Optimization with local vs. global arrays

[Jim West]

The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

There is considerable work being performed in the ... section.
This is on a Linux Itanium II system, compiled both with the Intel C++
compiler (V9.1) with interprocedural optimization enabled, and with the
GNU C V 3.3.5 compiler with -O3 optimization. (The performance change is
more dramatic with the Intel Compiler.) I tried declaring the local
FOO arrays static with

static FOO a[10], b[10], c[10];

which helped with the GNU compiler but was actually worse with the Intel
compiler. I also tried

FOO d[30];
FOO *a = d, *b = d+10, *c = d+20;

with a local d array, but that had no effect.

Is this just a compiler issue, or am I missing something? I want to avoid
the external arrays, obviously, but that code compiled by the Intel
compiler gives the fastest execution speed by far. I'd like to get the
equivalent performance with something less dangerous than global arrays.

[GeekBoy]

"Jim West" <eg***********@ yahoo.comwrote in message
news:9F******** ***********@new sfe20.lga...

The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

There is considerable work being performed in the ... section.
This is on a Linux Itanium II system, compiled both with the Intel C++
compiler (V9.1) with interprocedural optimization enabled, and with the
GNU C V 3.3.5 compiler with -O3 optimization. (The performance change is
more dramatic with the Intel Compiler.) I tried declaring the local
FOO arrays static with

static FOO a[10], b[10], c[10];

which helped with the GNU compiler but was actually worse with the Intel
compiler. I also tried

FOO d[30];
FOO *a = d, *b = d+10, *c = d+20;

with a local d array, but that had no effect.

Is this just a compiler issue, or am I missing something? I want to avoid
the external arrays, obviously, but that code compiled by the Intel
compiler gives the fastest execution speed by far. I'd like to get the
equivalent performance with something less dangerous than global arrays.

Faster processor?

[Mark P]

Jim West wrote:

The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

There is considerable work being performed in the ... section.
This is on a Linux Itanium II system, compiled both with the Intel C++
compiler (V9.1) with interprocedural optimization enabled, and with the
GNU C V 3.3.5 compiler with -O3 optimization. (The performance change is
more dramatic with the Intel Compiler.) I tried declaring the local
FOO arrays static with

static FOO a[10], b[10], c[10];

which helped with the GNU compiler but was actually worse with the Intel
compiler. I also tried

FOO d[30];
FOO *a = d, *b = d+10, *c = d+20;

with a local d array, but that had no effect.

Is this just a compiler issue, or am I missing something? I want to avoid
the external arrays, obviously, but that code compiled by the Intel
compiler gives the fastest execution speed by far. I'd like to get the
equivalent performance with something less dangerous than global arrays.

It's not especially surprising that the local arrays, which may be
pushed on the stack with each invocation of bar, would be slower than
the global arrays. If you want something "safer" you could try moving
the arrays to a namespace.

Mark

[Jim West]

On 2007-04-12, GeekBoy <ne*@nerdy.comw rote:

>
Faster processor?

No, all are run on the same system, OS etc. It is compiled with
the Intel compiler using

icc -O3 -ip -c foo.cc

and with the GNU compiler using

g++ -O3 -c foo.cc

[Jim West]

On 2007-04-12, Mark P <us****@fall200 5REMOVE.fastmai lCAPS.fmwrote:

It's not especially surprising that the local arrays, which may be
pushed on the stack with each invocation of bar, would be slower than
the global arrays. If you want something "safer" you could try moving
the arrays to a namespace.

OK, I had thought that the time needed to push the small arrays on the
stack (FOO isn't a very large class) would be small compared to the
heavy number crunching I do in the bar() routine. Guess not!

The namespace solution is what I needed, since some of the array names
are reused through-out the code. Seems obvious once it was pointed out.
:)

Thanks for the help.

[Ian Collins]

Jim West wrote:

The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

What is a FOO?

Does it require construction?

Do you call bar() in a loop?

--
Ian Collins.

[Jim West]

On 2007-04-12, Ian Collins <ia******@hotma il.comwrote:

Jim West wrote:

>The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

What is a FOO?

Does it require construction?

Do you call bar() in a loop?

FOO is actually a three-dimensional space vector:

class FOO {
float x, y, z;
FOO() : x_(0), y_(0), z_(0) { };
FOO(float x, float y, float z) : x_(x), y_(y), z_(z) { };
inline FOO& operator+=(cons t FOO& a);
/* Many more inline operators and member functions included */
};

bar() is called many times in a loop.

[Ian Collins]

Jim West wrote:

On 2007-04-12, Ian Collins <ia******@hotma il.comwrote:

>>Jim West wrote:

>>>The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

What is a FOO?

Does it require construction?

Do you call bar() in a loop?

FOO is actually a three-dimensional space vector:

class FOO {
float x, y, z;
FOO() : x_(0), y_(0), z_(0) { };
FOO(float x, float y, float z) : x_(x), y_(y), z_(z) { };
inline FOO& operator+=(cons t FOO& a);
/* Many more inline operators and member functions included */
};

bar() is called many times in a loop.

So there's your reason - FOO() gets called 30 times for each call of bar().

--
Ian Collins.

[GeekBoy]

"Ian Collins" <ia******@hotma il.comwrote in message
news:58******** *****@mid.indiv idual.net...

Jim West wrote:

>The execution speed of the following code is dramatically faster if I
declare some arrays globally rather than locally. That is

FOO a[10], b[10], c[10];

void bar() {
...
}

runs much faster (up to 33%) than

void bar() {
FOO a[10], b[10], c[10];
...
}

What is a FOO?

Foobar is a universal variable understood to represent whatever is being
discussed.
It's usually used in examples that illustrate concepts and ideas in computer
science.
For instance, a computer science professor may be discussing different file
formats. In this case, he would call the generic-example file foo or foobar,
then list the extensions associated with the file formats (e.g. foobar.txt,
foobar.gif, foobar.exe, foobar.tar).

When foo or foobar is used, everyone understands that these are just
examples, and they don't really exist.
Programmers and administrators also use foo and foobar in a similar context.
Files or program s named with foo or foobar are understood not to be
permanent and will be changed or deleted at anytime.
Foo, bar, and the compound foobar were commonly used at MIT, Stanford and
the Helsinki University of Technology, Finland. Other generic variables are
used other places, but only these three are considered universal.

Does it require construction?

Do you call bar() in a loop?

--
Ian Collins.

[Ian Collins]

GeekBoy wrote:

"Ian Collins" <ia******@hotma il.comwrote in message
news:58******** *****@mid.indiv idual.net...

>>
What is a FOO?

When foo or foobar is used, everyone understands that these are just
examples, and they don't really exist.

Not in this case, if you read the OP's reply.

>
Foo, bar, and the compound foobar were commonly used at MIT, Stanford and
the Helsinki University of Technology, Finland. Other generic variables are
used other places, but only these three are considered universal.

If you haven't done so already, research the origin of the term.

>>
--
Ian Collins.

*Please* don't quote signatures.

--
Ian Collins.