mesa3d-dev

[Mesa3d-dev] [Bug 27286] New: Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286

           Summary: Mod function returns wrong value
           Product: Mesa
           Version: unspecified
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
        AssignedTo: mes...@li...
        ReportedBy: kar...@ar...


The following shader returns incorrect output. It looks like the GLSL Compiler
doesnt implement mod correctly.

varying vec var1;
void main (void)
{
        gl_FragColor = vec4(mod(4.0, 2.0), 0.0, 0.0, 1.0);
}

It should be the same as,

varying vec var1;
void main (void)
{
        float var = 4.0 - (2.0 * (floor(4.0/2.0)) ); //According to spec
        gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
}


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.

[Mesa3d-dev] [Bug 27286] Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286


Karthik Hariharakrishnan <kar...@ar...> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         OS/Version|All                         |Linux (All)
           Platform|Other                       |x86 (IA32)
            Version|unspecified                 |7.6




--- Comment #1 from Karthik Hariharakrishnan <kar...@ar...>  2010-03-24 04:41:43 PST ---
Changing the version of Mesa, Hardware and OS info in bug


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.

[Mesa3d-dev] [Bug 27286] Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286





--- Comment #2 from Brian Paul <bri...@gm...>  2010-03-24 07:53:33 PST ---
Can you provide a stand-alone test program that demonstrates this?

I tried it myself and I got the expected result: mod(4.0, 2.0) returns 0.0

Mesa's mod() function is implemented just as the spec says:

float mod(const float a, const float b)
{
    float oneOverB;
    __asm float_rcp oneOverB, b;
    __retVal = a - b * floor(a * oneOverB);
}


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.

[Mesa3d-dev] [Bug 27286] Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286





--- Comment #3 from Karthik Hariharakrishnan <kar...@ar...>  2010-03-24 08:13:10 PST ---
(In reply to comment #2)
> Can you provide a stand-alone test program that demonstrates this?
> 
> I tried it myself and I got the expected result: mod(4.0, 2.0) returns 0.0
> 
> Mesa's mod() function is implemented just as the spec says:
> 
> float mod(const float a, const float b)
> {
>     float oneOverB;
>     __asm float_rcp oneOverB, b;
>     __retVal = a - b * floor(a * oneOverB);
> }
> 

Hi Brian,

  Could you try running the program against the library in lib/gallium.
export LD_LIBRARY_PATH=<blah>/Mesa-7.7/lib/gallium

The test passes when the following is set
export LD_LIBRARY_PATH=<blah>/Mesa-7.7/lib/

Thanks


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.

[Mesa3d-dev] [Bug 27286] Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286





--- Comment #4 from Brian Paul <bri...@gm...>  2010-03-24 08:37:42 PST ---
OK, looks like I wasn't running w/ X86/SSE codegen enabled.  Looks like a bug
in that code.  I'll try to take a look in a while.


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.

[Mesa3d-dev] [Bug 27286] Mod function returns wrong value

[<bug...@fr...>]

http://bugs.freedesktop.org/show_bug.cgi?id=27286





--- Comment #5 from Brian Paul <bri...@gm...>  2010-03-25 17:23:47 PST ---
Off-hand, I _think_ this is an issue with the SSE rcp (reciprocol) instruction.
 Per the comment in the code, I think we need to produce a more accurate
result:

static void
emit_rcp (
   struct x86_function *func,
   unsigned xmm_dst,
   unsigned xmm_src )
{
   /* On Intel CPUs at least, this is only accurate to 12 bits -- not
    * good enough.  Need to either emit a proper divide or use the
    * iterative technique described below in emit_rsqrt().
    */
   sse2_rcpps(
      func,
      make_xmm( xmm_dst ),
      make_xmm( xmm_src ) );
}

Maybe someone handier with SSE can try to fix this.  The emit_rsqrt() function
further down uses a Newton-Raphson step to improve the results.  That could
help.


-- 
Configure bugmail: http://bugs.freedesktop.org/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are the assignee for the bug.