Material Covered on the Final

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1 Material Covered on the Final On the final exam, you are responsible for: Anything covered in class, except for stories about my good friend Ken Kennedy All lecture material after the midterm ( below the line on the web page) Material in the book, by chapter: 6: all of it 7: all of it, but we won t ask a question on string assignments 8: 8.1, 8.2, 8.3, 8.4.1, : intro, 9.3 (intro), 9.3.1, plus high-level understanding of reaching definitions from page : : the draft material from EaC3e (on web page) plus 11.5 from EaC2e 12: 12.1, 12.2, 12.3, 12.4 COMP 412, Fall

COMP 412 FALL 2017 Instruction Selection via Peephole Optimization Comp 412 source code IR

Cooper & Linda Torczon, all rights reserved.

2 COMP 412 FALL 2017 Instruction Selection via Peephole Optimization Comp 412 source code IR Front End Optimizer Back End IR target code Copyright 2017, Keith D. Cooper & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make copies of these materials for their personal use. Faculty from other educational institutions may use these materials for nonprofit educational purposes, provided this copyright notice is preserved. New and Improved, with extra complications Chapter 11 in EaC3e

The Goal We want to automate generation of instruction

Front End Optimizer Back End IR target code Tables machine

Description-based retargeting Selector The machine

3 The Goal We want to automate generation of instruction selectors (as with parsers & scanners) source code IR Front End Optimizer Back End IR target code Tables machine description Back-End Generator Pattern Matching Engine Description-based retargeting Selector The machine description should also help with scheduling & allocation COMP 412, Fall

4 How do we perform this kind of matching? Tree-oriented IR suggests pattern matching on trees Process takes tree-patterns as input, matcher as output Each pattern maps to a sequence of target-machine ops Use dynamic programming or bottom-up rewrite systems Linear IR suggests using some sort of string matching Process takes strings as input, matcher as output Each string maps to a sequence of target machine ops Use text matching (Aho-Corasick) or peephole matching In practice, both work well & produce efficient matchers. Either can be implemented as a table-driven or hard-coded matcher. COMP 412, Fall

5 Peephole Matching Basic idea Compiler can discover local improvements locally Look at a small window of adjacent operations a peephole Slide the window over code & look for opportunities in the window Classic motivating example was a store followed by a load Consider the AHSDT code generator s actions for x w + z; a x + y; If x cannot be kept permanently in a register, the code generator will emit a store for x, followed immediately by a load. COMP A peephole 412, Fall 2017 was a pre-digital device to see who was on the other side of a door. 4

6 Peephole Matching Basic idea Compiler can discover local improvements locally Look at a small window of adjacent operations a peephole Slide the window over code & look for opportunities in the window Classic motivating example was a store followed by a load Original code add r w, r z Þ r 1 storeai r 1 Þ r 0,8 loadai r 0,8 Þ r 2 add r 2, r y Þ r 3 Improved code add r w, r z Þ r 1 storeai r 1 Þ r 0,8 i2i r 1 Þ r 2 add r 2, r y Þ r 3 1. Assume x is at offset 8 from the ARP 2. The store and load are long-latency operations that address the same memory location. The only way the scheduler can avoid the full latency is if it proves enough to eliminate the load. COMP 3. Copy 412, coalescing Fall 2017 is highly likely to eliminate the i2i. 5

7 Peephole Matching Basic idea Compiler can discover local improvements locally Look at a small window of adjacent operations a peephole Slide the window over code & look for opportunities in the window Classic motivating example was a store followed by a load Simple algebraic identities are another opportunity Original code Improved code addi r 2,0 Þ r 7 mult r 4,r 7 Þ r 10 mult r 4,r 2 Þ r 10 multi r 5,2 Þ r 7 add r 5,r 5 Þ r 7 COMP Algebraic 412, simplification Fall 2017 in LVN See Table on p 424 of EaC2e 6

8 Peephole Matching Basic idea Compiler can discover local improvements locally Look at a small window of adjacent operations a peephole Slide the window over code & look for opportunities in the window Classic motivating example was store followed by load Simple algebraic identities are another opportunity Jump to a jump Original code Improved code jumpi L 10 L 10 : jumpi L 11 L 10 : jumpi L 11 Second jump must be within the window COMP Merging 412, an Fall empty 2017 block in CLEAN 7

Peephole Matching Implementing the idea Early peephole systems used limited set of hand-coded patterns Small window size ensured quick processing If we assume window size is 3 or 4, then O(n 2 ) Þ

9 Peephole Matching Implementing the idea Early peephole systems used limited set of hand-coded patterns Small window size ensured quick processing If we assume window size is 3 or 4, then O(n 2 ) Þ O(n) Modern peephole instruction selectors Break problem into three tasks (Davidson) IR Expander LLIR Simplifier LLIR Matcher ASM IR LLIR LLIR LLIR LLIR ASM Apply symbolic interpretation & simplification systematically COMP 412, Fall

10 Peephole Matching Expander Turns IR code into a low-level IR (LLIR) such as RTL Operation-by-operation, template-driven rewriting LLIR form includes all direct effects Significant, albeit constant, expansion of size (e.g., setting CC) IR Expander LLIR Simplifier LLIR Matcher ASM IR LLIR LLIR LLIR LLIR ASM COMP 412, Fall

11 Peephole Matching Simplifier Looks at LLIR through window and rewrites it Uses forward substitution, algebraic simplification, local constant propagation, and dead-effect elimination Performs local optimization within window IR Expander LLIR Simplifier LLIR Matcher ASM IR LLIR LLIR LLIR LLIR ASM This is the heart of the peephole system Benefit of peephole optimization shows up in this step COMP 412, Fall

12 Peephole Matching Matcher Compares simplified LLIR against a library of patterns Picks low-cost pattern that captures effects Must preserve LLIR effects, may add new ones Generates the assembly code output (e.g., set CC) IR Expander LLIR Simplifier LLIR Matcher ASM IR LLIR LLIR LLIR LLIR ASM COMP 412, Fall

13 Finding Dead Effects The Simplifier must know which ops are useless (i.e., dead) Expander works in a context-independent fashion It can process the operations in any order Use a backward walk and compute local LIVE information Tag each use that is a last use What about non-local effects? Most useless effects are local defined & used in same block It can be conservative & assume LIVE until proven dead As in Lab 1 Assume that values LIVE on exit are LIVE ASM LLIR LLIR ASM mult r 5,r 9 Þ r 12 add r 12,r 17 Þ r 13 Þ expand r 12 r 5 * r 9 cc f(r 5 *r 9 ) r 13 r 12 + r 17 cc f(r 12 +r 17 ) simplify Þ r 12 r 5 * r 9 cc f(r 5 *r 9 ) r 13 r 12 + r 17 cc f(r 12 +r 17 ) Þ match madd r 5,r 9,r 17 Þ r 13 This effect would prevent multiply-add from matching COMP 412, Fall

14 Example a b - 2 * c becomes Original IR 1 t 1 2 * c 2 a b t 1 Assume that: Kind Base Offset a local variable ARP 4 b cbr parameter ARP -16 c global COMP 412, Fall

15 Example a b - 2 * c becomes LLIR Code Original IR 1 t 1 2 * c 2 a b t 1 Expand We loaded c before 2 following Ravi Sethi s most demanding r 18 r 17 x r subtree first rule. Same thing with 16 evaluating the expression before computing the address of a. Base Offset a ARP 4 b ARP -16 Assume is the label s offset 12 in the global constant pool, as in tree-pattern matching lecture. COMP 412, Fall 2017 MEM(r 25 ) r 23 14

16 Example Remember, one of the primary goals of instruction selection is to make effective use of the ISA s features, particularly address modes. LLIR Code a b - 2 * c becomes Original IR 1 t 1 2 * c Expand load c 2 a b t 1 Among the additional details that the Expander adds are all of the nitty-gritty computations involved in addressing a, b, and c. Base Offset a ARP 4 b ARP r 18 r 17 x r 16 load b store a COMP 412, Fall 2017 MEM(r 25 ) r 23 15

17 Example You will sometimes hear compiler writers talk about establishing addressability for a global variable. This code establishes addressability a b - 2 * for c becomes c by retrieving the label from the constant pool. Original If that IR label is used often Expand(> 2 times), 1 t 1 it 2 may * c be worthwhile to load it on 2 a entry b t 1 and spill it to the AR. Among the additional details that the Expander adds are all of the nitty-gritty computations involved in addressing a, b, and c. Base Offset a ARP 4 b ARP LLIR Code r 18 r 17 x r 16 COMP 412, Fall 2017 MEM(r 25 ) r load c load b store a

18 LLIR Code r 11 LLIR r r@cp 10 + r 11 r MEM(r ) r r r r r MEM(r ) r r r x r r r r arp - r r MEM(r ) r MEM(r ) r r r r r r arp + r Example Simplify Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r 23 MEM(r ) r COMP 412, Fall LLIR operations to 7 17

19 Example Remember, one of the primary goals of instruction selection is to make effective use of the ISA s features, particularly address modes. Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r 23 Match Final ILOC Code Þ r 10 loadai r Þ r 11 loadai r 11, 12 Þ r 12 add r 12, r 12 Þ r 13 subi r arp, 16 Þ r 14 load r 14 Þ r 15 load r 15 Þ r 16 sub r 16, r 13 Þ r 17 storeai r 17 Þ r arp, 4 Introduces all memory operations & temporary names Turns out pretty good code; equivalent to tree pattern matcher s code COMP 412, Fall LLIR operations to 9 final operations 18

20 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier Three operation window Fill the window 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 19

21 LLIR Code Steps of the Simplifier r Three operation window r 18 r 17 x r 16 Forward substitute Assignment to r 12 was the last use of r 10 & r 11 The assignments to r 10 & r 11 are now dead 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 20

22 LLIR Code Steps of the Simplifier r Three operation window r 18 r 17 x r 16 Discard and fill delete ops with dead results will combine with Forward substitute from here on out 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 21

23 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 13 MEM(@CP Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 22

24 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 13 MEM(@CP r 15 r Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 23

25 LLIR Code Steps of the Simplifier r 13 MEM(@CP r 16 MEM(r ) Three operation window r 18 r 17 x r 16 Forward substitute, discard, and fill nothing more to do in this window 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 24

26 LLIR Code Steps of the Simplifier r 16 MEM(r ) r 18 r 17 x r 16 Three operation window r 18 r 17 x r 16 Roll definition of r 13 out of the window and fill roll removes op from window and emits the op for the matcher 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 25

27 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 16 MEM(r ) r 18 2 x r 16 Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 26

28 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 16 MEM(r ) r 18 r 16 + r 16 Three operation window Apply algebraic identity 2 x a = a + a 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 27

29 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 18 r 16 + r 16 Three operation window Roll the definition of r 16 out of the window and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 28

30 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 18 r 16 + r 16 r 20 r arp - 16 Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 29

31 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 18 r 16 + r 16 r 21 MEM(r arp - 16) Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 30

32 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 21 MEM(r arp - 16) Three operation window Roll definition of r 18 out of the window and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 31

33 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier Three operation window Roll definition of r 21 out of the window and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 32

34 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier Three operation window Roll definition of r 22 out of the window and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 33

35 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier r 25 r arp + 4 MEM(r 25 ) r 23 Three operation window Forward substitute, discard, and fill 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 34

36 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier MEM(r arp + 4) r 23 empty Three operation window Forward substitute, discard, and try to fill out of ops 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 35

37 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier MEM(r arp + 4) r 23 empty Three operation window Roll the last two ops in the window 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 36

38 LLIR Code r 18 r 17 x r 16 Steps of the Simplifier empty empty empty Three operation window Roll the last two ops in the window 1. Forward substitution 2. MEM(r 25 ) r Algebraic simplification Local constant propagation COMP 412, Fall Dead-effect elimination 37

39 LLIR Code Results of the Simplifier r 18 r 17 x r 16 MEM(r 25 ) r 23 Simplify Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r 23 These are the ops that rolled out of the top of the window. COMP 412, Fall

40 Matcher Remember, one of the primary goals of instruction selection is to make effective use of the ISA s features, particularly address modes. Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r 23 Match Final ILOC Code Þ r 10 loadai r Þ r 11 loadai r 11, 12 Þ r 12 add r 12, r 12 Þ r 13 subi r arp, 16 Þ r 14 load r 14 Þ r 15 load r 15 Þ r 16 sub r 16, r 13 Þ r 17 storeai r 17 Þ r arp, 4 Introduced memory operations, address modes, & temporary names Similar code to tree pattern matcher, except for multi versus add different order of operations Used address modes and immediate operands where possible COMP 412, Fall

41 Matcher Remember, one of the primary goals of instruction selection is to make effective use of the ISA s features, particularly address modes. Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r Final ILOC Code Þ r 10 loadai r Þ r 11 loadai r 11, 12 Þ r 12 add r 12, r 12 Þ r 13 subi r arp, 16 Þ r 14 load r 14 Þ r 15 load r 15 Þ r 16 sub r 16, r 13 Þ r 17 storeai r 17 Þ r arp, 4 Introduced memory operations, address modes, & temporary names 1. Matcher must know about size of constants 2. Matcher had to reconstruct subi from MEM(r arp - 16) Other operations were a one-to-one match COMP 412, Fall

42 Matcher Remember, one of the primary goals of instruction selection is to make effective use of the ISA s features, particularly address modes. Improved LLIR Code r 13 MEM(@CP r 16 MEM(r ) r 18 r 16 + r 16 r 21 MEM(r arp - 16) MEM(r arp + 4) r 23 Match Final ILOC Code Þ r 10 loadai r Þ r 11 loadai r 11, 12 Þ r 12 add r 12, r 12 Þ r 13 subi r arp, 16 Þ r 14 load r 14 Þ r 15 load r 15 Þ r 16 sub r 16, r 13 Þ r 17 storeai r 17 Þ r arp, 4 Actual systems use different technologies for the matcher Hand-coded matcher for LLIR that emits target machine code (gcc) LR parser adapted to handle (wildly) ambiguous grammars (VPO) Some kind of formal matcher (e.g., Aho-Corasick) COMP 412, Fall

43 Making It All Work Details LLIR is largely machine independent Some compilers use LLIR as one of their IRs Eliminates the need for the Expander Target machine described with LLIR ASM patterns for Matcher Actual pattern matching (RTL in gcc) Use a hand-coded pattern matcher (gcc) Turn patterns into grammar & use LR parser (VPO) Several important compilers use this technology It seems to produce good portable instruction selectors Key strength appears to be late low-level optimization COMP 412, Fall

Other Considerations Control-flow operations Can clear simplifier s window at branch or label Predication has similar effects May want to special case predicated single operations so as not to

44 Other Considerations Control-flow operations Can clear simplifier s window at branch or label Predication has similar effects May want to special case predicated single operations so as not to disrupt the flow of the simplifier too often Physical versus logical windows Can run optimizer over a logical window k operations connected definition to use Expander can link definitions & uses Logical windows (within block) improve effectiveness Davidson & Fraser report 30% faster & 20% fewer ops with local logical window. COMP 412, Fall

45 Instruction Selection The Big Picture Selection by Tree-Pattern Matching Assumed a tree structure for both IR and machine description Note: trees not DAGs, and matches on syntax, not values Recursive bottom-up procedure to find tiling(s) Adding costs makes it behave like dynamic programming Selection by Peephole Optimization Assumed a linear IR and performed late-stage optimization Forward substitution, algebraic simplification, local constant propagation, & dead-effect elimination Relies on a small window (or peephole) to control complexity Both techniques automate a complex part of the process Both techniques can produce high-quality code COMP 412, Fall

46 Instruction Selection How good is the code that these techniques produce? The code is not optimal Tree-pattern matchers are limited by the available patterns Initial 1-to-1 mapping might produce mediocre code Adding more patterns can capitalize on complex ISA features Multiply-add, address modes, decrement & branch, call/ret Leads to larger tables The match is a syntactic match (cannot inspect values) Peephole technique is limited by size of the window & strength of its transformations Logical windows do better than physical windows Can examine values (e.g., for algebraic simplification) Adding more transformations would improve code & increase complexity The literature introduces the term locally optimal to describe quality As well as can be done with only local knowledge Both techniques can claim locally optimal code For non-local improvement, rely on the compiler s optimizer COMP 412, Fall

Instruction Selection

Compiler Design Instruction Selection Hwansoo Han Structure of a Compiler O(n) O(n) O(n log n) Scanner words Parser IR Analysis & Optimization IR Instruction Selection Either fast or NP-Complete asm asm