sundman@ttds.UUCP (Bo Sundman) (01/15/85)
C The example below reproduces a bug in f77.
C
C The error seems to be caused by using L both as loop variable and as the
C start value of the loop. It is clear from the output that the first loop
C is never executed as L obtains its large value, 2147479256, before the first
C test. The problem should thus have nothing to do with the question
C whether the loop variable is undefined or not after exiting the loop.
C
DIMENSION X(10)
L=1
DO 100 L=L,10
X(L)=L
100 CONTINUE
200 WRITE(*,*)L,X
I=1
DO 300 L=I,10
X(L)=L
300 CONTINUE
400 WRITE(*,*)L,X
END
C Output from the example:
C
C 2147479256 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.
C 11 1.00000 2.00000 3.00000 4.00000 5.00000 6.00000 7.00000
C 8.00000 9.00000 10.0000
--
Bo Sundman (..mcvax!enea!ttds!sundman)
Thermo-Calc group, Division of Physical Metallurgy
The Royal Institute of Technology
Stockholm, Swedenwoods@hao.UUCP (Greg Woods) (01/17/85)
> C The example below reproduces a bug in f77. > C > C The error seems to be caused by using L both as loop variable and as the > C start value of the loop. It is clear from the output that the first loop > C is never executed as L obtains its large value, 2147479256, before the first > C test. The problem should thus have nothing to do with the question > C whether the loop variable is undefined or not after exiting the loop. > C > DIMENSION X(10) > L=1 > DO 100 L=L,10 > X(L)=L > 100 CONTINUE This is not a bug. You are violating the standard. The standard does not allow this type of loop initialization. The order of evaluation in the DO 100 statement is not guaranteed to be what you think it will be. --Greg -- {ucbvax!hplabs | allegra!nbires | decvax!stcvax | harpo!seismo | ihnp4!stcvax} !hao!woods "...sometimes the light's all shining on me; other times I can barely see..."
west@sdcsla.UUCP (Larry West) (01/19/85)
In article <787@ttds.UUCP> sundman@ttds.UUCP (Bo Sundman) writes: >C > DIMENSION X(10) > L=1 > DO 100 L=L,10 > X(L)=L >100 CONTINUE >200 WRITE(*,*)L,X > I=1 > DO 300 L=I,10 > X(L)=L >300 CONTINUE >400 WRITE(*,*)L,X > END > >C Output from the example: >C >C 2147479256 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. >C 11 1.00000 2.00000 3.00000 4.00000 5.00000 6.00000 7.00000 >C 8.00000 9.00000 10.0000 Bo did not say what computer/OS this was done on, but I tried this out on a Sun and Vax-11/750, running nearly identical 4.2bsd. The bug only appeared on the Vax, withOUT the "-O" flag. According to "f77 -v", this is "Berkeley F77, version 1.0", while on the Sun it's "SMI F77, version 1.1". Looking at the assembly language output (VAX, w/o "-O"), it's easy to see the error: at the start of the loop, register 10 is loaded from itself: L13: movl $1,v.2-v.1(r11) movl r10,r10 cmpl r10,$10 jgtr L16 This is the first access to "r10" in the program. The correct code is emitted by the VAX f77 with "-O": L13: movl $1,v.2-v.1(r11) movl v.2-v.1(r11),r10 movl r10,r10 cmpl r10,$10 jgtr L16 Seems like optimization by comparison, of course... -- --| Larry West, UC San Diego, Institute for Cognitive Science --| UUCP: {decvax!ucbvax,ihnp4}!sdcsvax!sdcsla!west --| ARPA: west@NPRDC { NOT: <sdcsla!west@NPRDC> }
west@sdcsla.UUCP (Larry West) (01/19/85)
In article <787@ttds.UUCP> sundman@ttds.UUCP (Bo Sundman) writes: > DIMENSION X(10) > L=1 > DO 100 L=L,10 > X(L)=L >100 CONTINUE >200 WRITE(*,*)L,X > I=1 > DO 300 L=I,10 > X(L)=L >300 CONTINUE >400 WRITE(*,*)L,X > END > >C Output from the example: >C >C 2147479256 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. >C 11 1.00000 2.00000 3.00000 4.00000 5.00000 6.00000 7.00000 >C 8.00000 9.00000 10.0000 Bo did not say what computer/OS this was done on, but I tried this out on a Sun and Vax-11/750, running nearly identical 4.2bsd. The bug only appeared on the Vax, withOUT the "-O" flag. According to "f77 -v", this is "Berkeley F77, version 1.0", while on the Sun it's "SMI F77, version 1.1". Looking at the assembly language output (VAX, w/o "-O"), it's easy to see the error: at the start of the loop, register 10 is loaded from itself: L13: movl $1,v.2-v.1(r11) movl r10,r10 cmpl r10,$10 jgtr L16 This is the first access to "r10" in the program. The correct code is emitted by the VAX f77 with "-O": L13: movl $1,v.2-v.1(r11) movl v.2-v.1(r11),r10 movl r10,r10 cmpl r10,$10 jgtr L16 Seems like optimization by comparison, of course... -- --| Larry West, UC San Diego, Institute for Cognitive Science --| UUCP: {decvax!ucbvax,ihnp4}!sdcsvax!sdcsla!west --| ARPA: west@NPRDC { NOT: <sdcsla!west@NPRDC> }