Question

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.

Answer 1

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)