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The Most Underused Compiler Switches in Visual C++

, March 11, 2014 Microsoft's Visual C++ team explains the nine most underused switches and why you should make sure to include them in your build.
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/Gy and /Gw

A compiler can only optimize away data or a function if it can prove that the data or function will never be referenced. In a non-LTCG compile (i.e., a build with Whole Program Optimization (WPO) disabled), the compiler's visibility is limited to a single module (.obj), so for data and function that has global scope, the compiler will never know if other modules make use of them. As a result, the compiler can never optimize them away. The linker has a good view of all the modules that will be linked together, so the linker is in a good position to optimize away unused global data and unreferenced functions. The linker, however, manipulates on a section level, so if the unreferenced data/functions are mixed with other data or functions in a section, the linker won't be able to extract and remove. To equip the linker to remove unused global data and functions, we need to put each global data or function in a separate section. These small sections are called COMDAT.

Using the /Gy compiler switch instructs the compiler to package only individual functions in the form of packaged functions or COMDATs, each with its own section header information. This enables function-level linkage and linker optimizations ICF (folding together identical COMDATs) and REF(eliminating unreferenced COMDATs). In VS2013, we have introduced a new compiler switch — /Gw — that extends these linker-optimization benefits to data as well.

Additional comments:

These switches enable linker optimizations that greatly help in reducing the size of the application, thus providing better cache locality and overall "goodness" with respect to memory — which also results in runtime performance gains.

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