Editing Stack machine (section)

===Common subexpressions===
In register machines, a [[common subexpression]] (a subexpression which is used multiple times with the same result value) can be evaluated just once and its result saved in a fast register. The subsequent reuses have no time or code cost, just a register reference. This optimization speeds simple expressions (for example, loading variable X or pointer P) as well as less-common complex expressions.

With stack machines, in contrast, results can be stored in one of two ways. Firstly, results can be stored using a temporary variable in memory. Storing and subsequent retrievals cost additional instructions and additional data cache cycles. Doing this is only a win if the subexpression computation costs more in time than fetching from memory, which in most stack CPUs, almost always is the case. It is never worthwhile for simple variables and pointer fetches, because those already have the same cost of one data cache cycle per access. It is only marginally worthwhile for expressions such as {{code|X+1}}. These simpler expressions make up the majority of redundant, optimizable expressions in programs written in languages other than [[concatenative programming language|concatenative languages]]. An optimizing compiler can only win on redundancies that the programmer could have avoided in the source code.{{cn|date=May 2023}}

The second way leaves a computed value on the data stack, duplicating it as needed. This uses operations to copy stack entries. The stack must be depth shallow enough for the CPU's available copy instructions. Hand-written stack code often uses this approach, and achieves speeds like general-purpose register machines.<ref name="Koopman_1989"/><ref name="LaForest_2007"/> Unfortunately, algorithms for optimal "stack scheduling" are not in wide use by programming languages.