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/Os, /Ot (Favor Small Code, Favor Fast Code)

  • Article

The /Os and /Ot compiler options specify whether to favor size (/Os) or speed (/Ot) when optimizing code.

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. /Ot is the default when optimizations are enabled. 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 /O2 (Maximize speed) option. The /O2 option combines several options to produce faster code.

Note

Information that's gathered from profiling test runs overrides any optimizations that would otherwise be in effect if you specify /Ob, /Os, or /Ot. For more information, see Profile-Guided Optimizations.

x86-specific example

The following example code demonstrates the difference between the /Os (Favor small code) option and the /Ot (Favor fast code) option:

Note

This example 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

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Select the Configuration Properties > C/C++ > Optimization property page.

  3. Modify the Favor Size or Speed property.

To set this compiler option programmatically

See also

/O options (Optimize code)
MSVC compiler options
MSVC compiler command-line syntax