Loop Shifting and Compaction for the High-Level Synthesis of Designs with Complex Control Flow

Transcription

1 Shifting n Comption for the High-Level Synthesis of Designs with Complex Control low Sumit Gupt Nikil Dutt Rjesh Gupt Alexnru Niolu Center for Emee Computer Systems Shool of Informtion n Computer Siene Dept. of Computer Siene n Engineering University of Cliforni t Irvine University of Cliforni t Sn Diego {sumitg, utt, niolu}@es.ui.eu gupt@s.us.eu sprk Astrt Emerging emee system pplitions in multimei n imge proessing re hrterize y omplex ontrol flow onsisting of eeply neste onitionls n loops. We present tehnique lle loop shifting tht inrementlly exploits loop level prllelism ross itertions y shifting n ompting opertions ross loop itertions. Our experimentl results show tht loop shifting is prtiulrly effetive for the synthesis of esigns with omplex ontrol espeilly when resoure utiliztion is lrey high n/or uner tight resoure onstrints. In situtions when further loop unrolling (or inititing nother itertion of the loop oy) les to shrp inrese in the longest omintionl pth in the iruit n the iruit re, loop shifting is le to hieve up to 2 % reution in the input-to-output ely in the synthesize iruit. We implemente loop shifting within the SPARK prllelizing high-level synthesis frmework n present results for experiments on esigns erive from multimei n imge proessing pplitions. Introution he omputtionlly expensive portions of multimei n imge proessing pplitions typilly onsist of rithmeti opertions emee in eeply neste loops with omplex mix of onitionl (if-then-else) onstruts. he fous of our work is improving the synthesis results of these oes y exposing n inresing the prllelism ville in the lgorithmi esription []. In the pst, speultive oe motions hve een use to signifintly improve the performne, re, n resoure utiliztion of the synthesize iruits y moving opertions ross onitionl ounries [2, 3]. he presene of neste loops in our pplition oes, however, limits the sope of prllelizing oe motion trnsformtions to within one loop itertion. o hieve the next level of performne improvement, we must look eyon loop itertions. In this pper, we present loop trnsformtion, lle loop shifting, tht moves opertions from one itertion of the loop oy to its previous itertion. It oes this y shifting set of opertions from the eginning of the loop oy to the en of the loop oy; opy of these opertions is lso ple in the loop he or prologue. In ontrst to loop pipelining tehniques tht initite new 53-59/4 $2. () 24 IEEE itertion of the loop oy t onstnt time (initition) intervls, loop shifting shifts set of opertions one t time, therey, exposing just s muh prllelism s n e exploite y the ville resoures. Prllelizing trnsformtions n then operte on the shifte opertions to further ompt the loop oy. Although loop unrolling n pipelining hve een propose previously for high-level synthesis, we foun tht when the resoure utiliztion is lrey high euse of either high instrution level prllelism in the esign or s result of loop unrolling the ontrol n interonnet (multiplexing) osts of further loop unrolling or loop pipelining outweigh the gins hieve in performne. his is euse frequently the longest omintionl pth in the iruit (the ritil pth length) inreses so muh tht the input to output iruit ely eomes worse (we emonstrte this through experiments in Setion 5). In suh sitution, we propose pplying loop shifting to inrementlly move oe in struture wy so tht opertions n e ompte further without exessive inrese in ontroller osts. We implemente our loop shifting tehnique in the SPARK prllelizing high-level synthesis frmework. his frmework synthesizes ehviorl esription speifie in C (with restritions on pointers, irregulr jumps, n funtion reursion) using set of ggressive ompiler, prllelizing ompiler n synthesis tehniques n genertes resoure oun RL VHDL esription. We emonstrte the utility of loop shifting y presenting sheuling n logi synthesis results for experiments performe on esigns erive from the multimei n imge proessing omins. he rest of this pper is orgnize s follows: we first review previous relte work. In Setion 3, we present the loop shifting tehnique n its vrious spets. We riefly esrie loop unrolling in Setion 3.4. We present n lgorithm for loop shifting in Setion 4, followe y the experimentl setup n finlly the results in Setion 5. 2 Relte Work unrolling n loop pipelining (or softwre pipelining) hve een explore extensively in the softwre ommunity for exploiting inter-itertion prllelism ([4, 5, 6, 7, 8, 9] to nme few). shifting ws first propose s prt of the resoure-irete loop pipelining (RDLP) teh-

2 BB BB BB S BB BB BB S () () () igure. Shifting: () An exmple with loop. () Opertions n re shifte to the en of loop oy (si lok ) n opies re inserte in the loop he (BB). () Shorter sheule length fter oe omption. nique [7]. RDLP first unrolls the loop severl times n then ttempts loop shifting n omption. Erly work on loop pipelining in high-level synthesis fouse on the innermost loops of DSP pplitions with no onitionl onstruts. hese works inlue loop wining [], rottion sheuling [], peroltion se synthesis [2], n loop foling [3]. More reent work on esigns with onitionl rnhes inlues work on sheuling opertions on the most prole pth first [4], extening rottion sheuling [5], speultively exeuting opertions from future loop itertions [6], n extening loop foling for CDGs [7]. Although severl prllelizing oe trnsformtions n loop trnsformtions hve een propose in the prllelizing ompiler ommunity, we foun tht these trnsformtions re not iretly useful for high-level synthesis (HLS). he ost funtions n optimiztion riteri for ompilers re ifferent from those of HLS. Wheres, ompilers often pursue mximum prlleliztion, we foun tht in HLS, prllelizing trnsformtions hve to e tempere y their effets on the ontrol n re (in terms of interonnet) osts. Inee, the very nture of trnsformtions tht re useful for HLS is ifferent from those tht re useful for ompilers. In some ses, this mens tht we tully en up moving opertions into the onitionl loks y reverse n onitionl speultion [3]. Similrly, in this pper, we show tht loop unrolling n le to worse iruit elys n re; inste n inrementl loop trnsformtion tehnique suh s loop shifting is more useful for HLS. 3 Shifting shifting is tehnique wherey n opertion op is move from the eginning of the loop oy to the en of the loop oy, long the k-ege of the loop. o preserve the orretness of the progrm, opy op of opertion op is ple in the loop he/prologue. hus, op is exeute efore the first itertion of the loop oy n the originl opertion op is then exeute t the en of the loop oy. his exeution orrespons to the exeution of op from the next loop itertion s per the originl oe. We emonstrte loop shifting with n exmple in igure. In this exmple, si loks BB n form the oy S2 S3 S4 or BB i < n : = + 2: = +. 3: i = i + 4: f = + e () or 6: = + BB i < n 5: = 2: = +. 3: i = i + : = + 4: f = + e igure 2. () An exmple esign. () Copy opertion, = is left in ple of shifte op to ensure oe orretness. of loop n BB is the loop he n is the loop exit or til. Soli rrows inite t flow n she rrows inite ontrol flow. Consier tht we shift opertions n from the loop oy in the originl esign in igure () to the en of the loop oy () n opies of n re inserte in the loop he (BB). he resultnt esign is shown in igure (). We n now ompt the oe insie the shifte loop oy using prllelizing trnsformtions. In the shifte esign, it is possile to sheule opertion onurrently with opertion n onurrently with opertion. he resultnt, ompte esign is shown in igure (). he stte ssignments (S to S4) for these three esigns re emrte y she lines. Clerly, the esign in igure (), fter shifting n omption, hs shorter sheule length thn the originl esign in igure (). hus, s result of loop shifting n omption, the loop oy exeutes in fewer yles. hese fewer yles multiplie y the loop itertion ount give us the reution in exeution yles of the esign. However, loop shifting is useful only when the gins in performne of the loop oy is lrger thn the overhe of the opies of the shifte opertions tht re ple in the loop he. 3. Ensuring the Corretness of Coe Shifting n opertion les to one extr exeution of the opertion over the numer of times it is exeute in the originl oe. his n e unerstoo y the shifte esign shown erlier in igure (). In this esign, if the loop exeutes for 8 itertions, then the shifte opertion exeutes 8 times insie the loop oy plus one in the loop he (si lok BB). In ontrst, in the originl esign in igure (), opertion exeutes only 8 times insie the loop oy. o ensure tht exeuting the shifte opertion one extr time oes not hnge the ehvior of the progrm, we write the result of the shifte opertion, op, to new vrile, newvr n in ple of op, we leve opy opertion from newvr to the result vrile of the originl opertion op. We emonstrte this through n exmple in igure 2(). Here, the result of opertion in the loop oy (in si lok ) is re y opertion 4 fter the loop. Consier tht we shift opertion to the en of the loop oy n ple opy s opertion 6 in the loop he. Both these opertions write to new vrile n opy opertion = is left in ple of the originl opertion. his en- ()

3 If BB6 () BB BB7 BB g If BB6 () BB g BB BB7 If BB6 () BB BB " g " S S2 S3 S4 S5 BB7 S6 igure 3. () A esign with if-then-else onitionl lok insie loop. () Opertions n re shifte from the longer onitionl,. () Coe omption y upliting opertions n into oth onitionl rnhes. sures tht opertion 4 gets the orret vlue of fter loop shifting. he resultnt esign is shown in igure 2(). We lso hve to mintin the inter n intr-itertion t epenenies while pplying loop shifting sine shifte opertion my hve t epenenies ross loop itertions. In the exmple in igure 2(), opertion res the vrile tht is written y opertion 2. Hene, fter shifting opertion, we hve to t epeneny r from opertion 2 to shifte opertion. 3.2 Shifting s with Conitionl Brnhes In loops with onitionl onstruts, opertions n e shifte from within onitionl rnh. Sine the gol of our pproh is to minimize the length of the longest pth through the esign, we shift opertions from the rnh of the onitionl with the longer sheule length. Consier the exmple in igure 3(). his exmple hs n if-then-else onitionl lok within the oy of loop. Sine the true rnh (si lok ) of this if-lok hs longer sheule length (of 3) thn the flse rnh (), we hoose to shift opertions from si lok. Hene, onsier tht we shift opertions n from, s shown in igure 3(). he prllelizing oe trnsformtions n now ompt the shifte oe y upliting opertions n into oth rnhes of the if-lok, s opertions n n n. he resultnt esign is shown in igure 3(). Note tht, we employ oe motion lle onitionl speultion [] for opertion uplition. 3.3 Our Approh to Shifting We perform loop shifting fter sheuling the loop oy one. he sheuler my sheule some opertions to exeute onurrently in the sme yle. We term set of onurrent opertions in si lok s sheuling step. In our pproh, inste of shifting one opertion t time, we shift n entire sheuling step ross loop itertions. his is euse shifting only one of severl onurrent opertions will not eliminte the sheuling step n thus, the sheule length of the si lok (n loop oy) will not erese. In the esign in igure 3(), we hose to shift the first sheuling step in, i.e., oth opertions n (inste of just one of them). S7 /* Shifts one sheuling step in the loop oy */ ShiftBoy(Noe) : f irstbb irstbb(noe loopboy) 2: steposhi f t insteposhift( f irstbb) 3: BB(stepoShi f t) BB(stepoShi f t) - steposhi f t 4: lstbb LstBB(Noe loopboy) 5: lstbb lstbb steposhi ft 6: loophebb LstBB(Noe loophe) 7: loophebb loophebb Copy(stepoShi ft) 8: Resheule(Noe) () /* Reursive funtion tht returns step to shift from */ /* the si lok in the longer onitionl rnh */ insteposhift(urrbb) Returns: he step to shift : steposhi f t irstnonconstep(urrbb) 2: if (steposhi ft = /) { 3: in nextbb SUCCS(urrBB) with the mximum NumSteps(nextBB) 4: steposhi f t insteposhift(nextbb) 5: } 6: return steposhi f t () igure 4. Shifting Algorithm: shifts one sheuling step in loop oy. 3.4 Unrolling unrolling is oe trnsformtion in whih uplite of one or more itertions of the loop oy is ple t the en of the urrent loop oy. he loop ouns n loop inex vrile inrement re upte s neessry. unrolling is use for exposing prllelism ross loop itertions n thus, enle oe omption of the unrolle loop oy. However, loop unrolling n le to oe explosion; so, loops re usully unrolle one itertion t time. In our synthesis frmework, the numer of unrolls for eh loop is user-irete. Our synthesis tool first unrolls the loop s speifie y the esigner n then sheules the esign. We n thus stuy the effets of ifferent unrolling ftors on hrwre osts n iruit performne. 4 Shifting Algorithm Our loop shifting lgorithm is liste in igure 4(). his lgorithm tkes the loop noe to e shifte s input n shifts one sheuling step from the eginning of the loop oy to its en. We use n intermeite representtion lle Hierrhil sk Grphs (HGs) [8, ] tht enpsultes onstruts suh s loops, if-then-else loks, et eter in hierrhil noes tht in turn my hve su-noes. Using this intermeite representtion, we n ess the su-prts (loop he, oy, n til) of loop y referring to Noe loophe, Noe loopboy n Noe loopil. he loop he n loop til eh ontin one si lok, wheres the loop oy is hierrhil noe tht my ontin other hierrhil noes (inluing if-then-else loks n other loops). By efinition, eh HG noe HtgNoe hs Strt (or first) si

4 Design # # # # # Resoures Ifs s BBs Ops + == << [] / pre pre tiler le. Chrteristis of the esigns use in our experiments n the resoures llote for sheuling them. lok n Stop (or lst) si lok tht n e otine y irstbb(htgnoe) n LstBB(HtgNoe) []. he loop shifting lgorithm strts y looking for sheuling step to shift. o o this, it lls the funtion insteposhi ft with the first si lok in the loop s rgument. his funtion, liste in igure 4(), lls the funtion irstnonconstep for eh si lok urrbb. his funtion returns NULL step if urrbb is empty (ue to pst shift opertio or urrbb only hs sheuling steps with onitionl Boolen heks (enote y tringles in our figures). If irstnonconstep oes not fin sheuling step, the insteposhi f t funtion reursively trverses the si loks in the loop oy till it fins sheuling step in one of them. If si lok hs severl suessor si loks (rnhes), the lgorithm trverses to the rnh with the lrger numer of sheuling steps. One the insteposhi f t funtion returns sheuling step steposhi ft, this step is remove from its si lok, n e to the lst si lok in the loop oy (lines 3 to 5 in igure 4()). A opy of steposhi ft is lso e to the loop he (lines 6 n 7). We then resheule the loop y lling the funtion Resheule. Note tht, y ing or removing sheuling step, we men tht the opertions in tht step re e or remove from si lok. In the worst se, this lgorithm my en up trversing ll the si loks in loop. In prtie, it usully trverses not more thn 2 to 3 si loks. Resheuling the loop, on the other hn, n e omputtionlly expensive. However, in prtie, only the shifte opertions hve to e repke in the sheule. In our experiments, we fin tht the run times of our synthesis tool, on.6 Ghz PC running Linux, rnge from -3 uses (user seons) with no loop shifting, 2 to 6 uses with loop shifting n 5 to uses with loop unrolling (see next two setio. 6 % of this time is spent in resoure ining, sine it is formulte s network flow prolem. In ontrst, the logi synthesis tool tkes etween 2 to 8 hours for synthesizing these esigns. 5 Experimentl Setup n Results We implemente the loop unrolling n shifting trnsformtions, long with the shifting lgorithm in the SPARK high-level synthesis frmework []. he SPARK frmework tkes n input in ANSI-C (with restritions of no pointers, irregulr jumps, n reursive funtio n proues synthesizle RL VHDL. he frmework pplies rnge of ompiler trnsformtions (loop-invrint oe motion, CSE, opy propgtion, e oe elimintion) n prllelizing ompiler trnsformtions (speultive oe motions, ynmi CSE) []. So ll the results presente in this setion represent improvements over esign lrey optimize y these oe trnsformtions. rnsform MPEG- pre MPEG- pre2 Applie # Sttes # yles # Sttes # yles No Unrolls Unroll 48(+26.3%) 83(-.7%) 79(+4.5%) 23(-4.5%) 3 Unrolls 66(+37.5%) 723(-%) 97(+22.8%) 95(-3.9%) Reu % -9.6 % +4.6 % -8.3 % le 2. Sheuling results fter unrolling the inner loops in pre n pre2. rnsformtion GIMP tiler Applie # Sttes # yles No Unrolls Unroll 52 (+62.5 %) 2284 (-9.9 %) 4 Unrolls 97 (+86.5 %) 834 (-9.7 %) Reution +23. % % le 3. Sheuling Results fter unrolling for tiler. In this setion, we present results for experiments performe using esigns erive from two moertely omplex rel-life pplitions: the pre n pre2 funtions from the Preition lok of the MPEG- lgorithm n the tiler trnsform from the GIMP imge proessing tool [9]. or the experiments presente elow, we pply loop unrolling n loop shifting to inner loops of the three esigns. All three esigns hve ouly neste loop pirs tht form the min kernel of these pplition oes. le lists the hrteristis of the vrious esigns in terms of the numer of if-then-else loks, for-loops, nonempty si loks n opertions in the input esription. he resoures llote to sheule these esigns re lso liste: + oes n sutrt, == is omprtor, multiplier, / ivier, [ ] n rry ress eoer n << is shifter. he multiplier ( ) exeutes in 2 yles n the ivier (/) in 5 yles. All other resoures re single yle. 5. Synthesis Results for Unrolling We first present the sheuling results for loop unrolling in les 2 n 3. he unroll ftors re etermine s follows: for loop with n itertion ount of N, we llow unrolling the loop y M times suh tht N/( + M) is n integer n less thn or equl to. he loops tht re unrolle in pre, pre2 n tiler hve N equl to 8, 8 n respetively. Hene, for N=8, possile vlues of M re, 3 n 7, n for N=, M n e, 4 or 9. he sheuling results re in terms of the numer of sttes in the SM ontroller n the yles on the longest pth. Longest pth for loops is the yles on the longest pth through the loop oy multiplie y the numer of loop itertions. he first row in the two tles lists the results for the se when no loop unrolling is one, the seon row for one loop unroll, n the thir row for 3 loop unrolls for the pre n pre2 esigns n 4 unrolls for tiler. he perentge reutions of eh row over the previous row re given in prentheses. he lst row gives the totl reution of the thir row over the first row. he results in the seon row of les 2 n 3 show tht with one unroll, we n hieve improvements rnging from 4 % to % in the yles on the longest pth for the three esigns. Unrolling the loop further (three times for the pre n pre2 esigns n four times for tiler) les to further improvement of, 4 n 9.7 % respetively.

5 MPEG- Pre untion MPEG- Pre2 untion GIMP iler untion No Unrolls Unroll 3 Unrolls for MPEG/4 for tiler igure 5. Logi synthesis results fter loop unrolling. However, the results in les 2 n 3 lso show tht loop unrolling les to lrge inrese in the size of the SM ontroller (numer of sttes). his is euse when the loop oy is uplite, the numer of ontrol steps in the sheule inreses, even though the numer of exeutions of the loop oy my reue. o stuy the impt on iruit re n ely, we performe logi synthesis on the RL VHDL generte fter sheuling, ining, n ontroller genertion y the SPARK high-level synthesis tool. We use Synopsys Design Compiler with the SMC.3 miron tehnology lirry. he logi synthesis results re presente in the grphs in igure 5. he metris mppe in these grphs re the ritil pth length in nnoseons (s etermine y the stti timing nlysis tool), the longest input to output ely or lteny of the iruit (longest pth yles multiplie y the ritil pth length), n the unit re of the iruit (in terms of the tehnology lirry). All vlues re normlize y the no unrolling se. he ritil pth length is longest omintionl pth through the iruit n hene, ittes lok perio or frequeny. he results in these grphs show tht when the loops re unrolle, the ritil pth lengths inrese y to 25 %. his inrese works ginst the gins hieve in yles through the longest pth. As result, the longest input to output ely or lteny through the three esigns remins lmost onstnt s the loops re unrolle. However, there is sustntil inrese in re from 22 % to up to 5 %. he inreses in ritil pth length n re re ue to the lrger ontroller size n more omplex steering logi (multiplexers, e-multiplexers n ssoite ontrol logi). As the loops re unrolle, the numer of opertions in the esign inreses. Hene, lrger numer of opertions re mppe to the sme numer of resoures. his inreses resoure utiliztion, whih in turn les to n inrese in the size n omplexity of the steering logi. 5.2 Synthesis Results for Shifting les 4 n 5 list the sheuling results for the three esigns s the inner loops re shifte, strting from no loop shifting (first row) to three shifts (fourth row). he perentge reutions of eh row over the previous row re given in prentheses. he lst row gives the totl reution of the fourth row (3 loop shifts) over the first row (no loop shifts). he results in this tle show tht s the loops re shifte, the sheule length (yles on the longest pth) n sometimes inrese. his hppens when set of onurrent opertions is shifte from one rnh of n lrey lne onitionl lok. his mens tht, potentilly, fter shift rnsform MPEG- pre MPEG- pre2 Applie # Sttes # yles # Sttes # yles No Shifts Shift 36(-5.3 %) 77(-4%) 67(-2.9 %) 999(-6 %) 2 Shifts 37(+2.8 %) 779(+ %) 68(+.5 %) 27(+.4 %) 3 Shifts 36(-2.7 %) 75(-8.2 %) 67(-.5 %) 943(-3.2 %) Reu -5.3 % -2.5 % -2.9 % -8.7 % le 4. Sheuling results fter loop shifting. rnsform GIMP tiler Applie # Sttes # yles No Shifts Shift 3 (-6.3 %) 2334 (-7.9 %) 2 Shifts 3 ( %) 2244 (-3.9 %) 3 Shifts 29 (-3.3 %) 254 (-8.5 %) Reu -9.4 % -8.9 % le 5. Results fter shifting the inner loops. urther shifting oes not improve sheuling results. MPEG- Pre untion MPEG- Pre2 untion GIMP iler untion Pth( No Shifts Shifts 2 Shifts 3 Shifts igure 6. Logi synthesis results fter loop shifting. ing the sheuler is unle to ompt the loop oy to its size efore shifting. However, in suh se, we n usully get k to the originl sheule length y shifting one more; this time the sheuling step from the other rnh of the onitionl gets shifte. If two onseutive shifts proue worse results, this inites tht we shoul stop shifting. he worse results men tht it is not possile to ompt the loop oy with ny more shifte opertions. We re urrently eveloping more eterministi wys to etermine the numer of loop shifts. or now, we n experiment with ifferent loop shifts ue to low run times of our synthesis tool for firly lrge esigns. rom the results in les 4 n 5, we n see tht the est sheuling results re hieve for ll the esigns fter shifting the loop 3 times. he totl reutions (lst row) rnge from 8 to 2 % in the yles on the longest pth n 2 to 9 % in the sttes in the SM ontroller. hese sheuling results trnslte over to the ritil pth length n re results otine fter logi synthesis. hese re presente in igure 6. he rs in these grphs orrespon to no shifts, shift, 2 shift n 3 shifts. hese logi synthesis results show tht we n hieve 5-2 % improvements in the ely through the iruit y employing loop shifting, while inurring mrginl inreses in iruit re ompre to loop unrolling. Rell tht the se se (no shifts) represents esign tht is lrey optimize y rnge of prllelizing ompiler trnsformtions []. 5.3 Results with Higher Resoure Allotion We rn our experiments gin with higher resoure llotion for the MPEG esigns. We use 4 ers n 3 Due to lk of spe, we left out results for tiler in this setion

6 rnsform MPEG- pre MPEG- pre2 Applie # Sttes # yles # Sttes # yles No Unrolls Unroll 42(+6.7%) 6(-2.8%) 72(+5.9%) 775(-8.3%) 3 Unrolls 56(+33.3%) 563(-7.9%) 86(+9.4%) 727(-2.7%) Reu % -27 % % -.7 % le 6. Higher resoure llotion: Sheuling results fter unrolling the inner loops in pre n pre2. rnsform MPEG- pre MPEG- pre2 Applie # Sttes # yles # Sttes # yles No Shifts Shifts 36( %) 659(-4.5%) 66(-2.9%) 823(-5.8%) 5 Shifts 36( %) 63(-8.5%) 66( %) 767(-3.%) 7 Shifts 37(+2.8%) 555(-8 %) 67(+.5%) 79(-2.7%) Reu +2.8 % -28 % -.5 % -.2 % le 7. Higher resoure llotion: Sheuling results fter shifting loops in pre n pre2. rry eoers inste of 2 eh, with other resoures eing the sme s efore (inresing the other resoures i not ffet sheuling results). he sheuling results for loop unrolling n shifting re liste in les 6 n 7 respetively. With higher resoure llotion, the improvements re lrger for oth loop unrolling n loop shifting. Also, loop unrolling les to smller inrese in ontroller size euse the unrolle opertions get ompte more esily ue to the higher resoure llotion. Also, we re le to o more loop shifting: 7 shifts versus the erlier 3. he logi synthesis results for these experiments re presente in igure 7. here results gin emonstrte tht loop unrolling les to lrge inrese iruit re with only moest improvements in iruit ely. In ontrst, loop shifting gin les to 5 to 25 % improvement in iruit ely with firly onstnt iruit re. 6 Conlusions n uture Diretions We presente loop trnsformtion tehnique lle loop shifting tht inrementlly exposes prllelism ross loop itertions y shifting opertions from one itertion of the loop oy to the previous one. Experimentl results on esigns erive from multimei n imge pplitions show tht loop shifting results in up to 2 % lower elys with inreses in re etween -2 %. hese represent improvements over esigns lrey optimize y our synthesis frmework using rnge of prllelizing oe motions n oe trnsformtions. In ontrst, the ontrol n multiplexing overhes of loop unrolling uno the gins hieve in sheule lengths. he length of the ritil pth (longest omintionl pth) n thus, iruit ely inreses. Ciruit re inreses y up to 5 %. In future work, we wnt to evelop ost moels to guie our loop trnsformtion heuristis tht estimte the impt of the loop trnsformtions on iruit ontrol n interonnet osts. Referenes [] S. Gupt, N.D. Dutt, R.K. Gupt, n A. Niolu. SPARK: A high-level synthesis frmework for pplying prllelizing ompiler trnsformtions. VLSI Design, 23. [2] K. Wkyshi n H. nk. Glol sheuling inepenent of ontrol epenenies se on onition vetors. In Design Automtion Conferene, MPEG- Pre untion MPEG- Pre2 untion No Unrolls Unroll 3 Unrolls MPEG- Pre untion MPEG- Pre2 untion Pth( No Shifts 3 Shifts 5 Shifts 7 Shifts igure 7. Higher resoure llotion: Logi synthesis results fter loop unrolling n loop shifting. [3] S. Gupt, et l. Conitionl speultion n its effets on performne n re for high-level synthesis. In Intl. Symp. on System Synthesis, 2. [4] B. R. Ru n C. D. Gleser. Some sheuling tehniques n n esily sheulle horizontl rhiteture for high performne sientifi omputing. In Annul Workshop on Miroprogrmming, 98. [5] A. Aiken, A. Niolu. Perfet Pipelining: A new loop prlleliztion tehnique. Euro Symp. on Progrmming, 988. [6] M. Lm. Softwre pipelining: An effetive sheuling tehnique for VLIW mhines. PLDI, 988. [7] S. Novk n A. Niolu. An effiient, glol resoureirete pproh to exploiting instrution-level prllelism. In Conferene on PAC, 996. [8] A. Aiken, A. Niolu, n S. Novk. Resoure-onstrine softwre pipelining. IEEE rnstions on Prllel n Distriute Systems, 6(2), Deemer 995. [9] R. Jones n V. Alln. Softwre pipelining: A omprison n improvement. In Proeeings of the Miro-23, 99. [] E. Girzy. wining - t flow pproh to funtionl pipelining. Intl. Symp. of Ciruits n Systems, 987. [] L.-. Cho, A. S. LPugh, n E. H.-M. Sh. Rottion sheuling: A loop pipelining lgorithm. DAC, 993. [2] R. Potsmn, J. Lis, A. Niolu, n D. Gjski. Peroltion se synthesis. In Design Automtion Conferene, 99. [3] G. Goossens, J. Vnewlle, n H. De Mn. optimiztion in register-trnsfer sheuling for DSP-systems. In Design utomtion onferene, 989. [4] U. Holtmnn n R. Ernst. Comining MBP-speultive omputtion n loop pipelining in high-level synthesis. In Europen Design n est Conferene, 995. [5]. Z. Yu, E. H.-M. Sh, N. Pssos, n R. D.-C. Ju. Algorithms n hrwre support for rnh ntiiption,. In Gret Lkes Symposium on VLSI, 997. [6] G. Lkshminryn, A. Rghunthn, n N.K. Jh. Inorporting speultive exeution into sheuling of ontrolflow intensive ehviorl esriptions. DAC, 998. [7] I. Rivojevi n. Brewer. Anlysis of onitionl resoure shring using gur-se ontrol representtion. In Interntionl Conferene on Computer Design, 995. [8] M. Girkr n C.D. Polyhronopoulos. Automti extrtion of funtionl prllelism from orinry progrms. IEEE rns. on Prllel & Distriute Systems, Mr [9] GNU Imge Mnipultion Progrm. gimp.org.