Order of statements using if

[aeismail]

Hi, everybody:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

My question comes because I want to know if there is any advantage in
writing the test condition to favor one block over the other. (For the
code I'm working with, one case is definitely preferred over the
other--at least 2 to 1.)

Thanks,

--AEI

[Ron AF Greve]

Hi,

I believe the x86 type of procesors expect to fall through (thus if compiled
'litterally' the if condition being true).

However if your compiler does or doesn't optimize for that one can only
guess. I would recommend to just try it (and still then it might depend on
the condition).
Regards, Ron AF Greve

http://www.InformationSuperHighway.eu

<ae******@gmail.comwrote in message
news:11**********************@k35g2000prh.googlegr oups.com...

Hi, everybody:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

My question comes because I want to know if there is any advantage in
writing the test condition to favor one block over the other. (For the
code I'm working with, one case is definitely preferred over the
other--at least 2 to 1.)

Thanks,

--AEI

[Neelesh Bodas]

On Sep 19, 9:32 pm, [e-mail] wrote:

Hi, everybody:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

My question comes because I want to know if there is any advantage in
writing the test condition to favor one block over the other. (For the
code I'm working with, one case is definitely preferred over the
other--at least 2 to 1.)

The exact implementation of if-else construct is not specified by the
standard, and hence the answer would vary depending on the platform
and implementation. In other words, trying to write if-else clause
based on certain heuristic might not help if you are writing code that
runs across platforms and implementations.

-N

[Eric Johnson]

Another thing to consider is that modern CPUs perform branch
prediction, so it might not really matter that much whether your
compiler optimizes in a certain particular way. From what I
understand the branch prediction in hardware is quite good these
days. Check out this article for more info:

http://en.wikipedia.org/wiki/Branch_prediction

[Michal Nazarewicz]

Eric Johnson <er***********@gmail.comwrites:

Another thing to consider is that modern CPUs perform branch
prediction, so it might not really matter that much whether your
compiler optimizes in a certain particular way.

Yep, when code is run for the first time processor has no data about
execution of code and branches taken previously so it can only guess.
In x86 the guess is as follows: If the branch is forward it is guessed
not to be taken and if the branch is backward it is guessed to be taken.

--
Best regards, _ _
.o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
ooo +--<mina86*tlen.pl>---<jid:mina86*chrome.pl>--ooO--(_)--Ooo--

[Michael Angelo Ravera]

On Sep 19, 10:51 am, Eric Johnson <[e-mail]:

Another thing to consider is that modern CPUs perform branch
prediction, so it might not really matter that much whether your
compiler optimizes in a certain particular way. From what I
understand the branch prediction in hardware is quite good these
days. Check out this article for more info:

http://en.wikipedia.org/wiki/Branch_prediction

If there is code following both the "if" and the "else" case, there
has to be a branch to the common following code from the "if"
alternative and a branch "to" the conditional code for the "else"
alternative (maybe no branch to the common code, however).

If, however, there are a number of tests, the most common would be
most efficiently first.

So, if there is code following and only two alternatives and the the
excecution paths rejoin, there is likey to be no difference at all.

[Michal Nazarewicz]

Michael Angelo Ravera <ma******@prodigy.netwrites:

On Sep 19, 10:51 am, Eric Johnson <[e-mail]:

>Another thing to consider is that modern CPUs perform branch
prediction, so it might not really matter that much whether your
compiler optimizes in a certain particular way. From what I
understand the branch prediction in hardware is quite good these
days. Check out this article for more info:

http://en.wikipedia.org/wiki/Branch_prediction

If there is code following both the "if" and the "else" case, there
has to be a branch to the common following code from the "if"
alternative and a branch "to" the conditional code for the "else"
alternative (maybe no branch to the common code, however).

But the branch in the "if part" is unconditional and as such does not
need a branch prediction.

If, however, there are a number of tests, the most common would be
most efficiently first.

So, if there is code following and only two alternatives and the the
excecution paths rejoin, there is likey to be no difference at all.

There is, as conditional jumps are those that make the difference.

--
Best regards, _ _
.o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
ooo +--<mina86*tlen.pl>---<jid:mina86*chrome.pl>--ooO--(_)--Ooo--

[James Kanze]

On Sep 19, 6:32 pm, [e-mail] wrote:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

Why? Does your profiler say that this is causing a bottleneck
in your code?

My question comes because I want to know if there is any advantage in
writing the test condition to favor one block over the other. (For the
code I'm working with, one case is definitely preferred over the
other--at least 2 to 1.)

The usual rule is to write the shorter block first, provided
that this doesn't make the conditional expression harder to
read. Worrying about performance in this sort of thing is a
total waste of time.

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Jim Langston]

<ae******@gmail.comwrote in message
news:11**********************@k35g2000prh.googlegr oups.com...

Hi, everybody:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

My question comes because I want to know if there is any advantage in
writing the test condition to favor one block over the other. (For the
code I'm working with, one case is definitely preferred over the
other--at least 2 to 1.)

consider a large if.. else block
if ( a )
// do
else if ( b )
// do
else if ( c)
// do
else if ( d )
// do
else
// do

Normally they are quite fast, but if cycles are tight you want to ensure
that condition a will happen more than b, b more than the rest, etc...

For the program to check condition d, it will have had to check a,b and c.
If the conditional check is trivial it usually doesn't matter, a cycle or
two. If it is a more complicated conditional check then it can matter a
lot. A switch statement can be more effecient since it becomes pretty much
just a jump table, but then you have to be able to have an ordinal value to
check, which is not always possible.

Usually, however, people don't need to concern themselves with the
effecientcy of if else blocks as they don't take that much time. If you've
found that this is your bottle neck, the first thing to do is make sure the
conditions that happen most are at the top.

[Scott Gifford]

ae******@gmail.com writes:

Hi, everybody:

A quick question regarding how to write if/then blocks.

Do C++ compilers care in terms of execution (i.e., efficiency) which
block in an if/then/else loop is executed? That is, will going to the
"else" block be less efficient than using the "then" block?

Some compilers have an option to optimize the code depending on how
often particular branches are taken. In g++, you can use the
"-fprofile-arcs" option to instrument your code, run it on some sample
data to generate statistics, then re-compile with
"-fbranch-probabilities" to give the compiler a hint about how often
the various branches are taken. I haven't tried this, but I've heard
it gives a speedup in some cases. Perhaps your compiler has similar
options you could try.

Good luck!

---Scott.