Instruction Unit. Scheduling Unit. 8-Port Register File. Execution Unit

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1 PERFORMANCE ISSUES OF A SUPERSCALAR MICROPROCESSOR Svn Walla and Nadr Baghrzadh Dparmn of Elrial and Compr Enginring Univrsiy of California, Irvin Irvin, CA swalla or nadr@.i.d Absra: Cah, dynami shdling, bypassing, branh prdiion, and fh iny ar primary isss onrning prforman of a sprsalar miroprossor. This papr onsidrs all hs isss and shows hir impa on prforman by rnning or simlaor on svnn dirn programs. Or approah in handling branh prdiion is shown o signianly dras h bad branh pnaly. Frhrmor, rsls show ha h avrag insrion fh plas an ppr bond on spdp and is h mos riial faor in drmining ovrall prforman. Is prforman impa is grar han all ohr faors ombind. INTRODUCTION In rn yars, hr has bn a growing inrs in dsigning miroprossors basd on h noion of Insrion-Lvl Paralllism (ILP). Thr ar dirn approahs for xploiing ILP. On approah ss rnim shdling o vala h daa dpndnis and x insrions onrrnly. A miroprossor basd on his hniq is alld a sprsalar miroprossor [4]. Anohr approah, ommonly known as a Vry Long Insrion Word (VLIW) arhir [2], is nirly basd on ompil-im analysis o xra paralllism. Ths arhirs xploi ILP by issing mor han on insrion pr yl. VLIW prossors rqir a sophisiad ompilr whil sprsalars iliz dynami shdling o xra paralllism a rn-im, in addiion o sai shdling [6]. This papr analyzs h prforman isss of a sprsalar prossor, h SDSP (Sprsalar Digial Signal Prossor) dvlopd a h Univrsiy of California, Irvin. Th SDSP ss a nralizd window and dynami rgisr rnaming o allow o-of-ordr iss and omplion of insrions. Th GNU CC ompilr, assmblr, and linkr wr pord. A simlaor was wrin o x SDSP obj od on a SUN worksaion. Th simlaor is ongrabl for many dirn paramrs so i an vala h prforman araly via xion-drivn mhods. Afr olling rsls from dirn rns of svral programs, ah iss is valad o drmin is prforman impa. Finally, all faors ar onsidrd oghr. THE SDSP ARCHITECTURE Th Sprsalar Digial Signal Prossor (SDSP) is a 32-bi sprsalar prossor wih a RISC-syl insrion s. I is piplind and has a fh bandwidh of for insrions pr yl. Th SDSP rsarh proj bgan in 1992 a U.C. Irvin. Th prossor is spially sid for digial signal prossing appliaions, y is powrfl on any gnral prpos appliaion. I follows VLSI dsign sragis of VIPER and TinyRISC arhirs [1, 2]. Dsign disions wr mad wih fll onsidraion for VLSI implmnaion. For a daild dniion of h SDSP and is insrion s, rfr o [8]. Arhiral Organizaion Th arhir organizaion of h SDSP is shown in Figr 1. I is dividd ino hr basi nis: h Insrion Uni (IU), h Shdling Uni (SU), and h Exion Uni (EU). Insrion Uni Shdling Uni Exion Uni Insrion 1 Insrion 2 Insrion 3 Insrion 4 8-Por Rgisr Fil Insrion Cah Program Conr Insrion Dodrs Rordr Bffr ALU1 ALU2 ALU3 ALU4 Branh Prdior Main Shdling Uni Load Uni Sor Bffr Daa Cah Insrion Window CTU 16-bi Mliplir Figr 1: SDSP Arhiral Organizaion

2 Insrion Uni Th IU mainains h onrol ow of h prossor. Th basi omponns of h IU ar h Program Conr (PC) and branh prdior. Program Conr. Th PC handls h onrol ow of h prossor. Th mmory addrss sd by h PC is organizd in h forma shown in Figr Blok Addrss Word Addrss By Addrss Figr 2: Mmory Addrss Congraion Branh Prdior. SDSP ss a sprsalar branh prdior o sppor splaiv xion [3]. Th prdior is a modid branh arg br [5], and is apabl of prforming mlipl branh prdiions pr yl. I ss an p-down saraing onr o vala branh prdiion for ah branh insrion nonrd. Eah nry in h prdiion br is indxd by h insrion blok addrss whih onains prdiion informaion for p o for branh insrions in h blok. Th prdior valas all branh lds in h nry simlanosly o drmin h rs branh ha is prdid \akn", if any. Thmlipl prdiion far liminas h nd o r-fh h sam insrion blok if a branh is prdid \no-akn". Thrfor, p o for prdiions ar aomplishd in on yl. Whn h SU ommis a blok of rsls o h rgisr l, branh prdior saisis ar pdad basd on oom of branh prdiions in ha blok. Shdling Uni Th Shdling Uni dods and isss insrions rivd from h Insrion Uni o h Exion Uni. I mainains h propr sa of h mahin. Ths asks ar aomplishd sing an insrion window, rordr br, and rgisr l [4]. Dynami o-of-ordr shdling is ahivd sing a rordr br and a nral insrion window, whih opra as a singl FIFO (Firs in, Firs O) ni. Th SU dph is dnd o b h oal nmbr of nris dividd by h dod siz (for for h SDSP). I is qivaln o h nmbr of shif yls i aks for h insrion o b ommid o h rgisr l and lav h Shdling Uni. Nw insrions nr a h op of h SU. Compld insrions xi a h boom of h SU and ar ommid o h rgisr l. A SU sall ors whn h SU anno shif bas h rordr br is waiing for a rsl or h insrion window has no issd an insrion in h las blok. Rady-o-rn insrions, howvr, onin o b issd from h SU rgardlss of h yp of sall. Shdling Algorihm. Th SDSP isss insrions sing h \olds rs" shdling algorihm. An insrion is onsidrd for ponial shdling if is oprands ar rady, i will riv is rmaining oprand(s) from a fnional ni a h nd of h rrn yl,oriisr- rnly bing dodd by h rordr br. Propr ordring of loads and sors is aomplishd by in-ordr issing of sors b allowing o-of-ordr issing of loads wih rsp o sors. In addiion, a sor may no b ommid o mmory nil prvios branhs ar rsolvd. Iss Ra. Th SU may iss p o igh dirn insrions pr yl (4 ALU, 1 mliply, 1 load, 1 sor, and 1 onrol ransfr), wi h dod siz of for. Rsl Wri Poliy. Th SU an riv p o for rsl vals and orrsponding dsinaion ags ah yl from h Exion Uni. This ras a problm sin six rsls may b rady in a parilar yl (4 ALU, 1 mliply, and 1 load). This is rsolvd by giving prioriy o load and mliply rsls ovr ALU rsls. Exion Uni Th xion ni onsiss of for ingr Arihmi and Logi Unis (ALU), on ingr mliplir, on load ni, on sor ni, and on onrol ransfr ni. Eah fnional ni aks as inp h rqird oprands, onrol signals, and dsinaion rgisr ag. Simlaor SIMULATION METHODS Afr ompiling, assmbling, and linking h original C sor od, h obj od is loadd and xd. A yl by yl simlaion gahrs saisis abo is xion. Th simlaor maks h following assmpions and onsidraions: Insrion Uni. Insrions from an nalignd blok fh ar invalidad, as wll as insrions afr a onrol ransfr and no-ops. Branh Prdiion. Whn a branh isnonrd, h simlaor onins down is prdid pah nil i is drmind o b inorr. Shdling Uni. Insrions ar issd o-of-ordr sing h \olds rs" shdling algorihm, as long as h iss limi paramr has no bn xdd. Exion Uni. Eah fnional ni may wri bak is rsl if i dos no xd h maximm rsl wris pr yl paramr. Prioriy isgivn o load, mliply, and ALU insrions, in ha ordr. ALU opraions x in a singl yl. Mliply opraions ar piplind wih a wo yl lany. Load and sor opraions ak a singl yl on a daa ah hi (or prf ah). Sor br. Th sor br was no simlad. I is assmd a rasonabl sor br siz wold b sd (say, 8nris), so ha h probabiliy of salls wold b low and is ngligibl. Cah. Th insrion ah is dir mappd. Th daa ah is wri hrogh dir mappd. Th pnaly foraah miss is 6 yls. Floaing Poin. FP opraions ar implmnd as rap insrions. Whn a FP opraion is nonrd, h orrsponding yl lany rqird o mla his fnion sing ingr opraions is addd, and h blok is rfhd, if nssary. Opraing Sysm. OS alls ar implmnd as raps. Whn a rap o h OS is nonrd, h simlaor alls h orrsponding OS fnion on h hos sysm and hn rsms simlaion.

3 Bnhmarks Th rsls in his papr ar from svnn ingrinnsiv programs. All bnhmarks wr rn nil omplion. Eigh of h programs om from h Sanford si of bnhmarks (bbbl, inmm, prm, pzzl, qns, qiksor, owrs, r). Eqno oms from h SPEC '89 Ingr bnhmark si. DCT is a som bnhmark whih was xrad from h JPEG imag omprssion sandard sin i is widly sd in digial signal prossing appliaions. I prforms a disr osin ransform on a 8x8 pixl imag and hn prforms h invrs ransform a oal of 100 ims. Th MPEG, P64, and JPEG (omprssion only) wr wrin by h Porabl Vido Rsarh Grop. Th domprssion for JPEG am from h Indpndn JPEG Grop's sofwar, rlas 4. Exion from hs programs rangs from js a half a million o on and a half billion insrions. Dfal Congraion Unlss ohrwis nod, h bas ongraion of h simlaor ss for ALU nis, on mliplir, on load ni, on sor ni, 8 insrion iss limi, 4 rsl wri limi, prf ah, and a 64 nry mlipl branh prdiion br. PERFORMANCE FACTORS Fh Einy Th SDSP prossor fhs a blok a a im, whih onains for insrions. Howvr, all for insrions may no b valid bas h rs insrion is no a h rs dod posiion or a onrol ransfr is no a h las dod posiion. Th Avrag Insrion Fh (AIF) on plas an ppr bond on h spdp of a program, sin h prossor anno x mor insrions han i aally dods. Figr 3 shows h AIF on for ah program for a dod siz of 2 and a dod siz of F 3.0 h C o n d dhrysn qno 2.68 bbbl 3.45 inmm 2.83 jpg mpg. p64. jpg.d mpg.d prm qns owrs pzzl qiksr 2.75 Dod 2 Dod 4 Figr 3: Avrag Insrion Fh vs. Dod Siz r As xpd, h AIF for a dod siz of 4 is signianly largr han for a dod siz of 2. Also shown in h gr ar h avrag fh for a dod siz of 2 and 4. Alhogh h dod siz dobld, h AIF only inrasd by abo 73%. Th avrag insrion loss pr blok d o misalignmn inrass as h dod siz inrass. Thrfor, inrasing h dod siz has a marginal rrn o AIF. For a dod siz of 4, Figr 4 displays h prnag of insans whr 1, 2, 3, or 4 valid insrions wr fhd and xd. As an b obsrvd, ovr half h im 4 valid insrions wr fhd. Th ohr half is spli p roghly vnly bwn 1, 2, and 3 fhs. F h P r n 100% 90% 80% 70% 60% 50% 30% 20% 10% 0% d dhrysn qno bbbl inmm jpg. mpg. jpg.d mpg.d p64. 1 Fh 2 Fh prm 3 Fh 4 Fh qns owrs pzzl qiksr Figr 4: Insrion Fh Disribion Programs wih lss alignmn sally r mor onrol ransfr insrions and lowr AIF. In h as of DCT, h AIF is narly prf bas insrion rns ar long. If insrions old b rarrangd sh ha mos onrol ransfr insrions ar a h las dod posiion, and is dsinaion addrss is a h rs dod posiion, a sbsanial inras in AIF old b ahivd. Branh Prdiion Using a 64-nry prdiion branh br, h programs wr abl o ahiv an avrag branh prdiion aray of 88%. On h ohr hand, if a simpl \no akn" prdiion poliy wr sd, h programs ahivd a rlaivly poor avrag branh prdiion of 37%. Th programs' aray for boh shms is shown in Figr % 80% A 60% r a 41% y 20% 0% 92% d 61% 79% 15% dhrysn qno 96% 93% 95% 26% 3% bbbl inmm 19% 91% 92% jpg. 26% 84% 15% 92% 31% 87% mpg. p64. jpg.d mpg.d 51% 93% 63% 74% prm 8% 95% 52% 65% 89% 75% 96% qns owrs pzzl qiksr Figr 5: Branh Prdiion Aray 42% Prdi "no-akn" SDSP Prdior On way o handl branhs is o x hm in-ordr whn h branh rahs h boom of h SU. If a branh is misprdid, h nir rordr br and insrion window ar invalidad and xion rsms a h orr loaion. Th pnaly assoiad wih a misprdid branh wold b h dph of h SU. This pnaly is onsan and grows linarly wih rsp o h siz of h SU. Alrnaivly, if h SU rovrd from a misprdid branh as soon as h oom was known, sbsanial savings old b ahivd. Tabl 1 shows h avrag bad branh yl pnaly for Shdling Uni dph of 2, 89% r r

4 4, 8, and 16 sing his mhod wih a 64-nry branh prdiion br. Th bad branh yl pnaly is h nmbr of inorrly fhd bloks, pls an addiional yl pnaly for branhs misprdid akn ha ar no on h las dodr posiion, sin i ms rfh h sam blok pon rovry. On h ohr hand, yls ha salld d o daa dpndnis in h Shdling Uni ar no onsidrd par of h branh pnaly. For insan, if afr a misprdid branh h SU salls a yl and rsolvs ha branh, h Insrion Uni will rsm fhing h orr blok and will hav nvr fhd an inorr blok. Hn, hr is no pnaly assoiad wih ha misprdiion, sin a orr prdiion wold hav rsld in idnial prforman. This is how an avrag pnaly of lss han on yl an b ahivd. Tabl 1: Avrag Bad Branh Pnaly vs. SU Dph SU Dph Program d dhryson qno bbbl inmm jpg jpg.d mpg mpg.d p prm pzzl qns qiksor owrs r Avrag Th dph of h SU drmins h ponial of xraing paralllism and hiding h lany of longr lany opraions (sh as a daa ah miss on a load opraion). Figr 6 shows h ovrall spdp sing a SU dph of 2, 4, 8, and 16 for all h programs. Th spdp dirn bwn SU dph of 2 and 4 is sbsanial. Th inras from a dph of 4 o 8 is signian in mos ass. Comparing h spdp sing SU dph of 8 o h avrag fh on, as in Figr 3, shows ha mos programs hav om los o rahing hir maximm ponial spdp. Th dirn is sally from misprdid branh dlays. If hr is sill a signian dirn, inrasing h SU dph o 16 will rd his gap, as noid wih d, inmm, and jpg.d. Ohrwis i will mak no dirn and an aally rd spdp slighly as in h as wih bbbl, prm, pzzl, qiksor, and rsor. Th loss in spdp is from a dras in branh prdiion aray. Branh prdiion saisis ar pdad whn a blok of insrions is shifd o of h SU. Thrfor, branh aray is somims los whn inrasing h SU dph bas his inrass h lany rqird for pda S p d p SU Dph dhrysn bbbl jpg. mpg. p64. prm qns owrs d qno inmm jpg.d mpg.d pzzl qiksr r Bypassing Figr 6: Spdp Improvmn Th SDSP ss ompl bypassing of rsls o fnional nis o avoid xra lany. This, howvr, is osly in hardwar. Figr 7 shows h prforman wiho bypassing ompard o ompl bypassing sd in h bas mahin. As an b obsrvd, bypassing has a wo o hr ims savings in h nmbr of sall yls. This is a signian prforman savings. Anohr way of looking a i is ha h siz of h SU nds o b ovr wi as larg if bypassing is no sd o g qivaln prforman. For xampl, as shown in h gr, a a SU dph of 3 wih bypassing wold rqir a SU dph of 7 wiho bypassing for h sam prnag of SU salls. Hn, sing bypassing is worhwhil sin h xra spa in layo is signianly lss han a dobling of h siz of h SU W/O Bypassing % S a l l s Cah Figr 7: Prnag Salls vs. SU Dph Insrion fhing ms b ssaind, or i will b impossibl o ahiv a high IPC ra. In addiion, daa ms b qikly assibl, or ls poor prforman will rsl sin hr will b nohing o omp. Thrfor, h insrion and daa ah shold b dsignd o allow h prossor o ahiv los o idal prforman, ignoring ohr faors. Wih a 4 KB or 8 KB insrion and daa ah, mos programs ran wih a ah miss ra ndr 1% wih a oal dgradaion in prforman of abo 5%, on h avrag. Alhogh ah an no b ignord, is prforman impa is wll known and an b dsignd o m h prforman nds of mos programs [7]. Bas

5 Ovrall Analysis Th ovrall prforman is drmind by svral faors: insrion ah miss yls, Dlay i;ah yls dlayd from daa ah misss, Dlay d;ah bad branh prdiion pnaly yls, Dlay branh Shdling Uni sall yls, Sall SU and loss of ponial insrions from fh ininy, Loss fh. Th Insrions Pr Cyl, IPC, is rlad o h siz of h insrion fh blok, Siz blok, andu ilizaion by IPC = Siz blok U ilizaion whr Uilizaion 63.8% Fh 18.5% I-ah 2.1% D-ah 1.8% Branh 8.6% SU 5.3% Uilizaion =100%; %Dlay i;ah ; %Dlay d;ah ;%Dlay branh ; %Sall SU ; %Loss f h All prnags ar rlaiv o h oal nmbr of yls. Ths prnags hang dynamially, b shold no vary widly. Tabl 2 shows h disribion of hs faors ha onrib o prforman dgradaion. All programs sd dfal paramrs xp wih an 8 KB insrion ah and 8 KB daa ah. Tabl 2: Prforman Faors Program AIF %I- %D- %Br %SU %F IPC d dhrysn qno bbbl inmm jpg jpg.d mpg mpg.d p prm pzzl qns qiks owrs r Avrag Figr 8 is a pi har showing h faors onribing o ovrall prforman. On avrag, h dlays as by insrion and daa ah misss wr lss han 5%. Only 5% of h im was hr a Shdling Unis sall. Roghly, hr was a 9% pnaly d o misprdid branhs. A loss in spdp d o fhing ininly was h gras faor, laving 64% ilizaion of a ponial for ims spdp. CONCLUSION On riial faor in h prforman of any sprsalar is how onrol ow is handld. Using a mlipl branh prdior ovr a simpl \no akn" poliy signianly inrasd h aray of insrion ow. An vn mor signian improvmn was ahivd by rding h avrag bad branh pnaly by orring program Figr 8: Ovrall Prforman Faors ow and disarding h splaiv xion as soon as h branh is rsolvd. Th mos imporan iss of a sprsalar miroprossor is h Avrag Insrion Fh iny bas i is h gras faor limiing prforman. REFERENCES [1] A. Abnos, C. Chrisnsn, J. Gray, J. Lnll, A. Naylor, and N. Baghrzadh, \VLSI Dsign of h TinyRISC Prossor," Prodings of h 1992 IEEE Csom Ingrad Ciris Confrn, pp { , [2] J. Gray, A. Naylor, A. Abnos, and N. Baghrzadh, \VIPER: A 25MHz, 100 MIPS Pak VLIW Miroprossor," Prodings of h 1993 IEEE Csom Ingrad Ciris Confrn, San Digo, [3] Shng-Chih Hang, Analysis and Dsign of a Dynami Insrion Uni for a Sprsalar Compr Arhir, Masr's hsis, Univrsiy of California, Irvin, [4] Mik Johnson, Sprsalar Miroprossor Dsign, Prni Hall, Englwood Clis, [5] Johnny K. F. L and Alan J. Smih, \Branh Prdiion Sragis and Branh Targ Br Dsign," IEEE Compr, pp. 6{22, [6] John Lnll and Nadr Baghrzadh, \A Prforman Comparison of Svral Sprsalar Prossor Modls wih a VLIW Prossor," Prodings of Inrnaional Paralll Prossing Symposim, pp. 44{ 48, [7] Svn Przybylski, Mark Horowiz, and John Hnnssy, \Prforman Trados in Cah Dsign," Prodings of h 15h Annal Symposim on Compr Arhir, pp. 290{298, [8] Svn Walla, Prforman Analysis of a Sprsalar Arhir, Masr's hsis, Univrsiy of California, Irvin, 1993.