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C/C++ COMPILER->Compiler command line options->Extended Common Sub-Expression Elimination (/Oh)

By default, a certain amount of common sub-expression elimination is always performed.  The /Oh option tells the compiler to use extended common sub-expression elimination where possible.

Without extended common sub-expression elimination, the compiler performs a one pass transformation of the program, identifying common sub-expressions as it does so.

With extended common sub-expression elimination enabled, a two pass transformation is used allowing additional possibilities for identifying common sub-expressions.