https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63474
Bug ID: 63474
Summary: Optimizer bug causes crash on unaligned integer writes
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jaf at meyersound dot com
Created attachment 33660
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33660&action=edit
trivial example program to reproduce the fault
I think this is an optimizer bug, since Intel machines should be able to
perform non-word-aligned integer writes without crashing. Also, the crash only
occurs if optimization is enabled.
To reproduce:
1) On an Intel machine, compile and run the attached trivial example program
using gcc 4.7.2, like this:
root@debian7:/home/jaf/cs6/software# gcc --version
gcc (Debian 4.7.2-5) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
root@debian7:/home/jaf/cs6/software# gcc -O3 ./bug_in_gcc_4.7.2.c
root@debian7:/home/jaf/cs6/software# ./a.out
Testing for g++ unaligned-word-writes optimizer bug... (sizeof(int)==4)
Begin unaligned writes to 8 words...
Segmentation fault
Note the segmentation fault above, which is unexpected. Compiling with -O2
optimization or lower gives the expected (non-crashing) behavior:
root@debian7:/home/jaf/cs6/software# gcc -O2 ./bug_in_gcc_4.7.2.c
root@debian7:/home/jaf/cs6/software# ./a.out
Testing for g++ unaligned-word-writes optimizer bug... (sizeof(int)==4)
Begin unaligned writes to 8 words...
End unaligned writes. Resulting buffer is:
4a 4a 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a
4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a 4a
