From lisaev@umail.iu.edu Thu May  6 16:15:22 2010
Date: Thu, 6 May 2010 16:15:22 -0400
From: Leonid Isaev <lisaev@umail.iu.edu>
To: lapack@cs.utk.edu
Subject: Possible bug in BLAS ?rotm.f (and possibly other) subroutines
Message-ID: <20100506201522.GA25819@umail.iu.edu>
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Hello,

	While compiling and testing SVD routines in LAPack 3.2.1, I came across
a fortran runtime error, which complained about BLAS' s(d)rotm.f files. I used:
(i) gcc<4.5
    FC = gfortran
    FFLAGS = -fbounds-check
(ii) gcc=4.5
     FC = gfortran
     FFLAGS = -fcheck=all
The error message is:

SVD: Testing Singular Value Decomposition routines
./xeigtsts < svd.in > ssvd.out
At line 74 of file srotm.f
Fortran runtime error: Index '2' of dimension 1 of array 'sx' above upper bound
of 1
make: *** [ssvd.out] Error 2

and similar for drotm.f. Apparently, the program performs no bounds check for
the sx(*) array. I think, the situation is similar to something like this:

real::sx(1)
...
if ((.false.).and.(sx(2)>0)) then
...
end if

So, "if" will never work, but the compiler is allowed to evaluate the
condition in the parenthesis, thus sometimes resulting in a segfault, which
would be pretty hard to catch especially in a big application.
	I haven't seen similar errors about other BLAS routines, but it does
not mean there aren't any.

Thank you,
Leonid.
PS: I also attach the output from gdb.

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GNU gdb (GDB) 7.1
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/lisaev/web_buffer/lapack-3.2.1/TESTING/xeigtsts...(no debugging symbols found)...done.
(gdb) r <svd.in
Starting program: /home/lisaev/web_buffer/lapack-3.2.1/TESTING/xeigtsts <svd.in
 Tests of the Singular Value Decomposition routines

 LAPACK VERSION 3.2.1

 The following parameter values will be used:
    M:         0     0     0     1     1     1     2     2     3     3
               3    10    10    16    16    30    30    40    40
    N:         0     1     3     0     1     2     0     1     0     1
               3    10    16    10    16    30    40    30    40
    NB:        1     3     3     3    20
    NBMIN:     2     2     2     2     2
    NX:        1     0     5     9     1
    NS:        2     0     2     2     2

 Relative machine underflow is taken to be    0.117549E-37
 Relative machine overflow  is taken to be    0.340282E+39
 Relative machine precision is taken to be    0.596046E-07

 Routines pass computational tests if test ratio is less than   35.00


 SBD routines passed the tests of the error exits ( 43 tests done)


 SBD routines passed the tests of the error exits ( 43 tests done)



 SVD:  NB =   1, NBMIN =   2, NX =   1, NRHS =   2

 SBD -- Real Singular Value Decomposition
 Matrix types (see xCHKBD for details):
 Diagonal matrices:
   1: Zero                             5: Clustered entries
   2: Identity                         6: Large, evenly spaced entries
   3: Evenly spaced entries            7: Small, evenly spaced entries
   4: Geometrically spaced entries
 General matrices:
   8: Evenly spaced sing. vals.       12: Small, evenly spaced sing vals
   9: Geometrically spaced sing vals  13: Random, O(1) entries
  10: Clustered sing. vals.           14: Random, scaled near overflow
  11: Large, evenly spaced sing vals  15: Random, scaled near underflow

 Test ratios:  (B: bidiagonal, S: diagonal, Q, P, U, and V: orthogonal
                X: m x nrhs, Y = Q' X, and Z = U' Y)
   1: norm( A - Q B P' ) / ( norm(A) max(m,n) ulp )
   2: norm( I - Q' Q )   / ( m ulp )
   3: norm( I - P' P )   / ( n ulp )
   4: norm( B - U S V' ) / ( norm(B) min(m,n) ulp )
   5: norm( Y - U Z )    / ( norm(Z) max(min(m,n),k) ulp )
   6: norm( I - U' U )   / ( min(m,n) ulp )
   7: norm( I - V' V )   / ( min(m,n) ulp )
   8: Test ordering of S  (0 if nondecreasing, 1/ulp  otherwise)
   9: norm( S - S2 )     / ( norm(S) ulp ), where S2 is computed
                                            without computing U and V'
  10: Sturm sequence test (0 if sing. vals of B within THRESH of S)
  11: norm( A - (QU) S (V' P') ) / ( norm(A) max(m,n) ulp )
  12: norm( X - (QU) Z )         / ( |X| max(M,k) ulp )
  13: norm( I - (QU)'(QU) )      / ( M ulp )
  14: norm( I - (V' P') (P V) )  / ( N ulp )
 M=   40, N=   40, type 10, seed=3992,3027,2249, 789, test( 9)=  37.59    
 SBD:      1 out of   5510 tests failed to pass the threshold
At line 74 of file srotm.f
Fortran runtime error: Index '2' of dimension 1 of array 'sx' above upper bound of 1

Program exited with code 02.

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