Editing Static single-assignment form (section)

==Variations that reduce the number of Φ functions==
"Minimal" SSA inserts the minimal number of Φ functions required to ensure that each name is assigned a value exactly once and that each reference (use) of a name in the original program can still refer to a unique name. (The latter requirement is needed to ensure that the compiler can write down a name for each operand in each operation.)

However, some of these Φ functions could be ''[[dead code elimination|dead]]''. For this reason, minimal SSA does not necessarily produce the fewest Φ functions that are needed by a specific procedure. For some types of analysis, these Φ functions are superfluous and can cause the analysis to run less efficiently.

===Pruned SSA===
Pruned SSA form is based on a simple observation: Φ functions are only needed for variables that are "live" after the Φ function. (Here, "live" means that the value is used along some path that begins at the Φ function in question.) If a variable is not live, the result of the Φ function cannot be used and the assignment by the Φ function is dead.

Construction of pruned SSA form uses [[live-variable analysis|live-variable information]] in the Φ function insertion phase to decide whether a given Φ function is needed. If the original variable name isn't live at the Φ function insertion point, the Φ function isn't inserted.

Another possibility is to treat pruning as a [[dead-code elimination]] problem. Then, a Φ function is live only if any use in the input program will be rewritten to it, or if it will be used as an argument in another Φ function. When entering SSA form, each use is rewritten to the nearest definition that dominates it. A Φ function will then be considered live as long as it is the nearest definition that dominates at least one use, or at least one argument of a live Φ.

===Semi-pruned SSA===
Semi-pruned SSA form<ref>{{cite tech report |title=Practical Improvements to the Construction and Destruction of Static Single Assignment Form |year=1998 |last1=Briggs |first1=Preston |last2=Cooper |first2=Keith D. |last3=Harvey |first3=Timothy J. |last4=Simpson |first4=L. Taylor |url=http://www.cs.rice.edu/~harv/my_papers/ssa.pdf |url-status=dead |archive-url=https://web.archive.org/web/20100607003509/http://www.cs.rice.edu/~harv/my_papers/ssa.pdf |archive-date=2010-06-07 }}</ref> is an attempt to reduce the number of Φ functions without incurring the relatively high cost of computing live-variable information. It is based on the following observation: if a variable is never live upon entry into a basic block, it never needs a Φ function. During SSA construction, Φ functions for any "block-local" variables are omitted.

Computing the set of block-local variables is a simpler and faster procedure than full live-variable analysis, making semi-pruned SSA form more efficient to compute than pruned SSA form. On the other hand, semi-pruned SSA form will contain more Φ functions.

===Block arguments===

Block arguments are an alternative to Φ functions that is representationally identical but in practice can be more convenient during optimization. Blocks are named and take a list of block arguments, notated as function parameters. When calling a block the block arguments are bound to specified values. [[MLton]], [[Swift (programming language)|Swift]] SIL, and LLVM [[MLIR (software)|MLIR]] use block arguments.<ref>{{cite web |title=Block Arguments vs PHI nodes - MLIR Rationale |url=https://mlir.llvm.org/docs/Rationale/Rationale/#block-arguments-vs-phi-nodes |website=mlir.llvm.org |access-date=4 March 2022}}</ref>