/Os, /Ot (Favor Small Code, Favor Fast Code)
The latest version of this topic can be found at -Os, -Ot (Favor Small Code, Favor Fast Code).
Minimizes or maximizes the size of EXEs and DLLs.
Syntax
/Os
/Ot
Remarks
/Os (Favor Small Code) minimizes the size of EXEs and DLLs by instructing the compiler to favor size over speed. The compiler can reduce many C and C++ constructs to functionally similar sequences of machine code. Occasionally these differences offer tradeoffs of size versus speed. The /Os and /Ot options allow you to specify a preference for one over the other:
/Ot (Favor Fast Code) maximizes the speed of EXEs and DLLs by instructing the compiler to favor speed over size. (This is the default.) The compiler can reduce many C and C++ constructs to functionally similar sequences of machine code. Occasionally, these differences offer tradeoffs of size versus speed. The /Ot option is implied by the Maximize Speed (/O2) option. The /O2 option combines several options to produce very fast code.
If you use /Os or /Ot, then you must also specify /Og to optimize the code.
Note
Information that is gathered from profiling test runs will override optimizations that would otherwise be in effect if you specify /Ob, /Os, or /Ot. For more information, Profile-Guided Optimizations.
x86 Specific
The following example code demonstrates the difference between the Favor Small Code (/Os) options and the Favor Fast Code (/Ot) option:
Note
The following describes the expected behavior when using /Os or /Ot. However, compiler behavior from release to release may result in different optimizations for the code below.
/* differ.c
This program implements a multiplication operator
Compile with /Os to implement multiply explicitly as multiply.
Compile with /Ot to implement as a series of shift and LEA instructions.
*/
int differ(int x)
{
return x * 71;
}
As shown in the fragment of machine code below, when DIFFER.c is compiled for size (/Os), the compiler implements the multiply expression in the return statement explicitly as a multiply to produce a short but slower sequence of code:
mov eax, DWORD PTR _x$[ebp]
imul eax, 71 ; 00000047H
Alternately, when DIFFER.c is compiled for speed (/Ot), the compiler implements the multiply expression in the return statement as a series of shift and LEA instructions to produce a fast but longer sequence of code:
mov eax, DWORD PTR _x$[ebp]
mov ecx, eax
shl eax, 3
lea eax, DWORD PTR [eax+eax*8]
sub eax, ecx
END x86 Specific
To set this compiler option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see How to: Open Project Property Pages.
Click the C/C++ folder.
Click the Optimization property page.
Modify the Favor Size or Speed property.
To set this compiler option programmatically
- See FavorSizeOrSpeed.
See Also
/O Options (Optimize Code)
Compiler Options
Setting Compiler Options