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Does the C++ compiler optimize the multiply by two operation x*2 to a bitshift operation x<<1?
I would love to believe that yes.
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Compiler optimization is generally implemented using a sequence of optimizing transformations, algorithms which take a program and transform it to produce a semantically equivalent output program that uses fewer resources or executes faster.
Compilers are free to optimize code so long as they can guarantee the semantics of the code are not changed.
The C/C++ compiler compiles each source file separately and produces the corresponding object file. This means the compiler can only apply optimizations on a single source file rather than on the whole program. However, some important optimizations can be performed only by looking at the whole program.
Optimization of the code is often performed at the end of the development stage since it reduces readability and adds code that is used to increase the performance.
Actually VS2008 optimizes this to x+x:
01391000 push ecx int x = 0; scanf("%d", &x); 01391001 lea eax,[esp] 01391004 push eax 01391005 push offset string "%d" (13920F4h) 0139100A mov dword ptr [esp+8],0 01391012 call dword ptr [__imp__scanf (13920A4h)] int y = x * 2; 01391018 mov ecx,dword ptr [esp+8] 0139101C lea edx,[ecx+ecx] In an x64 build it is even more explicit and uses:
int y = x * 2; 000000013FB9101E mov edx,dword ptr [x] printf("%d", y); 000000013FB91022 lea rcx,[string "%d" (13FB921B0h)] 000000013FB91029 add edx,edx This is will the optimization settings on 'Maximize speed' (/O2)
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