Csdrhrt

This can be useful if there are #if statements, ala

   if (0)

   {

       // Empty block

   }

#if TEST1_ENABLED

   else if (test1())

   {

      action1();

   }

#endif

#if TEST2_ENABLED

   else if (test2())

   {

      action2();

   }

#endif

etc.

In this case, any (and all) of the tests can be #if'ed out, and the code will compile correctly. Almost all compilers will remove the if (0) {} part.
A simple autogenerator could generate code like this, as it is slightly easier to code - it doesn't have to consider the first enabled block separately.

I've seen a similar pattern used in generated code. For example, in SQL, I've seen libraries emit the following where clause.

where 1 = 1

This presumably makes it easier to just add on other criteria, because all additional criteria can be prepended with and instead of an additional check to see if it is the first criteria or not.

As written, the if (0) {} clause compiles out to nothing.

I suspect the function of the clause at the top of this ladder is to provide an easy place to temporarily disable all the other functionality at once (for debugging or comparison purposes) by changing the 0 to a 1 or true.

I am not sure of any optimizations, but my two cents:

This happened because of some code modification, where one primary condition was removed, (the function call in initial if block, let's say), but the developers/ maintainers

• were lazy to restructure the if-else block


• did not want to go down on the branch coverage count
  

so instead of removing the associated if block, they simply changed the condition to if(0) and moved on.

One possibility not yet mentioned: the if (0) { line could be providing a convenient spot for a breakpoint.

Debugging is often done on non-optimised code so the always-false test will be present and able to have breakpoint set on it. When compiled for production, the line of code would be optimised out. The seemingly useless line gives functionality for development and testing builds without impacting release builds.

There are other good suggestions above as well; the only way to really know what the purpose is, is to track down the author and ask. Your source code control system might help with that. (Look for blame-type functionality.)

It's code rot.

At some point that "if" did something useful, the situation changed, maybe the variable being evaluated was removed.

The person who was fixing/changing the system did as little as possible to affect the logic of the system so he just made sure the code would recompile. So he leaves an "if(0)" because that's quick and easy and he's not totally sure that's what he wants to do. He gets the system working and he doesn't go back to fix it completely.

Then the next developer comes along and thinks that was done deliberately and only comments out that part of the code (since it's not being evaluated anyway), then the next time the code is touched those comments are removed.

I think it's just bad code. Writing a quick example in Compiler Explorer, we see that in both gcc and clang no code is generated for the if (0) block, even with optimizations completely disabled:

https://godbolt.org/z/PETIks

Playing around with removing the if (0) causes no changes to the generated code, so I conclude that this is not an optimization.

It's possible that there used to be something in the top if block which was later removed. In short, it looks like removing it would cause the exact same code to be generated, so feel free to do that.

As it's been said, the zero is evaluated to false, and the branch will likely be optimized out by the compiler.

I've also seen this before in code where a new feature was added and a kill-switch was needed (if something goes wrong with the feature you can just turn it off), and some time later when the kill-switch was removed the programmer didn't also remove the branch, e.g.

if (feature_a_active()) {

    use_feature_a();

} else if (some_fn()) {

   ...

became

if (0) {

   // empty

} else if (some_fn()) {

   ...

I've seen non reachable code blocks in pre-expanded JavaScript that have been generated using a templating language.

For instance, the code you are reading could have been pasted from a server that pre-evaluated the first condition that at that time relied on a variable only available on server side.

if ( ${requestIsNotHttps} ){ ... }else if( ...

which once pre-compiled hences :

if ( 0 ){ ... }else if ( ...

hope this helps you relativise the potential low keyboard activity of the pro-recycling coders era for which i manifest enthusiasm !

I have seen this a few times, I think the most likely reason is it was evaluating something in an older/different version/branch of the code, or possibly for debugging, and changing it to if(0) is a somewhat lazy way of removing whatever was there.