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Code Motion of Control Structures From the paper by Cytron, Lowry, and Zadeck, COMP 512, Rice University Copyright 2011, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 512 at Rice University have explicit permission to make copies of these materials for their personal use. Faculty from other educational institutions may use these materials for nonprofit educational purposes, provided this copyright notice is preserved. Comp 512 Spring 2011

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Background Code Motion algorithms come in three basic flavors Ad-hoc techniques, such as code hoisting & sinking, & GCSE Opportunities found with data-flow information Little attention paid to placement Data-flow techniques, such as partial redundancy elimination ( PRE ) & lazy code motion ( LCM ) Opportunities found with data-flow information Placement computed with scrupulous care Graphical techniques built around the structure of the CFG Opportunities found using control dependeence Placement dictated by control dependence (structure of CFG ) This lecture looks at a technique from that third category Popularity of PRE/LCM has caused these techniques to be ignored COMP 512, Rice University2 This paper compares against PRE, because it predates LCM by about six years.

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COMP 512, Rice University3 Background Lazy Code Motion is (probably) the dominant technique Conditional control-flow inside loop causes problems LCM does not lengthen any path ( conservative not speculative ) … x + y … y + z LCM can move y + z, but not x + y … y + z … x + y After LCM Weaknesses of PRE/LCM (CLZ paper’s list ) 1. Control flow 2. Moving assignments 3. Speculative motion 4. Iterated code motion

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COMP 512, Rice University4 Background Cytron, Lowry, & Zadeck “Code motion of control structures in high-level languages” Paper predates the papers that define SSA form This paper includes most of the critical ideas of SSA form We will recast it into modern terminology Loops Algorithm operates on loops, inside out … Paper refers to intervals; think of them as SCCs of the CFG hanging from interval header ( loops, not maximal intervals ) Each interval (loop) has a landing pad on entry & on exit Guarded region at the loop’s entry, another on exit path Traverse each interval in dominated topological order Think of this as reverse postorder … Formally, a dominated topological order has the property that if n and m are both successors of l in topological order, and n dom m, then n precedes m

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COMP 512, Rice University5 Landing Pads A B C F D Interval in the original CFG E CFG from paper, redrawn to have test at both top and bottom of the loop; see EaC2e § 7.8.2

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COMP 512, Rice University6 Landing Pads A B C F D Interval in the original CFG E CFG from paper, redrawn to have test at both top and bottom of the loop; see EaC2e § Interval

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COMP 512, Rice University7 Landing Pads A B C F D Interval in the original CFG E CFG from paper, redrawn to have test at both top and bottom of the loop; see EaC2e § A BC F D E LP 1 LP 3 LP 2 Augmented with landing pads Modern view: split critical edges Landing pad for backward code motion Landing pads for forward code motion LP 4 Extra landing pad generated by splitting critical edge; not needed by CLZ

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COMP 512, Rice University8 Code Motion Two algorithms Strict — guaranteed not to lengthen paths Restrictions similar to PRE & LCM Nonstrict — can (and does) lengthen some paths “Speculative” code motion Can introduce unsafe behavior Big Picture Visit intervals in innermost to outermost order Copy movable control-flow code into entry or exit landing pad Copy movable statements into landing pad Interval is summarized and treated (in surrounding interval) as an atomic operation Need a carefully constructed name space ( SSA names ) Should not use this one unless you modify it to make it safe!

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COMP 512, Rice University9 Strict Algorithm ∀ interval I, in order from innermost to outermost ∀ statement s in I, in reverse postorder Test s to see if it is movable If s is movable then Copy s into the entry landing pad If s is a control structure then replace copy in loop with a bit test else delete original statement from the loop Need a usable test for movability Dominated total order

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COMP 512, Rice University10 Strict Algorithm S is movable if and only if s is not control dependent on any immovable test Definitions that reach uses in s originate outside the loop i.e., s is loop invariant s defines x and the algorithm has seen neither a for x in the loop nor an immovable definition for x y is control dependent on x if and only if a path from x to y such that every node in the path except x and y are post-dominated by y, and x is not post-dominated by y i … while(i

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COMP 512, Rice University11 Strict Algorithm ∀ interval I, in order from innermost to outermost ∀ statement s in I, in reverse postorder test each statement s to see if it is movable if s is movable & a control structure then copy s into the entry landing pad replace copy in loop with a bit test else if s is movable & not a control structure then move s to the equivalent place in entry landing pad clean up any unused control structures Details Copy control-structure to the landing pad and redirect any loop exits to the bottom of the landing pad

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COMP 512, Rice University12 Strict Algorithm How can they just delete the assignments in the loop? LCM moves expressions but not assignments It needs to ensure that the original name gets the invariant value on each iteration Those assignments would make control structures useful CLZ moves assignments & deletes empty control structures Remember their renaming scheme (SSA)? It creates a unique name for the definition & makes this particular kind of motion safe ! … if it could move the assignment out of the control structure … One key rationale for SSA: the assignment cannot be killed, so this kind of code motion becomes safe (1986 paper)

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COMP 512, Rice University13 Non-strict Algorithm Some control structures are not movable Their behavior varies from iteration to iteration Moving them might Raise an exception that the unmovable guard might prevent Lengthen the computation if the guard is always false Nonetheless, moving them might be profitable … CLZ also give a nonstrict algorithm that moves these operations If s depends on an immovable test, it is placed (in landing pad) where it will execute independent of the outcome of that test Algorithm can still guard s with any movable tests … Unsafe!

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COMP 512, Rice University14 Removing Redundancy Further improvement is possible After combining (hoisting), it needs no guard in landing pad May render control-flow structure useless CLZ present new algorithm for “combining” such statements Might just use hoisting, sinking, or adapt SSAPre Not moveable z x + y

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COMP 512, Rice University15 Reflections CLZ uses just one pass over each interval Accounts for 2 nd, 3 rd, … order effects ( dto, rpo ) Would need to iterate LCM to achieve same results.. SSA name space is critical to correctness Allows motion of assignments Allows nonstrict movement of invariants Other techniques can achieve similar effects to COMMON Peel the first iteration of the loop & apply strong techniques for redundancy & for constant propagation ( SCCP ) Hoisting, sinking, maybe a derivative of SSAPre SSA Pre ( Chow et al., PLDI 97 ) Produced same results as PRE or LCM on SSA Maybe they should have tried for better results, a la CLZ Peel each loop, use SSA Pre, then SCCP, DEAD, CLEAN

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COMP 512, Rice University16 Significance This paper is important Underpinnings and motivations of SSA, years before SSA paper Code motion of control structures is still not commonly done Community has seized on LCM as right paradigm CLZ algorithms challenge the wisdom of that opinion The SSA name space is a critical part of the big picture This work deserves wider recognition and use

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End of Lecture Extra Slides Start Here COMP 512, Rice University17

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COMP 512, Rice University18 Background Last lecture, we looked at Lazy Code Motion (LCM), which moves redundant and partially redundant code out of loops Conditional control-flow inside loop causes problems LCM does not lengthen any path ( conservative not speculative ) … x + y … y + z LCM can move y + z, but not x + y … y + z … x + y After LCM Weaknesses of PRE/LCM (CLZ paper’s list ) 1. Control flow 2. Moving assignments 3. Speculative motion 4. Iterated code motion

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