LLVM passes and Intro to Loop Transformation Frameworks

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1 LLVM passes and Intro to Loop Transformaton Frameworks Announcements Ths class s recorded and wll be n D2L panapto. No quz Monday after sprng break. Wll be dong md-semester class feedback. Today LLVM passes Skewng Smth-Waterman Automatng transformatons lke skewng Iteraton space representaton Transformaton representaton Applyng the transformaton to the teraton space Generatng code for the new teraton space CS 553 Intro to Automatng Loop Transformatons 1

2 LLVM Passes: Example Skeleton Pass LLVM documentaton on wrtng a pass assumes you have control over the llvm source herarchy Skeleton example (runnng t on rose computer) gt clone cd llvm-pass-skeleton Edt the Skeleton.cpp fle In llvm-pass-skeleton: mkdr buld; cd buld; cmake..; make VERBOSE=1 Create a test.c fle. clang -S -emt-llvm test.c opt -load buld/skeleton/lbskeletonpass.so - skeleton < test.ll > /dev/null CS 553 Intro to Automatng Loop Transformatons 2

3 Then what? LLVM documentaton Programmers Manual Helpful Hnts for Common Operatons Iterate over basc blocks n a functon Iterate n nstructons n a basc block Creatng and nsertng new Instructons Deletng nstructons Etc. The Core LLVM Class herarchy Instructon class Value class CS 553 Intro to Automatng Loop Transformatons 3

4 Automatng Loop Transformatons wth Frameworks Currently Frameworks used n compler to abstract loops, memory accesses, and data dependences n loop specfy the effect of a sequence of loop transformatons on the loop, ts memory accesses, and ts data dependences generate code from the transformed loop Loop transformatons affect the schedule of the loop Future How can framework technology be exposed n the programmng model? Frameworks we wll dscuss ths semester Unmodular Polyhedral Presburger Sparse Polyhedral CS 553 Intro to Automatng Loop Transformatons 4

5 Proten Strng Matchng Example (smthwaterman.c) for (=1;<=a[0];++) { for (j=1;j<=b[0];j++) { dag = h[-1][j-1] + sm[a[]][b[j]]; down = h[-1][j] + DELTA; rght = h[][j-1] + DELTA; max=max3(dag,down,rght); f (max <= 0) { h[][j]=0; xtraceback[][j]=-1; ytraceback[][j]=-1; } else f (max == dag) { h[][j]=dag; xtraceback[][j]=-1; ytraceback[][j]=j-1; } else f (max == down) { h[][j]=down; xtraceback[][j]=-1; ytraceback[][j]=j; } else { h[][j]=rght; xtraceback[][j]=; ytraceback[][j]=j-1; } f (max > Max){ Max=max; xmax=; ymax=j; } }} // end for loops CS 553 Intro to Automatng Loop Transformatons 5

6 Skewng (smthwaterman.c) // Let j =+j and =. for ( =1; <=a[0]; ++) { for (j = +1;j <= +b[0];j ++) { dag = h[ -1][j - -1] + sm[a[ ]][b[j - ]]; down = h[ -1][j - ] + DELTA; rght = h[ ][j - -1] + DELTA; max=max3(dag,down,rght); f (max <= 0) { h[ ][j - ]=0; xtraceback[ ][j - ]=-1; ytraceback[ ][j - ]=-1; } else f (max == dag) { h[ ][j - ]=dag; xtraceback[ ][j - ]= -1; ytraceback[ ][j - ]=j - -1; } else f (max == down) { h[ ][j - ]=down; xtraceback[ ][j - ]= -1; ytraceback[ ][j - ]=j - ; } else { h[ ][j - ]=rght; xtraceback[ ][j - ]= ; ytraceback[ ][j - ]=j - -1; } f (max > Max){ Max=max; xmax= ; ymax=j - ; } }} // end for loops CS 553 Intro to Automatng Loop Transformatons 6

7 Iteraton Space Representaton Orgnal code do = 1,6 do j = 1,5 A(,j) = A(-1,j+1)+1 j Represent the teraton space As an ntersecton of nequaltes The teraton space s the nteger tuples wthn the ntersecton Bounds: 1 <= <= 6 1 <= j j <= 5 CS 553 Intro to Automatng Loop Transformatons 7

8 Lexcographcal Order as Schedule Iteraton pont Integer tuple wth dmensonalty d ( 0, 1,..., d ) Lexcographcal Order Frst order the teraton ponts by _0, then _1, and fnally _d. ( 0, 1,..., d 1 ) (j 0,j 1,...,j d 1 ) ( 0 <j 0 ) _ ( 0 = j 0 ^ 1 <j 1 ) _...( 0 = j 0 ^ 1 = j 1 ^... d 1 <j d 1 ) CS 553 Intro to Automatng Loop Transformatons 8

9 Frameworks for Loop Transformatons Loop Transformatons as functons = f() Unmodular Loop Transformatons [Banerjee 90], [Wolf & Lam 91] can represent loop permutaton, loop reversal, and loop skewng unmodular lnear mappng (determnant of matrx s + or - 1) T s a matrx, and are teraton vectors example lmtatons apple 0 j 0 = only perfectly nested loops = T apple apple j all statements are transformed the same CS 553 Intro to Automatng Loop Transformatons 9

10 Loop Skewng Orgnal code do = 1,6 do j = 1,5 A(,j) = A(-1,j+1)+1 j Dstance vector: (1, -1) Skewng: j CS 553 Intro to Automatng Loop Transformatons 10

11 Transformng the Dependences and Array Accesses Orgnal code do = 1,6 do j = 1,5 A(,j) = A(-1,j+1)+1 Dependence vector: j A A New Array Accesses: apple apple 1 0 A 0 1 apple 1 0 A 0 1 j apple apple j apple + apple = A(, j) 1 1 apple 0 j 0 + apple 0 0 = A( 0,j 0 0 ) = A( 1,j+ 1) apple apple 0 1 j 0 + = A( 0 1,j ) 1 CS 553 Intro to Automatng Loop Transformatons 11 j

12 Transformng the Loop Bounds Orgnal code do = 1,6 Bounds: do j = 1,5 A(,j) = A(-1,j+1)+1 j Transformed code do = 1,6 do j = 1+,5+ A(,j - ) = A( -1,j - +1)+1 CS 553 Intro to Automatng Loop Transformatons 12 j

13 Revstng (smthwaterman.c) for (=1;<=a[0];++) { for (j=1;j<=b[0];j++) { dag = h[-1][j-1] + sm[a[]][b[j]]; down = h[-1][j] + DELTA; rght = h[][j-1] + DELTA; Let j =+j and =. for ( =1; <=a[0]; ++) { for (j = +1;j <= +b[0];j ++) { dag = h[ -1][j - -1] + sm[a[ ]][b[j - ]]; down = h[ -1][j - ] + DELTA; rght = h[ ][j - -1] + DELTA; CS 553 Intro to Automatng Loop Transformatons 13

14 Transformaton Legalty Recall A dependence vector s legal f t s lexcographcally non-negatve. Applyng the transformaton functon to each dependence vector produces a dependence vector for the new teraton space. When s a transformaton legal assumng a lexcographcal schedule? What about parallelsm? CS 553 Intro to Automatng Loop Transformatons 14

15 Skewng smthwaterman.c (Is ths transformaton legal?) for (=1;<=a[0];++) { for (j=1;j<=b[0];j++) { dag = h[-1][j-1] + sm[a[]][b[j]]; down = h[-1][j] + DELTA; rght = h[][j-1] + DELTA; Let j = and =+j. Thus =j and j= -j. for ( =2; <=a[0]+b[0]; ++) { for (j =max(1, -b[0]); j <=mn(a[0], -1); j ++) { dag = h[j -1][ -j -1] + sm[a[ ]][b[ -j ]]; down = h[j -1][ -j ] + DELTA; rght = h[j ][ -j -1] + DELTA; CS 553 Intro to Automatng Loop Transformatons 15

16 Convertng C loops to teraton space representaton Analyses needed Loop analyss Loop bounds from AST or control-flow graph Inducton varable detecton Ponter analyss Do ponters pont at same or overlappng memory? Note that n C can cast a ponter to an nteger and back and can do ponter arthmetc. In general requres whole program analyss. Dependence analyss Is ths even possble? Current tools make the optmstc ponter assumpton We need programmng models that smplfy or remove the need for such analyses CS 553 Intro to Automatng Loop Transformatons 16

17 Concepts Parallelsm and Memory Usage tradeoff Transformaton Frameworks Representng the teraton space Representng transformatons Applyng transformatons to the teraton space, dependences, and array accesses Testng the legalty of a transformaton Compler analyses needed n C to obtan an teraton space representaton References [Banerjee90] Uptal Banerjee, Unmodular transformatons of double loops, In Advances n Languages and Complers for Parallel Computng, [Wolf & Lam 91] Wolf and Lam, A Data Localty Optmzng Algorthm, In Programmng Languages Desgn and Implementaton, CS 553 Intro to Automatng Loop Transformatons 17

18 Next Tme Readng assgnment Unfyng Framework for Iteraton Reorderng Transformatons by Kelly and Pugh (read all but Secton 5) Lecture More loop transformaton frameworks CS 553 Intro to Automatng Loop Transformatons 18

Loop Transformations, Dependences, and Parallelization

Loop Transformations, Dependences, and Parallelization Loop Transformatons, Dependences, and Parallelzaton Announcements Mdterm s Frday from 3-4:15 n ths room Today Semester long project Data dependence recap Parallelsm and storage tradeoff Scalar expanson