Re: [Valgrind-users] Uninitialized values with fortran

[John R. <jr...@bi...>]

> I found that the mxn submatrix was always initialized at all its
> entries.  I did this several times, until "gdb continue" resulted
> in the program completing (because of looping, the breakpoint
> re-occurred several times).
> 
> This is why I am puzzled as to why valgrind is reporting uninitialised value(s).

Thank you for the test case.  After installing lapack and blas (Fedora 17),
then I see the behavior you describe.  I did not get the corresponding debuginfo
versions, so I have not seen inside iladr.

What I did do is binary search on:
----- tsvd3.cpp
  // Elliminate spurious valgrind uninitialized errors
#if 1
  for( int iii=38; iii<lwork; ++iii ) work[iii]=123.456;
#endif
-----
I see no complaints when starting the loop at iii=1,2,4,8,16,32;
then errors at 64,48,40; no complaint at 36; errors at 38;
no complaint at 37.  Hmmm...

At this point it's at least somewhat reasonable to file a bug report
and include the test case.  The rejoinder will be that memcheck
finds a real error, and the code "Eliminate ... uninit errors"
is the proof.  I'm not so sure.

I did notice that all the complaints on x86_64 are after a 'ucomisd'
instruction where the low 64 bits of the register have a double floating
point value, with the high 64 bits being zero.  When the other operand
to ucomids also is a register, then that register often has all bits
zero (128 zero bits).  In most ways the operating width is 64 bits,
yet the register has 128 bits.

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