automatic vectorization

[Lynn McGuire]

Do many of the prominent C++ compilers support automatic
vectorization, if at all ? I was wondering which of the
following code would get vectorized, if either ?

double x (1000);
for (int i = 0; i < 1000; i++)
x = 0;

-or-

double x (1000);
memset (x, 0, sizeof (x));

Thanks,
Lynn

 

[Victor Bazarov]

Do many of the prominent C++ compilers support automatic
vectorization, if at all ? I was wondering which of the
following code would get vectorized, if either ?

double x (1000);

You probably meant to declare 'x' as an array, not to initialize it to
1000, yes?

double x[1000];

for (int i = 0; i < 1000; i++)
x = 0;

-or-

double x (1000);
memset (x, 0, sizeof (x));

Regardless of the capabilities to vectorize anything, you should
probably consider initialization of an array using the language feature:

double x[1000] = {}; // initialized to "all zeros"

and let the compiler worry about speeding this up.

Unless, of course, your task is to micro-optimize your code, which is
usually compiler-specific, and as such is most likely OT here.

V

 

[Lynn McGuire]

double x (1000);

You probably meant to declare 'x' as an array, not to initialize it to 1000, yes?

double x[1000];

Sigh. Been doing too much fortran lately.

Regardless of the capabilities to vectorize anything, you should probably consider initialization of an array using the language
feature:

double x[1000] = {}; // initialized to "all zeros"

and let the compiler worry about speeding this up.

Cool ! I did not understand this. I will try to remember
this now.

Unless, of course, your task is to micro-optimize your code, which is usually compiler-specific, and as such is most likely OT here.

Not at all.

Thanks,
Lynn

 

[Jonathan Lee]

Do many of the prominent C++ compilers support automatic
vectorization, if at all ?  I was wondering which of the
following code would get vectorized, if either ?

At the risk of being off-topic, I know GCC has made
some headway wrt vectorization:

http://gcc.gnu.org/projects/tree-ssa/vectorization.html

I think it's enabled automatically nowadays (with -O3).

No idea about other compilers
--Jonathan

 

[Lynn McGuire]

http://gcc.gnu.org/projects/tree-ssa/vectorization.html

I saw that when I googled automatic vectorization and C++.
Also http://stackoverflow.com/questions/409329/practical-use-of-automatic-vectorization .

Automatic vectorization does not appear to be as much as a topic
for C++ compiler vendors as it does for fortran vendors:
http://absoft.com/Absoft_Vectorization.htm

Thanks,
Lynn

 

[Ian Collins]

At the risk of being off-topic, I know GCC has made
some headway wrt vectorization:

http://gcc.gnu.org/projects/tree-ssa/vectorization.html

I think it's enabled automatically nowadays (with -O3).

No idea about other compilers

Sun Studio has some support via the -xvector switch. It also (along
with gcc I think) has OpenMP support.