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Weird optimization bug
- From: Ignacio Fernández Galván <jellby at yahoo dot com>
- To: fortran at gcc dot gnu dot org
- Date: Mon, 1 Oct 2007 18:17:04 +0100 (BST)
- Subject: Weird optimization bug
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Hi all,
Before filling a bug report, I'd like to have some comments here, at
least so I can write a better test case and description.
This test case is from lapack. I noticed that I had to compile a
particular subroutine (dlamc1) with -O0, otherwise the programs using
it would hang. So I made this program that just uses dlamc1 (it's
fixed-format, I hope the mailing system preserves spaces):
C=============================================
PROGRAM Test
IMPLICIT NONE
LOGICAL :: LIEEE1, LRND
INTEGER :: LBETA, LT
CALL DLAMC1( LBETA, LT, LRND, LIEEE1 )
WRITE(6,*) LBETA, LT, LRND, LIEEE1
END PROGRAM Test
SUBROUTINE DLAMC1( BETA, T, RND, IEEE1 )
LOGICAL IEEE1, RND
INTEGER BETA, T
LOGICAL FIRST, LIEEE1, LRND
INTEGER LBETA, LT
DOUBLE PRECISION A, B, C, F, ONE, QTR, SAVEC, T1, T2
DOUBLE PRECISION DLAMC3
EXTERNAL DLAMC3
SAVE FIRST, LIEEE1, LBETA, LRND, LT
DATA FIRST / .TRUE. /
INTEGER I
IF( FIRST ) THEN
FIRST = .FALSE.
ONE = 1
A = 1
C = 1
10 CONTINUE
IF( C.EQ.ONE ) THEN
A = 2*A
C = DLAMC3( A, ONE )
C = DLAMC3( C, -A )
GO TO 10
END IF
B = 1
C = DLAMC3( A, B )
C WRITE(6,*) 'Hi'
20 CONTINUE
IF( C.EQ.A ) THEN
B = 2*B
C = DLAMC3( A, B )
GO TO 20
END IF
QTR = ONE / 4
SAVEC = C
C = DLAMC3( C, -A )
LBETA = C + QTR
B = LBETA
F = DLAMC3( B / 2, -B / 100 )
C = DLAMC3( F, A )
IF( C.EQ.A ) THEN
LRND = .TRUE.
ELSE
LRND = .FALSE.
END IF
F = DLAMC3( B / 2, B / 100 )
C = DLAMC3( F, A )
IF( ( LRND ) .AND. ( C.EQ.A ) )
$ LRND = .FALSE.
T1 = DLAMC3( B / 2, A )
T2 = DLAMC3( B / 2, SAVEC )
LIEEE1 = ( T1.EQ.A ) .AND. ( T2.GT.SAVEC ) .AND. LRND
LT = 0
A = 1
C = 1
30 CONTINUE
IF( C.EQ.ONE ) THEN
LT = LT + 1
A = A*LBETA
C = DLAMC3( A, ONE )
C = DLAMC3( C, -A )
GO TO 30
END IF
END IF
BETA = LBETA
T = LT
RND = LRND
IEEE1 = LIEEE1
RETURN
END
DOUBLE PRECISION FUNCTION DLAMC3( A, B )
DOUBLE PRECISION A, B
DLAMC3 = A + B
RETURN
END
C=============================================
$ gfortran -O0 test.f ; ./a.out
2 53 T T
This is fine, but then:
$ gfortran -O1 test.f ; ./a.out
[the program runs with 100% cpu for at least one minute, ctrl+C]
Oddly enough, if I now uncomment the WRITE just before line labeled 20:
$ gfortran -O1 test.f ; ./a.out
Hi
2 53 F F
So, I would say it was a too-sensitive-to-optimization thing, but
fixing it with a WRITE tells me it must be a bug. Don't you think so?
My system:
$ gfortran -v
Using built-in specs.
Target: i386-pc-linux-gnu
Configured with: /home/fx/gfortran_nightbuild/trunk/configure
--prefix=/home/fx/gfortran_nightbuild/irun-20070930
--enable-languages=c,fortran --build=i386-pc-linux-gnu
--enable-checking=release
--with-gmp=/home/fx/gfortran_nightbuild/software
Thread model: posix
gcc version 4.3.0 20070930 (experimental) [trunk revision 128889] (GCC)
$ uname -a
Linux djelibeibi 2.6.17-14mdv #1 SMP Wed May 9 21:11:43 MDT 2007 i686
AMD Athlon(tm) XP 2800+ GNU/Linux
Thanks
Ignacio
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