The program below compiled with gfortran 4.4.0 (and 4.3.2) and using the
-O3 optimization flag generates, in subroutine SMALL, sse instructions
which assume that the data array C is aligned on a 16 byte boundary.
For example: mulpd (%rax,%rcx,1),%xmm0
where register %rax may not be 16-byte aligned.
This leads to a segmentation violation at run time.
The test program deliberately forces array C to be unaligned before
being passed to subroutine SMALL. In a real situation one would
not do this, of course, but when the memory holding array C comes from
somewhere else (e.g. a program written in a different language) there
is no guarantee of alignment.
gfortran version 4.2.0 did not display this problem - the code
generated for SMALL did not assume alignment.
C Declare a REAL array as workspace. We pass it to a routine
C expecting a DOUBLE PRECISION array. This allows us to pass
C data aligned or not aligned on a 16 byte boundary.
real c(100)
integer m
external tt
m = 2
C Call tt with array C aligned on 16 byte boundary
call tt(m, c(1))
C Call tt with array C not aligned on 16 byte boundary
call tt(m, c(2))
end
subroutine tt(m, c)
double precision c(m,m)
external small
do i = 1, m
do j = 1, m
c(i,j) = 1.0d0
end do
end do
call small(m, c)
write (*,*) 'After call small(m, c), c(1,1) = ', c(1,1)
end
subroutine small(m, c)
integer m
double precision c(m,*)
integer i
do 70, i = 1, m
c(i,1) = c(i,1) + c(i,1)
70 continue
return
end
Here's the output from the -v flag of the compiler:
% gfortran -v -O3 align.f
Driving: gfortran -v -O3 align.f -lgfortranbegin -lgfortran -lm -shared-libgcc
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: /opt/gcc-4.4.0-build/gcc-4.4.0/configure
--enable-languages=c,fortran,c++ --prefix=/opt/gcc-4.4.0
--with-mpfr=/opt/mpfr-2.4.1 --with-gmp=/opt/gmp-4.1.4
Thread model: posix
gcc version 4.4.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
/opt/gcc-4.4.0/libexec/gcc/x86_64-unknown-linux-gnu/4.4.0/f951 align.f
-ffixed-form -quiet -dumpbase align.f -mtune=generic -auxbase align -O3
-version -fintrinsic-modules-path
/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/finclude -o
/tmp/cc9eHSWv.s
GNU Fortran (GCC) version 4.4.0 (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.4.0, GMP version 4.1.4, MPFR version 2.4.1.
warning: GMP header version 4.1.4 differs from library version 4.2.
warning: MPFR header version 2.4.1 differs from library version 2.2.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
as -V -Qy -o /tmp/cc05mKcU.o /tmp/cc9eHSWv.s
GNU assembler version 2.19 (x86_64-unknown-linux-gnu) using BFD version (GNU
Binutils) 2.19
COMPILER_PATH=/opt/gcc-4.4.0/libexec/gcc/x86_64-unknown-linux-gnu/4.4.0/:/opt/gcc-4.4.0/libexec/gcc/x86_64-unknown-linux-gnu/4.4.0/:/opt/gcc-4.4.0/libexec/gcc/x86_64-unknown-linux-gnu/:/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/:/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/
LIBRARY_PATH=/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/:/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
/opt/gcc-4.4.0/libexec/gcc/x86_64-unknown-linux-gnu/4.4.0/collect2
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2
/usr/lib/../lib64/crt1.o /usr/lib/../lib64/crti.o
/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/crtbegin.o
-L/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0
-L/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/../../../../lib64
-L/lib/../lib64 -L/usr/lib/../lib64
-L/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/../../..
/tmp/cc05mKcU.o -lgfortranbegin -lgfortran -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/opt/gcc-4.4.0/lib/gcc/x86_64-unknown-linux-gnu/4.4.0/crtend.o
/usr/lib/../lib64/crtn.o
--
Summary: gfortran -O3 optimization generates code that seg faults
on unaligned array data
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mick at nag dot co dot uk
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40037
