Is the C# Compiler Smart Enough to Optimize This Code

Is the C# compiler smart enough to optimize this code?

First off, the only way to actually answer performance questions is to actually try it both ways and test the results in realistic conditions.

That said, the other answers which say that "the compiler" does not do this optimization because the property might have side effects are both right and wrong. The problem with the question (aside from the fundamental problem that it simply cannot be answered without actually trying it and measuring the result) is that "the compiler" is actually two compilers: the C# compiler, which compiles to MSIL, and the JIT compiler, which compiles IL to machine code.

The C# compiler never ever does this sort of optimization; as noted, doing so would require that the compiler peer into the code being called and verify that the result it computes does not change over the lifetime of the callee's code. The C# compiler does not do so.

The JIT compiler might. No reason why it couldn't. It has all the code sitting right there. It is completely free to inline the property getter, and if the jitter determines that the inlined property getter returns a value that can be cached in a register and re-used, then it is free to do so. (If you don't want it to do so because the value could be modified on another thread then you already have a race condition bug; fix the bug before you worry about performance.)

Whether the jitter actually does inline the property fetch and then enregister the value, I have no idea. I know practically nothing about the jitter. But it is allowed to do so if it sees fit. If you are curious about whether it does so or not, you can either (1) ask someone who is on the team that wrote the jitter, or (2) examine the jitted code in the debugger.

And finally, let me take this opportunity to note that computing results once, storing the result and re-using it is not always an optimization. This is a surprisingly complicated question. There are all kinds of things to optimize for:

  • execution time

  • executable code size -- this has a major effect on executable time because big code takes longer to load, increases the working set size, puts pressure on processor caches, RAM and the page file. Small slow code is often in the long run faster than big fast code in important metrics like startup time and cache locality.

  • register allocation -- this also has a major effect on execution time, particularly in architectures like x86 which have a small number of available registers. Enregistering a value for fast re-use can mean that there are fewer registers available for other operations that need optimization; perhaps optimizing those operations instead would be a net win.

  • and so on. It get real complicated real fast.

In short, you cannot possibly know whether writing the code to cache the result rather than recomputing it is actually (1) faster, or (2) better performing. Better performance does not always mean making execution of a particular routine faster. Better performance is about figuring out what resources are important to the user -- execution time, memory, working set, startup time, and so on -- and optimizing for those things. You cannot do that without (1) talking to your customers to find out what they care about, and (2) actually measuring to see if your changes are having a measurable effect in the desired direction.

Is the C# or JIT compiler smart enough to handle this?

The C# compiler will not optimize this, and will emit just all operations for calling the property getters.

The JIT compiler on the other hand, might do a better job by inlining those method calls, but won't be able to optimize this any further, because it has no knowledge of your domain. Optimizing this could theorethically lead to wrong results, since your object graph could be constructed as follows:

var father = new Father
{
Child = new Child
{
Father = new Father
{
Child = new Child
{
Father = new Father { ... }
}
}
};

Or does it take longer to run codesnippet #1?

The answer is "Yes", it would take longer to run the first code snippet, because neither C# nor the JIT can optimize this away.

Will C# compiler and optimization break this code?

No, neither the compiler, nor JIT, will optimize your method call away.

There is a list of what the JIT compiler does. It does optimize away if (false) { ... } blocks for example, or unused variable assignments. It does not just optimize away your method calls. If that was true, every call to a void method should be gone too.

How smart is the compiler at optimizing string concatenation.

I took your code and compiled it in a simple Console application. Then, I examined the IL (using ILSpy). No optimizations were made in either Debug or Release mode.

In this case, the code is probably simple enough that the compiler didn't make any optimizations. However, more complicated versions might yield different results.

Also, note that there are very few differences between the two examples. In both cases, the runtime will end up making four different string objects. In the first example, assignment to a variable is only done twice, whereas in the second example assignment is done three times. However, the four strings are created, regardless. They are as follows:

  • "abcdefghijklmnopqrstuvwxyz"
  • "abcdefghijklmno" (from Substring)
  • "..."
  • "abcdefghijklmno..." (from concatenation)

The first three strings will be almost immediately eligible for garbage collection in both cases. I'm guessing the compiler didn't see any significant ways to improve what you have.

c#: Will this code get optimized out?

So, as you've found via Reflector, the C# compiler will not optimize it out. Whether the JIT compiler will is another question. But, I would guess the answer is almost certainly not.

Why? Because the JIT compiler doesn't know that IndexOf is a boring method. In other words, as far as the JIT compiler knows, string.IndexOf could be defined as

public int IndexOf()
{
CallAWebService();
}

Obviously, in that case optimizing out that line would be bad.

At what level C# compiler or JIT optimize the application code?

You may want to take a look at these articles:

JIT Optimizations - (Sasha Goldshtein - CodeProject)

Jit Optimizations: Inlining I (David Notario)

Jit Optimizations: Inlining II (David Notario)

To be honest you shouldn't be worrying too much about this level of micro-detail. Let the compiler/JIT'er worry about this for you, it's better at it than you are in almost all cases. Don't get hung up on Premature Optimisation. Focus on getting your code working, then worry about optimisations later on if (a) it doesn't run fast enough, (b) you have 'size' issues.

Compiler optimization of repeated accessor calls

From what I know, the C# compiler doesn't optimize this, because it can't be certain of side-effects (e.g. what if you have accessCount++ in the getter?) Take a look here at an excellent answer by Eric Lippert

From that answer:

The C# compiler never ever does this sort of optimization; as noted, doing so would require that the compiler peer into the code being called and verify that the result it computes does not change over the lifetime of the callee's code. The C# compiler does not do so.

The JIT compiler might. No reason why it couldn't. It has all the code sitting right there. It is completely free to inline the property getter, and if the jitter determines that the inlined property getter returns a value that can be cached in a register and re-used, then it is free to do so. (If you don't want it to do so because the value could be modified on another thread then you already have a race condition bug; fix the bug before you worry about performance.)

Just a note, seeing as Eric's on the C# compiler team, I trust his answer :)

Is the C# compiler smart enough to optimize this code?

First off, the only way to actually answer performance questions is to actually try it both ways and test the results in realistic conditions.

That said, the other answers which say that "the compiler" does not do this optimization because the property might have side effects are both right and wrong. The problem with the question (aside from the fundamental problem that it simply cannot be answered without actually trying it and measuring the result) is that "the compiler" is actually two compilers: the C# compiler, which compiles to MSIL, and the JIT compiler, which compiles IL to machine code.

The C# compiler never ever does this sort of optimization; as noted, doing so would require that the compiler peer into the code being called and verify that the result it computes does not change over the lifetime of the callee's code. The C# compiler does not do so.

The JIT compiler might. No reason why it couldn't. It has all the code sitting right there. It is completely free to inline the property getter, and if the jitter determines that the inlined property getter returns a value that can be cached in a register and re-used, then it is free to do so. (If you don't want it to do so because the value could be modified on another thread then you already have a race condition bug; fix the bug before you worry about performance.)

Whether the jitter actually does inline the property fetch and then enregister the value, I have no idea. I know practically nothing about the jitter. But it is allowed to do so if it sees fit. If you are curious about whether it does so or not, you can either (1) ask someone who is on the team that wrote the jitter, or (2) examine the jitted code in the debugger.

And finally, let me take this opportunity to note that computing results once, storing the result and re-using it is not always an optimization. This is a surprisingly complicated question. There are all kinds of things to optimize for:

  • execution time

  • executable code size -- this has a major effect on executable time because big code takes longer to load, increases the working set size, puts pressure on processor caches, RAM and the page file. Small slow code is often in the long run faster than big fast code in important metrics like startup time and cache locality.

  • register allocation -- this also has a major effect on execution time, particularly in architectures like x86 which have a small number of available registers. Enregistering a value for fast re-use can mean that there are fewer registers available for other operations that need optimization; perhaps optimizing those operations instead would be a net win.

  • and so on. It get real complicated real fast.

In short, you cannot possibly know whether writing the code to cache the result rather than recomputing it is actually (1) faster, or (2) better performing. Better performance does not always mean making execution of a particular routine faster. Better performance is about figuring out what resources are important to the user -- execution time, memory, working set, startup time, and so on -- and optimizing for those things. You cannot do that without (1) talking to your customers to find out what they care about, and (2) actually measuring to see if your changes are having a measurable effect in the desired direction.

C# compiler optimizations for benchmarking purposes

Compiler (JIT) may optimize whole function call out if it finds out that it has no side effects. It likely need to be able to inline function to detect that.

I tried small function that only acts on input arguments and see it is optimized out by checking resulting assembly (make sure to try Release build with "Suppres optimization on module load" unchecked).

 ...
for (int i = 0; i < 1000; i++)
{
int res = (int) Func(i);
}
...

static int Func(int arg1)
{
return arg1 * arg1;
}

Disassembly:

      for (int i = 0; i < 1000; i++) 
00000016 xor eax,eax
00000018 inc eax
00000019 cmp eax,3E8h
0000001e jl 00000018
}


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