.NET multiplication optimization

问题:

Does the compiler optimize out any multiplications by 1? That is, consider:

int a = 1;
int b = 5 * a;

Will the expression 5 * a be optimized into just 5? If not, will it if a is defined as:

const int a = 1;

回答1:

It will pre-calculate any constant expressions when it compiles, including string concatenation. Without the const it will be left alone.

Your first example compiles to this IL:

.maxstack 2
.locals init ([0] int32, [1] int32)

ldc.i4.1   //load 1
stloc.0    //store in 1st local variable
ldc.i4.5   //load 5
ldloc.0    //load 1st variable
mul        // 1 * 5
stloc.1    // store in 2nd local variable 

The second example compiles to this:

.maxstack 1
.locals init ( [0] int32 )

ldc.i4.5 //load 5 
stloc.0  //store in local variable

回答2:

Constant propagation is one of the most common and easiest optimisations.

回答3:

Looking at the code generated by the mono compiler, the version with the non-const a performs the multiplication at run time. That is, the multiplication is not optimized out. If you make a const, then the multiplication is optimized out.

The Microsoft compiler might have a more aggressive compiler, the best solution is to look at the code generated by the compiler to see what it is doing.

回答4:

What the compiler would optimise here is not multiplication by 1 per-se, but rather arithmetic with values known at compile-time. So yeah, a compiler would optimise out all the maths in your example, with or without the const.

Edit: A competent compiler, I should say.