Research in Compilers and Introduction to Loop Transformations Part I: Compiler Research Tomofumi Yuki EJCP 2017 June 29, Toulouse

Background Defended Ph.D. in C.S. on October 2012 Colorado State University Advisor: Dr. Sanjay Rajopadhye Currently Inria Chargé de Recherche Bretagne-Atlantique, Rennes, CAIRN team Optimizing compiler + programming language static analysis (polyhedral model) parallel programming models High-Level Synthesis I will start with a short reminder of my background. I did my PhD work at CSU under the supervision of Dr.Sanjay Rajopadhye. Currently, I am a post-doc at Inria Rennes, within the CAIRN team. My past work is in optimizing compilers and programming languages. In particular, I have worked on static analyses, parallel programming models, and high-level synthesis. EJCP 2017, June 29, Toulouse

What is this Course About? Research in compilers a bit about compiler itself Understand compiler research what are the problems? what are the techniques? what are the applications? may be do research in compilers later on! Be able to (partially) understand work by “compiler people” at conferences. Many domain is related to compilation, much more than a typical starting student may think of, and when you meet ppl at conferences, basic understanding of the context. Not going into parsing/lexing blah blah EJCP 2017, June 29, Toulouse

Compiler Advances Old compiler vs recent compiler modern architecture gcc -O3 vs gcc -O0 How much speedup by compiler alone after 45 years of research? EJCP 2017, June 29, Toulouse

Proebsting’s Law HW gives 60%/year Compiler Advances Double Computing Power Every 18 Years http://proebsting.cs.arizona.edu/law.html Someone actually tried it: On Proebsting’s Law, Kevin Scott, 2001 SPEC95, compared against –O0 3.3x for int 8.1x for float HW gives 60%/year EJCP 2017, June 29, Toulouse

Compiler Advances Old compiler vs recent compiler Not so much? modern architecture gcc -O3 vs gcc -O0 3~8x difference after 45 years Not so much? EJCP 2017, June 29, Toulouse

Compiler Advances Old compiler vs recent compiler Not so much? modern architecture gcc -O3 vs gcc -O0 3~8x difference after 45 years Not so much? “The most remarkable accomplishment by far of the compiler field is the widespread use of high-level languages.” by Mary Hall, David Padua, and Keshav Pingali [Compiler Research: The Next 50 Years, 2009] EJCP 2017, June 29, Toulouse

Earlier Accomplishments Getting efficient assembly register allocation instruction scheduling ... High-level language features object-orientation dynamic types automated memory management EJCP 2017, June 29, Toulouse

What is Left? Parallelism Security/Reliability Power/Energy multi-cores, GPUs, ... language features for parallelism Security/Reliability verification certified compilers Power/Energy data movement voltage scaling EJCP 2017, June 29, Toulouse

Agenda for today Part I: What is Compiler Research? Part II: Compiler Optimizations Lab: Introduction to Loop Transformations EJCP 2017, June 29, Toulouse

What is a Compiler? Bridge between “source” and “target” source target EJCP 2017, June 29, Toulouse

Compiler vs Assembler What are the differences? source target compile object/machine code assembly object assemble EJCP 2017, June 29, Toulouse

Compiler vs Assembler Compiler Assembler Many possible targets (semi-portable) Many decisions are taken Assembler Specialized output (non-portable) Usually a “translation” EJCP 2017, June 29, Toulouse

Goals of the Compiler Higher abstraction Performance No more writing assemblies! enables language features loops, functions, classes, aspects, ... Performance while increasing productivity speed, space, energy, ... compiler optimizations EJCP 2017, June 29, Toulouse

Productivity vs Performance Higher Abstraction ≈ Less Performance Python Java Abstraction C Fortran Assembly Performance EJCP 2017, June 29, Toulouse

Productivity vs Performance How much can you regain? Python Python Java C Fortran Java Abstraction C Fortran Assembly Performance EJCP 2017, June 29, Toulouse

Productivity vs Performance How sloppy can you write code? Python Java C Fortran Python Java Abstraction C Fortran Assembly Performance EJCP 2017, June 29, Toulouse

Compiler Research Branch of Programming Languages Program Analysis, Transformations Formal Semantics Type Theory Runtime Systems Compilers ... EJCP 2017, June 29, Toulouse

New HW Needs New Compiler New Architecture IBM Cell, GPU, Xeon-Phi, Kalray MPPA, ... which ones succeeded? Good prog. model and compiler easier to fully utilize new HW crucial for success HW vendors invest a lot into compilers EJCP 2017, June 29, Toulouse

Examples Two classical compiler optimizations register allocation instruction scheduling EJCP 2017, June 29, Toulouse

Case 1: Register Allocation Classical optimization problem 3 registers 8 instructions 2 registers 6 instructions C = A + B; D = B + C; load %r1, A load %r2, B add %r3, %r1, %r2 store %r3, C load %r1, B load %r2, C store %r3, D naïve translation load %r1, A load %r2, B add %r1, %r1, %r2 store %r1, C add %r1, %r2, %r1 store %r1, D smart compilation EJCP 2017, June 29, Toulouse

Register Allocation in 5min. Often viewed as graph coloring Live Range: when a value is “in use” Interference: both values are “in use” e.g., two operands of an instruction Coloring: conflicting nodes to different reg. a b c d Live Range Analysis a b d c Interference Graph b c = a + b; d = b + c; add %r1, %r1, %r2 add %r1, %r2, %r1 Assume unbounded number of registers and load memory into “virtual” registers. EJCP 2017, June 29, Toulouse

Register Allocation in 5min. Registers are limited a b c d x y a b d c y x a b d c y x c = a + b; d = b + c; x = c + d; y = a + x; Live Range Splitting a b c d x y z Assume unbounded number of registers and load memory into “virtual” registers. a b d c z x a b d c z x a = load A; c = a + b; d = b + c; x = c + d; z = load A; y = z + x; EJCP 2017, June 29, Toulouse 23

Research in Register Allocation How to do a good allocation which variables to split which values to spill How to do it fast? Graph-coloring is expensive Just-in-Time compilation “Solved” EJCP 2017, June 29, Toulouse

Case 2: Instruction Scheduling Another classical problem X = A * B * C; Y = D * E * F; R = A * B; X = R * C; S = D * E; Y = S * F; naïve translation R = A * B; S = D * E; X = R * C; Y = S * F; smart compilation Pipeline Stall (if mult. takes 2 cycles) Also done in hardware (out-of-order) EJCP 2017, June 29, Toulouse

Research in Instruction Scheduling Not much anymore for speed/parallelism beaten to death hardware does it for you Remains interesting in specific contexts faster methods for JIT energy optimization “predictable” execution in-order cores, VLIW, etc. EJCP 2017, June 29, Toulouse

Case 1+2: Phase Ordering Yet another classical problem practically no solution Given optimization A and B A after B vs A before B which order is better? can you solve the problem globally? Parallelism requires more memory trade-off: register pressure vs parallelism EJCP 2017, June 29, Toulouse

Job Market Where do they work at? Many opportunities in France Intel / IBM Research Apple Mathworks amazon Xilinx Many opportunities in France Mathworks @ Grenoble Many start-ups EJCP 2017, June 29, Toulouse