[Bug target/39085] Floating point errors on Arm9 processor

[rearnsha at gcc dot gnu dot org]

------- Comment #6 from rearnsha at gcc dot gnu dot org  2009-03-25 21:54 -------
(In reply to comment #2)
>           str       r3, [fp, #-32]
>           str       r4, [fp, #-28]
>           ldfd      f0, [fp, #-32]
>           sqtd      f0, f0
>           stfd      f0, [fp, #-40]
>           ldfd      f0, [fp, #-40]
>           ldfd      f0, [fp, #-40]
>           cmf       f0, f0
>           beq       .L2    <============
>           sub       r0, fp, #32
>           ldmia     r0, {r0-r1}
>           bl        sqrt
>           stfd      f0, [fp, #-40]

You need to understand what this code is doing to work out the source of the
bug.  The compiler is inlining a copy of the square-root function with an FPA
co-processor instruction

    sqtd      f0, f0

which, if successful will replace the library call.  If the square root
succeeds it will not return a NaN, so the comparison of the result

    cmf       f0, f0

will be that the two results are equal (a NaN never compares equal to itself).

So the square root function (sqrt) will only be called if a NaN was generated
by the machine instruction; it does this in order to set errno.

Of course, the ARM9 does not have an FPA co-processor so the original sqtd
instruction will be emulated by your operating system -- I strongly suspect
that that is what is generating the incorrect results and you now need to
investigate why.

If you still believe the bug is in the compiler you need to provide some
additional information, including how you configured the compiler and what
options you used when compiling your code.


-- 

rearnsha at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at gcc dot gnu dot
                   |                            |org
             Status|WAITING                     |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39085