/OPT (Optimizations)
The latest version of this topic can be found at -OPT (Optimizations).
Controls the optimizations that LINK performs during a build.
Syntax
/OPT:{REF | NOREF}
/OPT:{ICF[=iterations] | NOICF}
/OPT:{LBR | NOLBR}
Arguments
REF | NOREF
/OPT:REF eliminates functions and data that are never referenced; /OPT:NOREF keeps functions and data that are never referenced.
When /OFT:REF is enabled, LINK removes unreferenced packaged functions and data. An object contains packaged functions and data (COMDATs) if it was compiled by using the /Gy option. This optimization is known as transitive COMDAT elimination. By default, /OPT:REF is enabled in non-debug builds. To override this default and keep unreferenced COMDATs in the program, specify /OPT:NOREF. You can use the /INCLUDE option to override the removal of a specific symbol.
When /OPT:REF is enabled either explicitly or by default, a limited form of /OPT:ICF is enabled that only folds identical functions. If you want /OPT:REF but not /OPT:ICF, you must specify either /OPT:REF,NOICF or /OPT:NOICF.
If /DEBUG is specified, the default for /OPT is NOREF, and all functions are preserved in the image. To override this default and optimize a debugging build, specify /OPT:REF. Because /OPT:REF implies /OPT:ICF, we recommend that you also specify /OPT:NOICF to preserve identical functions in debugging builds. This makes it easier to read stack traces and set breakpoints in functions that would otherwise be folded together. The /OPT:REF option disables incremental linking.
You have to explicitly mark const data as a COMDAT; use __declspec(selectany).
Specifying /OPT:ICF does not enable the /OPT:REF option.
ICF[= iterations ] | NOICF
Use /OPT:ICF[=iterations] to perform identical COMDAT folding. Redundant COMDATs can be removed from the linker output. The optional iterations parameter specifies the number of times to traverse the symbols for duplicates. The default number of iterations is two. Additional iterations may locate more duplicates that are uncovered through folding in the previous iteration.
The linker behaves differently when /OPT:REF is specified—and ICF is in effect by default—than it does when /OPT:REF,ICF is specified explicitly. The form of ICF that's enabled with /OPT:REF alone does not fold read-only data—this includes .rdata, .pdata, and .xdata. Therefore, fewer functions are folded when images are produced for x64 because functions in these modules are more dependent on read-only data—for example, .pdata and .xdata. To get full ICF folding behavior, explicitly specify /OPT:ICF.
To place functions in COMDATs, you use the /Gy compiler option; to place const data, you declare it __declspec(selectany). For information about how to specify data for folding, see selectany.
By default, ICF is on if REF is on. To override this default if REF is specified, use NOICF. When /OPT:REF is not specified in a debug build, you must explicitly specify /OPT:ICF to enable COMDAT folding. However, because /OPT:ICF can merge identical data or functions, it can change the function names that appear in stack traces. It can also make it impossible to set breakpoints in certain functions or to examine some data in the debugger, and take you into unexpected functions when you single-step through code. Therefore, we do not recommend that you use /OPT:ICF in debug builds unless the advantages of smaller code outweigh these disadvantages.
Note
Because /OPT:ICF can cause the same address to be assigned to different functions or read-only data members (const variables compiled by using /Gy), it can break a program that depends on unique addresses for functions or read-only data members. For more information, see /Gy (Enable Function-Level Linking).
LBR | NOLBR
The /OPT:LBR and /OPT:NOLBR options apply only to ARM binaries. Because certain ARM processor branch instructions have a limited range, if the linker detects a jump to an out-of-range address, it replaces the branch instruction’s destination address with the address of a code "island" that contains a branch instruction that targets the actual destination. You can use /OPT:LBR to optimize the detection of long branch instructions and the placement of intermediate code islands to minimize overall code size. /OPT:NOLBR instructs the linker to generate code islands for long branch instructions as they are encountered, without optimization.
By default, the /OPT:LBR option is set when incremental linking is not enabled. If you want a non-incremental link but not long branch optimizations, specify /OPT:NOLBR. The /OPT:LBR option disables incremental linking.
Remarks
Optimizations generally decrease the image size and increase the program speed at a cost of increased link time.
You can use the /VERBOSE option to see the functions that are removed by /OPT:REF and the functions that are folded by /OPT:ICF.
To set the OPT:ICF or OPT:REF linker option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see Working with Project Properties.
In the left pane, expand Configuration Properties, Linker, Optimization.
Modify one of these properties:
Enable COMDAT Folding
References
To set the OPT:LBR linker option in the Visual Studio development environment
Open the project's Property Pages dialog box. For details, see Setting Visual C++ Project Properties.
Select the Linker folder.
Select the Command Line property page.
Enter the option in Additional Options:
/opt:lbr
To set this linker option programmatically
- See EnableCOMDATFolding and OptimizeReferences properties.