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Overview

Comment: | Fix a bunch of Markdown formatting issues in callframe motion implementation plan |
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Timelines: | family | ancestors | descendants | both | kbk-refactor-callframe |

Files: | files | file ages | folders |

SHA3-256: |
ab89bbf87963cfd1cfed4b955de20555 |

User & Date: | kbk 2019-07-21 19:56:02 |

Context

2019-11-23
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23:02 | merge trunk check-in: c3fc2d4137 user: kbk tags: notworking, kbk-refactor-callframe | |

2019-07-21
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19:56 | Fix a bunch of Markdown formatting issues in callframe motion implementation plan check-in: ab89bbf879 user: kbk tags: kbk-refactor-callframe | |

2019-06-08
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22:21 | Remove cfRedundancy' header - added prematurely before method structure actually designed. check-in: 74a853f4cd user: kbk tags: kbk-refactor-callframe | |

Changes

Changes to doc/20190216callframe/callframe.md.

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optimization. In particular, LOAD-LOAD and STORE-LOAD redundancies can be eliminated by checking each `moveFromCallFrame` operation in turn for whether it is accessing an available value. If it is, it is eliminated and its result is replaced with the available value. ### Availability analysis: equivalence sets LOAD-STORE pairs can also be detected, by examining 'moveToCallFrame' instructions and eliminating them if the variable being written is available, and known to be equal to the Tcl value in the callframe. Therefore, in the availability analysis, as well as accumulating the sets of available Tcl values and the names in the callframe that correspond to them, we must track the sets of names in a given callframe that correspond correspond to each SSA value. That way, ................................................................................ Availability by itself is not sufficient for loop-invariant code motion. In addition, we need the concept of _anticipability_. A (_cf_, _name_) pair is _anticipable_ at a given point in the program if every execution path forward from that point contains a `moveFromCallFrame` retrieving that value prior to the value's being modified. Calculating anticipability requires multiple traversals of the program in postdominator order. Nothing is anticipable on 'return'. For blocks that do not return, we begin by taking the intersection of values anticipable at their successors: ![\texttt{ANTIC\_OUT}[B] = \bigcap_{P\in \texttt{succ}[B]} \texttt{ANTIC\_IN}[P] \right)](./anticout.svg) ................................................................................ predecessor), in dominator order. For each (_cf_, _name_) that is anticipable at the entry to the block, we check availability in the predecessor blocks. If the expression is available in at least one predecessor, but not all predecessors, we insert `moveFromCallFrame` instructions for it in the predecessors where it is unavailable. This is another analysis that must be iterated to convergence. Inserting these 'moveFromCallFrame' operations produces new available variables, which can in turn expose further opportunities for code motion. \[VAND04\] discusses this in more detail in chapter 4. ### Possible combination of passes This analysis closely mirrors what happens in partial redundancy ................................................................................ `doSomething cfout cfin moreArgs` the input callframe will never again be used. ## TASK 4: Loop-invariant stores We have seen in Task 3 that'moveFromCallFrame' instructions can be placed speculatively to convert partial availability of a needed expression to full availability (and remove a 'moveFromCallFrame' later in the program on a 'busier' execution path). In a similar manner, 'moveToCallFrame' instructions can be placed speculatively at the head of blocks in order to make partially dead assignments ("faint" assignments in the terminology of \[KNOO94\]) fully dead and eliminate a 'moveToCallFrame' on an eariler path of the program. \[LO98\] demonstrates how this partial liveness analysis is precisely dual to partial availability analysis, and introduces a dual form to Static Single Assignment (SSA), called Static Single Use (SSU). This latter form has similar advantages to SSA in that the dependency chain, here from definition to use rather than from use to definition, is factored so that there are explicit deviation points (dual to the merge ................................................................................ reason, the more computationally complex analysis of \[KNOO94\] will be followed, but modified to operate on basic blocks rather than individual instructions. We begin with the liveness analysis of Task 2. We augment 'liveness' with the concept of partial liveness. The data flow equations will look like ![\textt{LIVE\_OUT}[B] = \begin{Bmatrix} \displaystyle \bigcap_{S \in \textt{succ}[B]} \textt{LIVE\_IN}[S],& S \text{ is not an exit block} \\ \displaystyle \left\{ v \in \textt{LOCALVARS}~|~v~\text{might alias a nonlocal variable} \right\},&\text{otherwise}\end{matrix}](liveout.svg) ![\textt{PLIVE\_OUT}[B] = \begin{Bmatrix} \displaystyle \bigcup_{S \in \textt{succ}[B]} \textt{PLIVE\_IN}[S],& S \text{ is not an exit block} \\ \displaystyle \left\{ v \in \textt{LOCALVARS}~|~v~\text{might alias a nonlocal variable} \right\},&\text{otherwise}\end{matrix}](pliveout.svg) ![\textt{LIVE\_IN}[B] = \textt{process}(B, \textt{LIVE\_OUT}[B])](livein.svg) ![\textt{PLIVE\_IN}[B] = \textt{process}(B, \textt{PLIVE\_OUT}[B])](plivein.svg) In these equations, _process_ represents the basic-block-level upward analysis from Task 2. The _LIVE_ and _PLIVE_ sets are sets of (callframe, variable) pairs, and it is assumed that the arguments to the intersection and union are translated if the callframe appears in a φ operation. Liveness corresponds to the 'availability' of the partial redundancy elimination. Just as a 'moveFromCallFrame' may not be hoisted over the point where it is available, a 'moveToCallFrame' may not be sunk past the point where it is live. Similarly, 'moveToCallFrame' creates values and corresponds to anticipability. Just as it is of no benefit to insert a 'moveFromCallFrame' for a value that is not anticipated, it is of no benefit to insert a 'moveToCallFrame' for a value that is already present. ## References \[DREC93\] Karl-Heinz Drechsler and Manfred P. Stadel. "A variation of Knoop, Rüthing, and Steffen's _Lazy Code Motion_". _ACM SIGPLAN |
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optimization. In particular, LOAD-LOAD and STORE-LOAD redundancies can be eliminated by checking each `moveFromCallFrame` operation in turn for whether it is accessing an available value. If it is, it is eliminated and its result is replaced with the available value. ### Availability analysis: equivalence sets LOAD-STORE pairs can also be detected, by examining `moveToCallFrame` instructions and eliminating them if the variable being written is available, and known to be equal to the Tcl value in the callframe. Therefore, in the availability analysis, as well as accumulating the sets of available Tcl values and the names in the callframe that correspond to them, we must track the sets of names in a given callframe that correspond correspond to each SSA value. That way, ................................................................................ Availability by itself is not sufficient for loop-invariant code motion. In addition, we need the concept of _anticipability_. A (_cf_, _name_) pair is _anticipable_ at a given point in the program if every execution path forward from that point contains a `moveFromCallFrame` retrieving that value prior to the value's being modified. Calculating anticipability requires multiple traversals of the program in postdominator order. Nothing is anticipable on `return`. For blocks that do not return, we begin by taking the intersection of values anticipable at their successors: ![\texttt{ANTIC\_OUT}[B] = \bigcap_{P\in \texttt{succ}[B]} \texttt{ANTIC\_IN}[P] \right)](./anticout.svg) ................................................................................ predecessor), in dominator order. For each (_cf_, _name_) that is anticipable at the entry to the block, we check availability in the predecessor blocks. If the expression is available in at least one predecessor, but not all predecessors, we insert `moveFromCallFrame` instructions for it in the predecessors where it is unavailable. This is another analysis that must be iterated to convergence. Inserting these `moveFromCallFrame` operations produces new available variables, which can in turn expose further opportunities for code motion. \[VAND04\] discusses this in more detail in chapter 4. ### Possible combination of passes This analysis closely mirrors what happens in partial redundancy ................................................................................ `doSomething cfout cfin moreArgs` the input callframe will never again be used. ## TASK 4: Loop-invariant stores We have seen in Task 3 that `moveFromCallFrame` instructions can be placed speculatively to convert partial availability of a needed expression to full availability (and remove a `moveFromCallFrame` later in the program on a 'busier' execution path). In a similar manner, `moveToCallFrame` instructions can be placed speculatively at the head of blocks in order to make partially dead assignments ("faint" assignments in the terminology of \[KNOO94\]) fully dead and eliminate a `moveToCallFrame` on an eariler path of the program. \[LO98\] demonstrates how this partial liveness analysis is precisely dual to partial availability analysis, and introduces a dual form to Static Single Assignment (SSA), called Static Single Use (SSU). This latter form has similar advantages to SSA in that the dependency chain, here from definition to use rather than from use to definition, is factored so that there are explicit deviation points (dual to the merge ................................................................................ reason, the more computationally complex analysis of \[KNOO94\] will be followed, but modified to operate on basic blocks rather than individual instructions. We begin with the liveness analysis of Task 2. We augment 'liveness' with the concept of partial liveness. The data flow equations will look like ![\textt{LIVE\_OUT}[B] formula](liveout.svg) ![\textt{PLIVE\_OUT}[B] formula](pliveout.svg) ![\textt{LIVE\_IN}[B] = \textt{process}(B, \textt{LIVE\_OUT}[B])](livein.svg) ![\textt{PLIVE\_IN}[B] = \textt{process}(B, \textt{PLIVE\_OUT}[B])](plivein.svg) In these equations, _process_ represents the basic-block-level upward analysis from Task 2. The _LIVE_ and _PLIVE_ sets are sets of (callframe, variable) pairs, and it is assumed that the arguments to the intersection and union are translated if the callframe appears in a φ operation. Liveness corresponds to the 'availability' of the partial redundancy elimination. Just as a `moveFromCallFrame` may not be hoisted over the point where it is available, a `moveToCallFrame` may not be sunk past the point where it is live. Similarly, `moveToCallFrame` creates values and corresponds to anticipability. Just as it is of no benefit to insert a `moveFromCallFrame` for a value that is not anticipated, it is of no benefit to insert a `moveToCallFrame` for a value that is already present. ## References \[DREC93\] Karl-Heinz Drechsler and Manfred P. Stadel. "A variation of Knoop, Rüthing, and Steffen's _Lazy Code Motion_". _ACM SIGPLAN |