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Topic: Common subexpression elimination

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Compiler optimization

	Common Subexpression Elimination in Titanium
		Common subexpression elimination needs to know what expressions are available for use.
		Common subexpression elimination is a transformation that removes the recomputations of common subexpressions and replaces them with uses of the saved values of those common subexpressions.
		As the live ranges of common subexpressions are unanalyzed, all declarations of compiler generated temporaries are placed at the top level of the method in which they are used.
	www.cs.berkeley.edu /projects/titanium/src/titaniumc/notes/CSE-paper.html (2713 words)

	Real World Technologies - Compilers - Code Optimization
		In this case the common subexpression is the "sqrt(M)" part, the CSE stage will store the result of the first sqrt(M) and reuse it instead of recalculating it.
		Common subexpression elimination is very important in array address calculations since some parts of the calculations can be reused.
		a + b is a common to d = a + b and g = a + b, so instead of creating a new node for it, the node is reused, so common subexpressions are found.
	www.realworldtech.com /page.cfm?ArticleID=RWT110302111309&p=2 (533 words)

	An Overview of the Intel IA-64 Compile
		Partial redundancy elimination (PRE) [1, 2, 11] is a well known scalar optimization technique that subsumes global common subexpression elimination (CSE) and loop invariant code motion.
		CSE removes expressions that are always redundant (redundant on all control flow paths).
		Also, elimination of the redundant load may expose further opportunities for redundancy elimination in the instructions that depend on the load.
	www.intel.com /technology/itj/q41999/articles/art_1j.htm (1204 words)

	Computer Science: Publication: Common Subexpression Elimination in a Lazy Functional Language
		Common subexpression elimination is a well-known compiler optimisation that saves time by avoiding the repetition of the same computation.
		Second, more common subexpressions can be recognised because they can be of arbitrary type whereas standard common subexpression elimination only shares primitive values.
		However, because lazy functional languages decouple program structure from data space allocation and control flow, analysing its effects and deciding under which conditions the elimination of a common subexpression is beneficial proves to be quite difficult.
	www.cs.kent.ac.uk /pubs/1997/1904 (229 words)

	Common subexpression elimination
		In computer programming, common subexpression elimination (CSE) is the practice of finding repeated redundant expression evaluations, and replacing them with a single computation assigned to a temporary variable.
		Although it can be and is done manually by human programmers, usage of the term usually refers to a compiler optimization.
		CSE is usually advantageous, but the compiler needs to be judicious about the number of subexpressions it saves in temporaries; a excessive number of temporary values creates register pressure possibly resulting in register allocation saving and loading temporaries to/from memory, which may take longer than simply recomputing an arithmetic result when it is needed.
	www.sciencedaily.com /encyclopedia/common_subexpression_elimination (287 words)

	GNU Compiler Collection - Wikipedia, the free encyclopedia
		The so-called gimplifier then lowers this more complex form into the simpler SSA-based GIMPLE form which is the common language for a large number of new powerful language- and architecture-independent global (function scope) optimizations.
		GCC developers have given this part of the compiler the somewhat contradictory name the "middle end." These optimizations include dead code elimination, partial redundancy elimination, global value numbering, sparse conditional constant propagation, and scalar replacement of aggregates.
		The exact set of GCC optimizations varies from release to release as it develops, but includes the standard algorithms, such as jump optimization, jump threading, common subexpression elimination, instruction scheduling, and so forth.
	en.wikipedia.org /wiki/GCC_Abstract_Syntax_Tree (1113 words)

	Encyclopedia: GNU Compiler Collection (Site not responding. Last check: 2007-10-08)
		It was the machine that made time-sharing common; it looms large in hacker folklore because of its adoption in the 1970s by many...
		Dead code elimination is a technique used in computer science to reduce program size by removing code which can never be executed.
		Partial redundancy elimination (PRE) is a compiler optimization that eliminates expressions that are redundant on some but not necessarily all paths through a program.
	www.nationmaster.com /encyclopedia/GNU-Compiler-Collection (4372 words)

	Introduction
		Partial redundancy elimination is the most general form of redundancy elimination, that encompasses both common-subexpression elimination and loop-invariant code motion.
		The expression computation in the first block is eliminated, but is retained in the second block.
		In addition, unlike problems with other PRE approaches, it will find all eliminations, and since we don't insert the expression along paths that don't compute it, there is no need for complex bidirectional analysis to determine this insertion.
	www.cs.cmu.edu /~girish/15745/node1.html (487 words)

	Common Subexpressions are Uncommon in Lazy Functional Languages (ResearchIndex) (Site not responding. Last check: 2007-10-08)
		Common Subexpressions are Uncommon in Lazy Functional Languages (1998)
		Abstract: Common subexpression elimination is a well-known compiler optimisation that saves time by avoiding the repetition of the same computation.
		More common subexpressions can be recognised because they can be of arbitrary type whereas standard common subexpression elimination only shares primitive values.
	citeseer.ist.psu.edu /chitil98common.html (365 words)

	SA-C compiler optimizations (Site not responding. Last check: 2007-10-08)
		Common Subexpression Elimination (CSE) is an old and well known compiler optimization that eliminates redundancies by looking for identical subexpressions that compute the same value.
		In such cases, the redundant computation can be eliminated by holding the values in registers so that they are available later and need not be recomputed.
		A useful phenomenon often occurs with Temporal CSE: one or more columns in the left part of the window are unreferenced, making it possible to eliminate those columns.
	www.cs.colostate.edu /cameron/optimizations.html (459 words)

	Read about Common subexpression elimination at WorldVillage Encyclopedia. Research Common subexpression elimination and ... (Site not responding. Last check: 2007-10-08)
		Read about Common subexpression elimination at WorldVillage Encyclopedia.
		In compiler theory, common subexpression elimination (CSE) is the practice of finding repeated redundant
		CSE is usually advantageous, but the compiler needs to be judicious about the number of subexpressions it saves in temporaries; a excessive number of temporary values creates
	encyclopedia.worldvillage.com /s/b/Common_subexpression_elimination (250 words)

	COMMON SUBEXPRESSION INDUCTION
		Whereas CSE is clearly beneficial when the execution time is directly proportional to the number of instructions executed, CSI is most beneficial when the execution time is proportional to the sum of the execution times for all control-flow paths -- as in single instruction stream multiple data stream (SIMD) parallel computers.
		For each inter-thread common subexpression, the remaining operation is given the guard that is the "or" of the guards for all operations absorbed by that optimization.
		After inter-thread CSE, the cost of the example drops from 1263 to 891 clocks(d); (Actually, 891 is the cost obtained after conversion to a linear schedule in step 6 (section 3.2.6), but this number reflects only the benefit gained by the use of inter-thread CSE.
	dynamo.ecn.purdue.edu /~hankd/CARP/CSI/paper.html (6682 words)

	redhat.com \| GCC Optimization
		In particular the information produced by alias analysis is used by Common Subexpression Elimination (CSE), Loop, scheduling and the register allocator.
		This involves eliminating instructions, which compute a value that is a known compile time constant.
		GCC supports loop inversion, which eliminates some test, and branch code inside loops and, supports inversion of loop counter to count down to zero instead of up to a constant value.
	www.redhat.com /software/gnupro/technical/gnupro_gcc.html (1168 words)

	A Handbook on Writing Better C Programs with GNU CC
		When they are not common subexpressions, instruction combination should eliminate the separate register-load.
		In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path.
		For example, when CSE encounters an `if' statement with an `else' clause, CSE will follow the jump when the condition tested is false.
	www.kernelthread.com /publications/faq/optimize.html (4161 words)

	Serializing parallel programs by removing redundant computation
		The basis for these methods is the detection of loop common expressions, or common subexpressions in different iterations of a parallel loop.
		Unrolling and application of the two-phase common subexpression elimination algorithm, which we introduce and which significantly outperforms other common subexpression elimination algorithms, can reduce its cost to less than 2 operand evaluations and 3 combining operations per result.
		Loop common expression elimination decreases these costs to 1 and log w, respectively; when combined with unrolling they drop to 1 operand evaluation and 4 combining operations per result.
	pag.csail.mit.edu /~mernst/pubs/serialize-tr638-abstract.html (574 words)

	Global CSE/Partial Redundancy Elimination - GNU Project - Free Software Foundation (FSF)
		Classic GCSE will only eliminate the computation of "exp1" found in block B3 since the evaluation in block B6 can be reached via a path which does not have an earlier computation of "exp1" (entry, B1, B7, B8, B5, B6).
		PRE will eliminate the evaluations of "exp1" in blocks B3 and B6; to make the evaluation in B6 redundant, PRE will insert an evaluation of "exp1" in block B8.
		Global constant/copy propagation and global common subexpression elimination are enabled by default with the -O2 optimization flag.
	gcc.gnu.org /news/gcse.html (466 words)

	[No title]
		It performs elimination of unused code, branch optimization, common subexpression elimination etc. -Olimit 2000 Limit for the number of basic blocks in a subroutine that will be optimized by the global optimizer (Default: 1000).
		It is common programming practice to use such strings as read-only constants.
		It includes: Level 1: Optimizations local to subroutines (register, branch, integer constant folding) Level 2: Optimizations over all subroutines in a compilation unit (common subexpression elimination, elimination of partial redundancies, copy propagation, strength reduction, loop optimization, dead code elimination) Level 3: Inline expansion of user subroutines (subject to compiler-internal heuristics), global register allocation.
	www.spec.org /cpu95/flags/Siemens-960504.txt (1276 words)

	EECS 583 – Homework 1
		Your first mission is to implement the optimization local common subexpression elimination.
		Subsequent copy propagation and dead code elimination will remove the copies (you don’t need to implement these).
		Several compiler switches will need to be adjusted so that other optimizations done in trimaran do not subsume (and thus take away all the opportunities) for your optimizations (both CSE and renaming).
	www.eecs.umich.edu /~mahlke/eecs583web/homework/583hw2.htm (1020 words)

	CS212 Fall 1999 - Problem Set 6: Part 2
		The process of eliminating this type of redundancies is called common subexpression elimination.
		Make sure your code is written in such a way that useless variable elimination will clean up useless variables at the end.
		Now, use this function and extend any other function you might need to in the source code to handle this extended case of common subexpression elimination.
	www.cs.cornell.edu /Info/People/ozan/ps6-part2.html (1111 words)

	+O2 level optimizations (Site not responding. Last check: 2007-10-08)
		The common subexpression elimination optimization identifies expressions that appear more than once and have the same result, computes the result, and substitutes the result for each occurrence of the expression.
		The subexpression types include instructions that load values from memory, as well as arithmetic evaluation.
		Total unrolling involves eliminating the loop structure completely by replicating the loop body a number of times equal to the iteration count and replacing the iteration variable with constants.
	docs.hp.com /en/B6056-96002/ch03s05.html (1421 words)

	Code Improvement (Site not responding. Last check: 2007-10-08)
		As you may have noticed while figuring information about live variables, reaching definitions and available expressions, we would do this to be able to find things so we can improve upon them.
		Usually, these are extensions of local block optimizations, like common subexpression elimination and copy propagation.
		In a loop, finding statements that do not change in value during the execution of the loop are good candidates for optimization.
	www.csd.uwo.ca /~wozniak/cs447a-fall2001/notes/18_-_Code_Improvement (906 words)

	Goals and Motivation
		For instance the common subexpression elimination pass now does de-facto global value numbering, or instruction combiner utilizes powerful algebraic simplifier of expressions.
		A nice example of these unsuccessful integrated optimization passes is the global common subexpression elimination pass (GCSE) dated to 1997.
		Even today in the official tree, GCSE is not able to do partial redundancy elimination on i386 instructions modifying flag registers (majority of them) and store motion implementation is disabled due to design problems, so the old pseudo-global CSE and GCSE are both run resulting in large compilation time overhead.
	www.ucw.cz /~hubicka/papers/proj/node3.html (1804 words)

	Just-in-Time Compilation
		Most cases are simply heuristic approaches to existing algorithms; for example, the CSE technique involves assigning variables to array elements and indices inside loops and then hoisting loop-invariant code.
		The authors have improved CACAO's efficiency by eliminating linked lists wherever possible and by reducing the number of passes to three (by combining register allocation and code generation).
		The major take-home lessons of this paper are: (a) copy elimination and register allocation are key problems for JIT compilers, and (b) some basic engineering (unrelated to compilation algorithms) can be a big win.
	www.cs.berkeley.edu /~jcondit/cs265/expert.html (1594 words)

	Code Optimization Using the GNU C Compiler LG #71
		Lines 30 to 108 show the unoptimized assembly code that is a straightforward translation of the C code and should be easy to understand.
		Thus it can be seen that common subexpressions save a lot of computations and also space that is used up by the code for these redundant computations.
		At this stage, one can understand that the compiler when analyzing the program for optimization must keep a track of how and when the variables change, the expressions that are evaluated and also which registers are allocated to variables.
	www.linuxgazette.com /issue71/joshi.html (2868 words)

	[No title] (Site not responding. Last check: 2007-10-08)
		An optimizing compiler for a high-level language such as ML may make as many as 20 passes over a single program, performing sophisticated analyses and transformations such as CPS conversion, closure conversion, unboxing, subsumption elimination, or region inference.
		"We quantify the effectiveness of these techniques...which include common-subexpression elimination of types, higher-order type abbreviations, and selective reverification, [and which] can dramatically change certificate size and verification time." "Support for arrays in TALx86 is perhaps the most complicated feature in the language.
		The critical issue is that array sizes and array indices cannot always be determined statically, yet to preserve type-safety, we must ensure that any index lies between 0 and the physical size of the array.
	www.digitalmars.com /drn-bin/wwwnews?D/11299 (651 words)

	/Og - Enable Global Optimization (Windows CE .NET Application Development) (Site not responding. Last check: 2007-10-08)
		In this optimization, the value of a common subexpression is calculated once.
		For local common subexpression optimization, the compiler examines short sections of code for common subexpressions.
		For global common subexpression optimization, the compiler searches entire functions for common subexpressions.
	msdn.microsoft.com /library/en-us/wcechp40/html/ccrefOg-EnableGlobalOptimization.asp?frame=true (309 words)

	Computer Science: Publication: Common Subexpressions are Uncommon in Lazy Functional Languages
		Computer Science: Publication: Common Subexpressions are Uncommon in Lazy Functional Languages
		Common Subexpressions are Uncommon in Lazy Functional Languages
		The reason is that common subexpressions whose elimination could speed up programs are uncommon in lazy functional languages.
	www.cs.kent.ac.uk /pubs/1998/1905 (230 words)

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