[Bug rtl-optimization/21809] New: [3.4/4.0/4.1 Regression] Floating Optimization Bug

[Bug rtl-optimization/21809] New: [3.4/4.0/4.1 Regression] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

This problems occurs with GCC 3.4.4, gcc-4.0-20050521 snapshot and
gcc-4.1-20050528 snapshot.


test-case.c:
#include <assert,h>
volatile float x = 3;
int main()
{
float a = 1 / x;
x = a;
assert(a == x);
}


This case (test-case.c) works with gcc -O0 without a problem.

But with gcc {-O1,-O2,-O3,-Os} causes the following error:
test: test.c:9: main: Assertion 'a == x' failed'

-- 
           Summary: [3.4/4.0/4.1 Regression] Floating Optimization Bug
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: themis_hv at yahoo dot co dot uk
                CC: gcc-bugs at gcc dot gnu dot org,themis_hv at yahoo dot
                    co dot uk
 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=21809

[Bug rtl-optimization/21809] [3.4/4.0/4.1 Regression] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 16:24 -------


*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug rtl-optimization/21809] [3.4/4.0/4.1 Regression] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 16:39 -------
Note every GCC from 2.95.3 gives the same result.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug rtl-optimization/21809] [3.4/4.0/4.1 Regression] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 16:44 -------
x and a should be identical, so assertion should not fail at all.

since a is assigned to x, it has *SAME* rounding precision.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 17:04 -------
This is a target problem, as the RTL is correct.
Looks like there is a forgotten rounding back to float.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |target
  GCC build triplet|i686-pc-linux-gnu           |
   GCC host triplet|i686-pc-linux-gnu           |
            Summary|[3.4/4.0/4.1 Regression]    |Floating Optimization Bug
                   |Floating Optimization Bug   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 17:58 -------
This also occurs with double, using test-case.c but with float replaced with
double, so code fragment looks like:

test-case.c:
#include <assert,h>
volatile double x = 3;
int main()
{
double a = 1 / x;
x = a;
assert(a == x);
}


Should I put this as separate PR?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 18:27 -------
(In reply to comment #5)
> Should I put this as separate PR?

Actually this is all a dup of bug 323.  The "problem" is excessive pression, which most non fp 
developers will not know about, read the full PR 323 and the references to papers which talk about this.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:06 -------
when you store it to memory the precission goes down (aka rounding) read 323 and all the rest of the 
problems related to it.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:01 -------
It should be logical equivalent regardless of how it stored in memory.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:24 -------
Please go read the papers.  Basically x87 is broken in this respect, use either a different machine or use 
SSE.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:18 -------
Surely assigning a float value to another float variable should not cause any
rounding as they are same data type.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:28 -------
Read the code carefully:

test-case.c:
#include <assert,h>
volatile float x = 3;
int main()
{
float a = 1 / x;
x = a;
assert(a == x);
}


Notice x = a before assertion, both of these variables are of the same data type.

This is *not* related to precission.

This is behaviour, you would expect from a compiler.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:32 -------
(In reply to comment #10)
> Please go read the papers.  Basically x87 is broken in this respect, use
eithera different machine or use 
> SSE.
It be good idea to do that by default then?

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:31 -------
(In reply to comment #11)
> This is *not* related to precission.
> 
> This is behaviour, you would expect from a compiler.

For non floating point, yes but floating point is different with respect with precission.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:35 -------
(In reply to comment #13)
> (In reply to comment #10)
> > Please go read the papers.  Basically x87 is broken in this respect, use
> eithera different machine or use 
> > SSE.
> It be good idea to do that by default then?
It is on x86_64, remember SSE is not every where.

Just use -ffloat-store instead.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:36 -------
> > It be good idea to do that by default then?
> It is on x86_64, remember SSE is not every where.
> 
x86-64 has support for SSE3 so it would use that instead.




-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[joseph at codesourcery dot com]

------- Additional Comments From joseph at codesourcery dot com  2005-05-29 19:37 -------
Subject: Re:  New: [3.4/4.0/4.1 Regression]
 Floating Optimization Bug

On Sun, 29 May 2005, themis_hv at yahoo dot co dot uk wrote:

> This case (test-case.c) works with gcc -O0 without a problem.
> 
> But with gcc {-O1,-O2,-O3,-Os} causes the following error:
> test: test.c:9: main: Assertion 'a == x' failed'

Not if you use -ffloat-store, as is required if you wish to avoid such 
instances of paradoxical excess precision.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:43 -------
(In reply to comment #15)
> x86-64 has support for SSE3 so it would use that instead.

Actually that is wrong, the subset which is supported by AMD and Intel (EM64T) only have SSE, SSE2, 
and MMX.   AMD's x86_64 which in my mind the true x86_64 has also support for 3dnow.  Intel's EM64t 
have SSE3.

Again just use -ffloat-store as required not get the excessive precision.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:47 -------
That is not going to change, the assert is allowed to fail by the standard by the way.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:46 -------
> Again just use -ffloat-store as required not get the excessive precision.
> 

This should included in gcc spec file by defaults.




-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-05-29 19:56 -------
(In reply to comment #20)
> Yes, assert fails in some cases (I think of a hundred at moment!).

See now you did not read my comment, I said it is _____ALLOWED____ by the standard to ___FAIL___.
How much clearer do you need it?

ALSO READ UP on excessive precision.  You seem like someone who does not want to do the leg work 
of getting your programs fixed so you don't depend on this.

Note this is how x87 works, live with it.  Just stop buying x86 machines if you don't like this.

*** This bug has been marked as a duplicate of 323 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 19:51 -------
(In reply to comment #19)
> That is not going to change, the assert is allowed to fail by the standard by
the way.
> 

Yes, assert fails in some cases (I think of a hundred at moment!).



-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|DUPLICATE                   |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809

[Bug target/21809] Floating Optimization Bug

[themis_hv at yahoo dot co dot uk]

------- Additional Comments From themis_hv at yahoo dot co dot uk  2005-05-29 20:09 -------
> You seem like someone who does not want to do the leg work 
> of getting your programs fixed so you don't depend on this.
Regardless, other poeple dependant on it.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21809