Outside of the ensuring that they cannot be changed (to the tune of a compiler error), does the JIT make any optimisations for const locals?
Eg.
public static int Main(string[] args)
{
const int timesToLoop = 50;
for (int i=0; i<timesToLoop; i++)
{
// ...
}
}
The generated IL is different (using Release mode):
using constant local using normal local
---------------------------------------------------------------------
.entrypoint .entrypoint
.maxstack 2 .maxstack 2
.locals init ( .locals init (
[0] int32 i) [0] int32 timesToLoop,
L_0000: ldc.i4.0 [1] int32 i)
L_0001: stloc.0 L_0000: ldc.i4.s 50
L_0002: br.s L_0008 L_0002: stloc.0
L_0004: ldloc.0 L_0003: ldc.i4.0
L_0005: ldc.i4.1 L_0004: stloc.1
L_0006: add L_0005: br.s L_000b
L_0007: stloc.0 L_0007: ldloc.1
L_0008: ldloc.0 L_0008: ldc.i4.1
L_0009: ldc.i4.s 50 L_0009: add
L_000b: blt.s L_0004 L_000a: stloc.1
L_000d: ret L_000b: ldloc.1
L_000c: ldloc.0
L_000d: blt.s L_0007
L_000f: ret
As you can see the compiler replaces all variable usages by the value of the constant which results in a smaller stack.
I gave the code a quick performance test, using Snippet Compiler. The code I used is as follows:
public static void Main()
{
DateTime datStart = DateTime.UtcNow;
const int timesToLoop = 1000000;
for (int i=0; i < timesToLoop; i++)
{
WL("Line Number " + i.ToString());
}
DateTime datEnd = DateTime.UtcNow;
TimeSpan tsTimeTaken = datEnd - datStart;
WL("Time Taken: " + tsTimeTaken.TotalSeconds);
RL();
}
Note, WL and RL are simply helper methods to read and write to the console.
To test the non-constant version, I simply removed the const keyword. The results were surprising:
Time Taken (average of 3 runs)
Using const keyword 26.340s
Without const keyword 28.276s
I'm aware that this is very rough'n'ready test, but the use of the const keyword appears to count as a valid micro-optimization.
Your code (using const) will actually be compiled as:
public static int Main(string[] args){
for (int i=0; i < 50; i++)
{
}
}
while a variable will compile as a variable:
public static int Main(string[] args){
int timesToLoop = 50;
for (int i=0; i < timesToLoop; i++)
{
}
}
This is not anywhere near an answer, just thought it would be nice to share this however the article does not mention runtime benefits explicitly:
Coding Standard Rule #2: Use const Wherever Possible
Excerpt:
Reasoning: The upside of using const as much as possible is compiler-enforced protection from unintended writes to data that should be read-only.
One difference is that if you had an assembly that referenced a const field in another assembly and you later changed that value, the referencing assembly would still use the old value until it was rebuilt.
