= Static single-assignment form
<Static single-assignment form> is an <intermediate representation> in which each named value has one static definition. Repeated source assignments receive different names: `x=1; x=x+2` becomes definitions such as `x1=1; x2=x1+2`. This makes def-use relationships explicit without requiring a search backward for which assignment supplied a value.
At a join in a <control-flow graph>, a <phi node> selects the value associated with the edge actually taken. If one branch defines `x1=1` and another defines `x2=2`, the join defines `x3=phi(x1,left; x2,right)`. The node selects an incoming value, not their arithmetic average. Loop-carried values similarly use entry and back-edge arguments. LLVM specifies that phi <machine instruction>[instructions] occur first in a <basic block>, with an incoming value for each predecessor.<LLVM Language Reference Manual>[phi instruction, arguments and semantics]{ref} Static uniqueness concerns program definitions, not the number of times a loop <machine instruction>[instruction] executes.
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