Module DefUse

Module containing the predicates that connect uses to their reaching definition. The reaching definitions are computed separately for each unique use MemoryLocation. An instruction is treated as a definition of a use location if the defined location overlaps the use location in any way. Thus, a single instruction may serve as a definition for multiple use locations, since a single definition location may overlap many use locations.

Definitions and uses are identified by a block and an integer “offset”. An offset of -1 indicates the definition from a Phi instruction at the beginning of the block. An offset of 2i indicates a definition or use on the instruction at index i in the block. An offset of 2i+1 indicates a definition or use on the Chi instruction that will be inserted immediately after the instruction at index i in the block.

For a given use location, each definition and use is also assigned a “rank” within its block. The rank is simply the one-based index of that definition or use within the list of definitions and uses of that location within the block, ordered by offset. The rank allows the various reachability predicates to be computed more efficiently than they would if based solely on offset, since the set of possible ranks is dense while the set of possible offsets is potentially very sparse.

Import path

import semmle.code.cpp.ir.implementation.aliased_ssa.internal.SSAConstruction

Predicates

definitionReachesEndOfBlock	Holds if the definition that overlaps `useLocation` at `(defBlock, defRank)` reaches the end of block `block` without any intervening definitions that overlap `useLocation`.
definitionReachesUse	Holds if a definition that overlaps `useLocation` at (`defBlock`, `defRank`) reaches the use of `useLocation` at (`useBlock`, `useRank`) without any intervening definitions that overlap `useLocation`.
getDefinitionOrChiInstruction
hasDefinitionAtRank	Holds if there is a definition at offset `offset` in block `block` that overlaps memory location `useLocation`. `rankIndex` is the rank of the definition as computed by `defUseRank()`.
hasPhiOperandDefinition	Holds if the `Phi` instruction for location `useLocation` at the beginning of block `phiBlock` has an operand along the incoming edge from `predBlock`, where that operand’s definition is at offset `defOffset` in block `defBlock`.
hasUseAtRank	Holds if there is a use of memory location `useLocation` on instruction `use` in block `block`. `rankIndex` is the rank of the use use as computed by `defUseRank`.
locationLiveOnEntryToBlock	Holds if the specified `useLocation` is live on entry to `block`. This holds if there is a use of `useLocation` that is reachable from the start of `block` without passing through a definition that overlaps `useLocation`. Note that even a partially-overlapping definition blocks liveness, because such a definition will insert a `Chi` instruction whose result totally overlaps the location.
locationLiveOnExitFromBlock	Holds if the specified `useLocation` is live on exit from `block`.