http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49103
Summary: local variables exchange values / wrong code with -O3
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: thomas.or...@awi.de
Host: x86_64-unknown-linux-gnu
Target: x86_64-unknown-linux-gnu
Build: x86_64-unknown-linux-gnu
Created attachment 24321
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24321
test program exposing the bug
After upgrading to gfortran 4.6.0, I noticed crashes in my codebase. This seems
to boil down to some local messup of variable values. I observe this with -O3,
but not with -O2 plus all available flags that are included in -O3. So, this
must be one of the internal optimizations that are not accessible via
individual flags.
The example is ripped out of a larger codebase and it shows. I spent over a
whole day diagnosing and trimming down already. Hopefully, you will mainly have
to look at the lower 100 lines. The routine grid_faces is the point of
interest, with a call to base_new triggering the issue.
The bad example:
shell$ gfortran -O3 gcc46gridbug.f90
shell$ ./a.out
LOG: Coordinate mapping: 1 2 3
LOG: back mapping: 1 2 3
LOG: Base point: 0.0000000000000000 0.0000000000000000
0.0000000000000000
generating base functions
grid: faces
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 1 1
Compare the base grades: 1 1
grades again 2 2
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 1 1
Compare the base grades: 1 1
grades again 2 2
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 1 1
Compare the base grades: 1 1
grades again 2 2
grid finished
PASS
And the good example:
shell$ gfortran -O2 gcc46gridbug.f90
shell$ ./a.out
LOG: Coordinate mapping: 1 2 3
LOG: back mapping: 1 2 3
LOG: Base point: 0.0000000000000000 0.0000000000000000
0.0000000000000000
generating base functions
grid: faces
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
grades again 2 2
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
grades again 2 2
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
Here is the bug. The grades should be (2 2): 2 2
Compare the base grades: 1 1
grades again 2 2
grid finished
PASS
The code worked with gcc 4.3.3 and 4.5.1, as well as the intel compiler 11.1. I
hesitate to count in open64 and the sun/oracle compilers as, while they might
not show this bug, they have enough others that stopped me from using them:-/
If this still should be an error on my side, I'd appreciate to be enlightened.
