The role of thread-level speculation in the manycore era

Au oma ic Pa alleliza ion in he Manyco e E a Diego R. Llanos, A u o Gonzalez-Esc ibano Dp o. In o m´a ica, Uni e sidad de Valladolid, Spain {diego,a u o}@in o .u a.es Ex ended abs ac Du ing he las decade, ad ances bo h in manyco e p ocesso s design and pa - allel p og amming models ha e led o a ise o pa allel de elopmen o egula applica ions. One success ul example is he ubiquous use o GPUs o pa allel compu ing. I egula applica ions, on he o he side, a e s ill di icul o pa - allelize by hand, because hey equi e a deep knowledge o he p oblem i sel , he pa allel p og amming model being used, and he unde lying a chi ec u e. Ha ing a sys em ha au oma ically pa allelize hem would be highly desi able. Fo an inc easing numbe o applica ions, he use o ad anced compile- ime echniques such as he polyhed al model allows o ob ain pa allel e sions au o- ma ically. Howe e , many o hem s ill de y au oma ic pa alleliza ion a compile ime, due o hei complex con ol pa h, and o he lack o un ime in o ma ion ha would be needed a compile ime o ensu e co ec ness. This is why a mech- anism o au oma ically de elop a pa allel e sion o any sequen ial p og am is s ill a desi able goal. Th ead-le el specula ion (TLS) is a well-known echnique ha allows o au- oma ically ex ac pa allelism o sequen ial applica ions, mos ly a loop le el. Many so wa e-based TLS p oposals ha e been published so a , equi ing di - e en deg ees o human in e en ion. Ou con ibu ion o he ield is ATLaS, a so wa e-based TLS un ime lib a y o sa ely execu e in pa allel any loop, in- cluding hose ha may su e om dependence iola ions. ATLaS is in ended o be used in he con ex o OpenMP p og ams, by o e ing a non-s anda d specula i e clause o le he use ma k he a iables whose use may lead o a dependence iola ion, le ing he un ime lib a y o gua an ee co ec ness in any case. In ac , ATLaS can be used in ully-au oma ic mode, jus by labeling all a iables o he loop as specula i e, a he cos o a po en ial pe o mance loss. The pu pose o his con ibu ion is o discuss he ole o so wa e-based TLS solu ions in he ollowing yea s. F om he so wa e side, au oma ic pa alleliza- ion echniques such as hose based on he polyhed al model ex ac s pa allelism om an inc easing numbe o applica ions. The ques ion he e is whe he his educes he need om specula i e un ime echniques. F om he ha dwa e side, he ad en o manyco e sys ems wi h dozens o hund eds o p ocesso s makes classic TLS echniques o ha e diminished e u ns. To deal wi h his scena io, an upda e o TLS un ime a chi ec u es may be desi able. 1