[Bug tree-optimization/58039] New: -ftree-vectorizer make a loop crash on non-aligned memory

[bar at mariadb dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58039

Bug ID: 58039
   Summary: -ftree-vectorizer make a loop crash on non-aligned
memory
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: major
  Priority: P3
 Component: tree-optimization
  Assignee: unassigned at gcc dot gnu.org
  Reporter: bar at mariadb dot org

Created attachment 30578
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30578&action=edit
The program that repeats the report crash

If I compile the attached program using:

gcc -Wall -O2 -fno-inline -ftree-vectorize -ftree-vectorizer-verbose=2 a.c

it crashes with "segmentation fault".


$ gcc --version
gcc (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2)

Processor: Intel® Core™ i7-3520M CPU @ 2.90GHz × 4


The program is a minimal extract from the MariaDB-10.0 sources
that reproduces the crash.

The GCC flags that are actually used in the debug build of MariaDB are:
gcc -Wall -O3 -fno-inline a.c

but after tracking it down we noticed that the actually reason is
-ftree-vectorize.

[Bug tree-optimization/58039] -ftree-vectorizer make a loop crash on non-aligned memory

[bar at mariadb dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58039

--- Comment #1 from Alexander Barkov  ---
The bug is known to repeat on the following operating systems:

- Fedora 17
- Ubuntu 13.04
- OpenSUSE 11.1

[Bug tree-optimization/58039] -ftree-vectorizer makes a loop crash on a non-aligned memory

[bar at mariadb dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58039

--- Comment #2 from Alexander Barkov  ---
Any updates? Thanks.

[Bug tree-optimization/58039] -ftree-vectorizer makes a loop crash on a non-aligned memory

[bar at mariadb dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58039

--- Comment #4 from Alexander Barkov  ---
Mikael, thanks for  your comment on this.

(In reply to Mikael Pettersson from comment #3)
> Your code performs mis-aligned uint16_t stores, which x86 allows.

Right, this is done for performance purposes.


> The
> vectorizer turns those into larger and still mis-aligned `movdqa' stores,
> which x86 does not allow, hence the SEGV.

Can you please clarify: is it a bug in the recent gcc versions?

Note, we've used such performance improvement tricks for years.
It worked perfectly fine until now.
Has anything changed in how the gcc vectorizer works recently?


> 
> Replace the non-portable mis-aligned stores with portable code like
> 
> #define int2store_little_endian(s,A) memcpy((s), &(A), 2)
> 
> or gcc-specific code like
> 
> struct __attribute__((__packed__)) packed_uint16 {
> uint16_t u16;
> };
> #define int2store_little_endian(s,A) ((struct packed_uint16*)(s))->u16 = (A)
> 
> and then the vectorizer generates large `movdqu' stores, which is pretty
> much the best you can hope for unless you rewrite the code to avoid
> mis-aligned stores.


Unfortunately it's not possible to avoid mis-aligned stores due to the
project architecture.


I've read somewhere that gcc vectorizer generates two code branches,
for aligned memory and for non-aligned memory (but can't find
the reference now). Can you please confirm this?

Thanks.