, P3
and P4, and "fld m80" consumes 3 cycles on P1, PMMX and P4 and two cycles on
PPRO, P2 and P3.

This means, that code in all examples will be faster, because there is less fxch
instructions. Also, more fp-stack could be used to store temporary variables if
arguments are taken from stack when needed instead of copying their value
between fp stack registers.

There is a question if it is worth to special-case "fld m80" instructions to use
fp register copies instead of memory load. Again, a lot of fxch instructions
would be needed and fp stack space could be wasted with register copies.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-08-19 13:12 -------
Some discussion is here: 
  http://gcc.gnu.org/ml/gcc/2004-08/msg00939.html 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-09-13 13:06 -------
With a patch from Jan Hubicka:
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01178.html

'gcc -O2 -ffast-math -S -march=pentium4 -fomit-frame-pointer' now produces:

test1:
        fldl    4(%esp)
        fldl    16(%esp)
        fldl    28(%esp)
        fsin
        fld     %st(1)
        fptan
        fstp    %st(0)
        fxch    %st(3)
        fsqrt
        fmulp   %st, %st(3)
        faddp   %st, %st(2)
        fabs
        fimull  12(%esp)
        faddp   %st, %st(1)
        fiaddl  24(%esp)
        ret


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-09-13 13:13 -------
So, it looks like it's fixed?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |giovannibajo at libero dot
                   |                            |it


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-09-13 14:49 -------
(In reply to comment #7)
> So, it looks like it's fixed?

Hm, first testcase still produces (gcc -O2 -ffast-math -march=pentium4
-fomit-frame-pointer).
test:
        fldl    4(%esp)
        fldl    12(%esp)
        fxch    %st(1)
        fmul    %st(0), %st
        fxch    %st(1)
        fmul    %st(0), %st
        faddp   %st, %st(1)
        ret

It looks that gcc does not know that arguments to add can be exchanged and is
emitting fxch insns to get arguments in the top stack position.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2005-06-21 11:24 -------
reg-stack has been reworked quite a bit recently.  Where do we stand on 
this one now? 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-21 11:31 -------
The first one is still producing the same old stupid code.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
   Last reconfirmed|2005-06-20 02:33:16         |2005-06-21 11:31:39
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492

Uros Bizjak <ubizjak at gmail dot com> changed:

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

--- Comment #12 from Uros Bizjak <ubizjak at gmail dot com> 2013-01-13 11:28:23 UTC ---
*** Bug 55952 has been marked as a duplicate of this bug. ***

[Bug target/15492] floating-point arguments are loaded too early to x87 stack

[hubicka at gcc dot gnu dot org]

------- Comment #11 from hubicka at gcc dot gnu dot org  2006-07-21 17:07 -------
Unless reg-stack is told how to reschedule the instructions to match register
stack constraints, I don't think we can get much closer to solving the problem
that some instruction orderings needs a lot more fxchs than others...

Honza


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15492