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Register Allocation by Graph Coloring Under Full Live-Range Splitting Sandrine Blazy, Benoît Robillard

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Register Allocation by Graph Coloring Under Full Live-Range Splitting Sandrine Blazy, Benoît Robillard Register allocation by graph coloring under full live-range splitting Sandrine Blazy, Benoît Robillard To cite this version: Sandrine Blazy, Benoît Robillard. Register allocation by graph coloring under full live-range splitting. ACM SIGPLAN/SIGBED Conference on Languages, Compilers, and Tools for Embedded systems (LCTES’2009), ACM, Jun 2009, Dublin, Ireland. inria-00387749 HAL Id: inria-00387749 https://hal.inria.fr/inria-00387749 Submitted on 2 Jun 2009 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Live-Range Unsplitting for Faster Optimal Coalescing Sandrine Blazy Benoit Robillard ENSIIE, CEDRIC {blazy,robillard}@ensiie.fr Abstract be stored in registers. Coalescing allocates unspilled variables to Register allocation is often a two-phase approach: spilling of regis- registers in a way that leaves as few as possible move statements ters to memory, followed by coalescing of registers. Extreme live- (i.e. register copies). Both spilling and coalescing are known to be range splitting (i.e. live-range splitting after each statement) en- NP-complete [24, 6]. ables optimal solutions based on ILP, for both spilling and coa- Classically, register allocation is modeled as a graph coloring lescing. However, while the solutions are easily found for spilling, problem, where each register is represented by a color, and each for coalescing they are more elusive. This difficulty stems from the variable is represented by a vertex in an interference graph. Given huge size of interference graphs resulting from live-range splitting. a number k of available registers, register allocation consists in This paper focuses on coalescing in the context of extreme live- finding (if any) a k-coloring of the interference graph. When there range splitting. It presents some theoretical properties that give rise is no k-coloring, some variables are spilled to memory. When there to an algorithm for reducing interference graphs. This reduction is a k-coloring, coalescing consists in choosing a k-coloring that consists mainly in finding and removing useless splitting points. It removes most of the move statements. is followed by a graph decomposition based on clique separators. ILP-based approaches have been applied to register allocation The reduction and decomposition are general enough, so that any in order to provide optimal solutions. Register allocation is one of coalescing algorithm can be applied afterwards. the most important passes of a compiler. A compiler performing an Our strategy for reducing and decomposing interference graphs optimal register allocation can thus generate efficient code. Having preserves the optimality of coalescing. When used together with such a code is often of primary concern for embedded software, an optimal coalescing algorithm (e.g. ILP), optimal solutions are even if the price to pay is to use an external ILP-solver. Appel and much more easily found. The strategy has been tested on a standard George formulate spilling as an integer linear program (ILP) and benchmark, the optimal coalescing challenge. For this benchmark, provide optimal and efficient solutions [3]. Their process to find the cutting-plane algorithm for optimal coalescing (the only opti- optimal solutions for spilling requires live-range splitting, an opti- mal algorithm for coalescing) runs 300 times faster when combined mization that leaves more flexibility to the register allocation (i.e. with our strategy. Moreover, we provide all the optimal solutions of avoids to spill a variable everywhere). While the solutions are easily the optimal coalescing challenge, including the three instances that found for spilling in this context, for coalescing they are more elu- were previously unsolved. sive. Indeed, live-range splitting generates huge interference graphs (due to many move statements) that make the coalescing harder to Categories and Subject Descriptors D.3.4 [Programming lan- solve. guages]: Processors - compilers, optimization; G.2.2 [Graph the- Splitting the live-range of a variable v consists in renaming v ory]: Graph algorithms to different variables having shorter live-ranges than v and adding move statements connecting the variables originating from v. Re- General Terms algorithms, languages cent spilling heuristics benefit from live-range splitting: when a Keywords register allocation, coalescing, graph reduction variable is spilled because it has a long live-range, splitting this live-range into smaller pieces may avoid to spill v. If the live-range of v is short, it is easier to store v in a register, as the register needs 1. Introduction to hold the value of v only during the live-range of v. Register allocation determines at compile time where each variable There exists several ways of splitting live-ranges (e.g. region will be stored at execution time: either in a register or in memory. spilling, zero cost range splitting, load/store range analysis) [10, 7, Register allocation is often a two-phase approach: spilling of reg- 18, 19, 4, 11, 20]. Splitting live-ranges often reduces the interfer- isters to memory, followed by coalescing of registers [3, 8, 16, 5]. ences with other live-ranges. Thus, most of the splitting heuristics Spilling generates loads and stores for live variables1 that cannot have been successful in improving the spilling phase. The differ- ences between these heuristics stem from the number of splitting 1 A variable that may be potentially read before its next write is called a live points (i.e. program points where live-ranges are split) as well as variable. the sizes of the split live-ranges. These heuristics are sometimes difficult to implement. The most aggressive live-range splitting is extreme live-range splitting, where live-ranges are split after each statement. Its main Permission to make digital or hard copies of all or part of this work for personal or advantage is the accuracy of the generated interference graph. In- classroom use is granted without fee provided that copies are not made or distributed deed, a variable is spilled only at the program points where there is for profit or commercial advantage and that copies bear this notice and the full citation no available register for that variable. As in a SSA form, each vari- on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. able is defined only once. Furthermore, contrary to other splitting LCTES’09, June 19–20, 2009, Dublin, Ireland. heuristics, extreme live-range splitting is easy to implement. Copyright c 2009 ACM 978-1-60558-356-3/09/06. $5.00. Extreme live-range splitting helps in finding optimal and ef- An affinity edge between two variables represents a move statement ficient solutions for spilling [3]. However, it generates programs between these variables (that should be stored in a same register or with huge interference graphs. Each renaming of a variable v to at the same memory location). Weights are associated to affinity v1 results in adding a vertex in the interference graph for v1 and, edges, taking into account the frequency of execution of the move consequently, some edges incident to that vertex. Thus, interfer- statements. ence graphs become so huge that coalescing heuristics often fail. Given a number k of registers, register allocation consists in The need for a better algorithm for the optimal coalescing problem satisfying all interference edges as well as maximizing the sum of gave rise to a benchmark of interference graphs called the optimal weights of affinity edges such that the same color is assigned to coalescing challenge (written OCC in the sequel of this paper) [2]. both endpoints. Satisfying most of the affinity edges is the goal of Recently, Grund and Hack have formulated coalescing as an register coalescing. ILP [16]. They introduce a cutting-plane algorithm that reduces Interference graphs are built after a liveness analysis [9]. In an the search space (i.e. the space of potential solutions). Thus, their interference graph, a variable is described by a unique live-range. ILP formulation needs less time to compute optimal solutions. As Consequently, spilling a variable means spilling it everywhere in a result, they provide the first optimal and efficient solutions of the the program, even if it could have been spilled on a shorter live- OCC. These solutions are 50% better than the solutions computed range. Figure 1 illustrates this problem on a small program consist- by the best coalescing heuristics. ing of a switch statement with 3 branches (see [21] for more de- Grund and Hack conclude in [16] that their cutting-plane al- tails). The program has 3 variables but only 2 variables are updated gorithm fails when applied to the largest graphs of the OCC. Be- in each branch of the switch statement. Thus, its corresponding cause of extreme live-range splitting (that was required for opti- interference graph is a 3-clique, that is not 2-colorable, although mal spilling), optimal solutions for coalescing cannot be efficiently only 2 registers are needed. computed on these graphs. Owing to the extreme amount of copies, coalescing is hard to solve optimally. In this paper, we study the switch(. ){ impact of extreme live-range splitting on coalescing and provide case 0: case 1: case 2: a strategy for reducing interference graphs before coalescing. ILP l1 : a := . l4 : a := . l7 : b:= . is commonly composed with algorithms in order to let the solver, l2 : b := .
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