Register Allocation

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Efficient code has to use the available registers on the target machine as much as possible: Accessing registers is much faster then

accessing memory (the same holds for cache).

Two Aspects:

• Register Allocation: Determine which variables are implemented by registers at which positions.

• Register Assignment: Determine which register implements which variable at which positions.

With register allocation, we mean both aspects.

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Register Allocation

Register Allocation (2)

Goals of Register Allocation

1. Generate code that requires as little registers as possible

2. Avoid unnecessary memory accesses, i.e., not only temporaries, but also program variables are implemented by registers.

3. Allocate registers such that they can be used as much as possible, i.e., registers should not be used for variables that are only rarely accessed.

4. Obey programmer’s requirements.

Ina Schaefer Selected Aspects of Compilers 101

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Outline

• Algorithm interleaving code generation and register allocation for nested expressions (cf. Goal 1)

• Algorithm for procedure-local register allocation (cf. Goals 2 and 3)

• Combination and other aspects

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Register Allocation Evaluation Ordering with Minimal Registers

Evaluation Ordering with Minimal Registers

The algorithm by Sethi and Ullmann is an example of an integrated approach for register allocation and code generation.

(cf. Wilhelm, Maurer, Sect. 12.4.1, p. 584 ff) Input:

An assignment with a nested expression on the right hand side

4.3.1 Auswertungsordnung mit minimalem Registerbedarf minimalem Registerbedarf

Der Algorithmus von Sethi-Ullman ist ein Beispiel für eine integriertes Verfahren zur Registerzuteilung und Codeerzeugung.

Eingabe:

Eine Zuweisung mit zusammengesetztem Ausdruck g g auf der rechten Seite:

Assign ( Var, Exp ) Exp = BinExp | Var BinExp ( Exp Op Exp ) BinExp ( Exp, Op, Exp ) Var ( Ident )

Ausgabe:

Zugehörige Maschinencode bzw Zwischensprachen Zugehörige Maschinencode bzw. Zwischensprachen- code mit zugewiesenen Registern. Wir betrachten hier Zwei-Adresscode, d.h. Code mit maximal einem

Speicherzugriff:

i [ ]

Ri := M[V]

M[V] := Ri

Ri := Ri op M[V]

Ri := Ri op Rj

28.06.2007 © A. Poetzsch-Heffter, TU Kaiserslautern 346

(vgl. Wilhelm/Maurer 12.4.1, Seite 584 ff)

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Evaluation Ordering with Minimal Registers (2)

Output:

Machine or intermediate language code with assigned registers.

We consider two-address code, i.e., code with one memory access at maximum. The machine hasr registers represented byR₀, . . . ,R_r−1.

Der Algorithmus von Sethi-Ullman ist ein Beispiel für eine integriertes Verfahren zur Registerzuteilung und Codeerzeugung.

Eingabe:

Eine Zuweisung mit zusammengesetztem Ausdruck g g auf der rechten Seite:

Assign ( Var, Exp ) Exp = BinExp | Var BinExp ( Exp Op Exp ) BinExp ( Exp, Op, Exp ) Var ( Ident )

Ausgabe:

Zugehörige Maschinencode bzw Zwischensprachen Zugehörige Maschinencode bzw. Zwischensprachen- code mit zugewiesenen Registern. Wir betrachten hier Zwei-Adresscode, d.h. Code mit maximal einem

Speicherzugriff:

i [ ]

Ri := M[V]

M[V] := Ri

Ri := Ri op M[V]

Ri := Ri op Rj

(vgl. Wilhelm/Maurer 12.4.1, Seite 584 ff)

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Example: Code Generation w/Register Allocation

Considerf := (a+b)−(c−(d+e))

Assume that there are two registersR0 andR1 available for the translation.

Result of direct translation:

Beispiel: (Codeerzeugung mit Registerzuteil.)

Betrachte: f:= (a+b)-(c-(d+e)) Betrachte: f:= (a+b) (c (d+e))

Annahme: Zur Übersetzung stehen nur zwei Register zur Verfügung.

Ergebnis der direkten Übersetzung:

R0 := M[a]

R0 := R0 + M[b]

R1 := M[d]

R1 := R1 + M[e]

M[t1] := R1 R1 := M[c]

R1 := R1 – M[t1]

R0 := R0 – R1 M[f] := R0 Ergebnis von Sethi-Ullman:

R0 := M[c]

R1 := M[d]

R1 := R1 + M[e]

R0 := R0 – R1 R1 : M[a]

R1 := M[a]

R1 := R1 + M[b]

R1 := R1 – R0 M[f] := R1

Besser, weil ein Befehl weniger und keine Zwischen- Speicherung nötig.

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Example: Code Generation w/Register Allocation (2)

Result of Sethi-Ullmann-Algorithm:

zur Verfügung.

Ergebnis der direkten Übersetzung:

R0 := M[a]

R0 := R0 + M[b]

R1 := M[d]

R1 := R1 + M[e]

M[t1] := R1 R1 := M[c]

R1 := R1 – M[t1]

R0 := R0 – R1 M[f] := R0 Ergebnis von Sethi-Ullman:

R0 := M[c]

R1 := M[d]

R1 := R1 + M[e]

R0 := R0 – R1 R1 : M[a]

R1 := M[a]

R1 := R1 + M[b]

R1 := R1 – R0 M[f] := R1

Besser, weil ein Befehl weniger und keine Zwischen- Speicherung nötig.

More efficient, because it uses one instruction less and does not need

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Sethi-Ullmann Algorithm

Goal: Minimize number of registers and number of temporaries.

Idea: Generate code for subexpression requiring more registers first.

Procedure:

• Define functionregbed that computes the number of registers needed for an expression

• Generate code for an expressionE = BinExp(L,OP,R);

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We use the following notations:

• v_reg(E): the set of available registers for the translation of E

• v_tmp(E): the set of addresses where values can be stored temporarily when translating E

• cell(E): register/memory cell where the result of E is stored

• vr = | v_reg(E)| denotes the number of available registers

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Sethi-Ullmann Algorithm (3)

We distinguish the following cases:

1. regbed(L)<vr

2. regbed(L)≥vr and regbed(R)<vr 3. regbed(L)≥vr and redbed(R)≥vr

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Case 1: regbed(L)<vr

• Generate code for R using v_reg(E) and v_tmp(E) with result in cell(R)

• Generate code for L using v_reg(E)\{cell(R)}and v_tmp(E) with result in cell(L)

• Generate code for the operation cell(L) := cell(L) OP cell(R)

• Set cell(E) = cell(L)

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Sethi-Ullmann Algorithm (5)

Case 2: regbed(L)≥vr and regbed(R)<vr

• Generate code for L using v_reg(E) and v_tmp(E) with result in cell(L)

• Generate code for R using v_reg(E)\{cell(L)}and v_tmp(E) with result in cell(R)

• Generate code for the operation cell(L) := cell(L) OP cell(R)

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Case 3: regbed(L)≥vr and redbed(R)≥vr

• Generate code for R using v_reg(E) and v_tmp(E) with result in cell(R)

• Generate code M[first(v_tmp(E))] := cell(R)

• Generate code for L using v_reg(E) and rest(v_tmp(E)) with result in cell(L)

• Generate code for the operation cell(L) := cell(L) OP M[first(v_tmp(E))]

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Sethi-Ullmann Algorithm (7)

Function regbed in MAX Notation (can be realized by S-Attribution):

3. Fall: regbed( L ) ! vr und regbed( R ) ! vr Generiere zunächst Code für R

Generiere zunächst Code für R

unter Verwendung von v_reg(E) und v_tmp(E) mit Ergebnis in zelle(R)

Generiere Code: M[ first(v_tmp(E)) ] := zelle(R) Generiere Code für L

unter Verwendung von v_reg(E) und rest( v_tmp(E) ) mit Ergebnis in zelle(L)

G i C d fü di O ti

Generiere Code für die Operation:

zelle(L) := zelle(L) OP M[ first(v_tmp(E)) ] Setze zelle(E) = zelle(L)

Die Funktion regbed (in MAX-Notation):

ATT regbed( Exp@ E ) Nat:

IF Assign@< Var@ E> : 0 IF Assign@<_,Var@ E> : 0

| BinExp@< Var@ E,_,_> : 1

| BinExp@<_,_,Var@ E > : 0

| BinExp@< L,_, R > E : IF regbed(L)=regbed(R)

THEN regbed(L) + 1

ELSE max( regbed(L), regbed(R) ) ELSE nil // Fall kommt nicht vor

(In ML wäre die Definition von regbed etwas aufwendiger, da der Kontext von Var-Ausdrücken nicht direkt berücksichtigt werden kann.)

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Considerf:= (( a + b ) - (c + d)) * (a - (d+e)) Attributes:

Beispiel: (Ablauf Sethi-Ullman)

Betrachte: f:= ((a+b)-(c+d)) * (a-(d+e)) Betrachte: f: ((a+b) (c+d)) (a (d+e)) Attribute: v_reg | v_tmp 12T

regbed zelle zelle

Assign

Var f

BinExp

* (3.)

12T 31

T BinExp BinExp

- (1.)

2 2

12 2 12T 1

Var a

BinExp BinExp

BinExp

+ ¹ + ¹ ¹ + ¹

2 2 12 1 12T 2

Var Var

Var

Var Var Var

(2.) (1.) (1.)

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Example: Sethi-Ullman Algorithm (2)

Beispiel: (Ablauf Sethi-Ullman)

Betrachte: f:= ((a+b)-(c+d)) * (a-(d+e)) Betrachte: f: ((a+b) (c+d)) (a (d+e)) Attribute: v_reg | v_tmp 12T

regbed zelle zelle

Assign

Var f

BinExp

* (3.)

12T 31

T BinExp BinExp

- (1.)

2 2

12 2 12T 1

Var a

BinExp BinExp

BinExp

+ 1 + 1 1 + 1

2 2 12 1 12T 2

Var Var

d Var

d

Var Var Var

b

(2.) (1.) (1.)

1 0 1 0 1 0

28.06.2007 © A. Poetzsch-Heffter, TU Kaiserslautern 351 e d d

a ¹ b ⁰ c ¹ ⁰ ¹ ⁰

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For formalizing the algorithm, we realize the set of available registers and addresses for storing temporaries with lists, where

• the list RL of registers is non-empty

• the list AL of addresses is long enough

• the result cell is always a register which is the first in RL, i.e., first(RL)

• the function exchange switches the first two elements of a list, fst returns the first element of the list,

rest returns the tail of the list

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Example: Sethi-Ullman Algorithm (4)

Remarks:

• The algorithm generates 2AC which is optimal with respect to the number of instructions and the number of temporaries if the expression has no common subexpressions.

• The algorithm shows the dependency between code generation and register allocation and vice versa.

• In a procedural implementation, register and address lists can be realized by a global stack.

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In the following, the functionexpcodefor code generation is given in MAX Notation (functional).

Note: The application of the functions exchange, fst and expcode satisfy their preconditions length(RL)>1 or length(RL)>0, resp.

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Example: Sethi-Ullman Algorithm (6)

FCT expcode( Exp@ E, RegList RL, AdrList AL ) CodeList: // pre: length(RL)>0 IF Var@<ID> E:

[ fst(RL) := M[adr(ID)] ]

| BinExp@< L,OP,Var@<ID> > E:

expcode(L,RL,AL)

++ [ fst(RL) := fst(RL) OP M[adr(ID)] ]

| BinExp@< L,OP,R > E:

LET vr == length( RL ) : IF regbed(L) < vr :

expcode(R,exchange(RL),AL) ++ expcode(L,rst(exchange(RL)),AL) ++ [ fst(RL):= fst(RL) OP fst(rst(RL))]

| regbed(L)>=vr AND regbed(R)<vr : expcode(L,RL,AL)

++ expcode(R,rst(RL),AL)

++ [ fst(RL):= fst(RL) OP fst(rst(RL))]

| regbed(L)>=vr AND regbed(R)>=vr : expcode(R,RL,AL)

[ [ f ( ) ] f ( ) ] ++ [ M[ fst(AL) ] := fst(RL) ] ++ expcode(L,RL,rst(AL))

++ [ fst(RL):= fst(RL) OP M[fst(AL)] ] ELSE nil

ELSE []

Beachte:

Die Anwendungen der Funktionen exchange, fst und

expcode erfüllen jeweils ihre Vorbedingungen length(RL) > 1 bzw. length(RL) > 0 .

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Register allocation by graph coloring is a procedure (with many variants) for allocation of registers beyond expressions and basic blocks.

Register Allocation for 3AC

• Input: 3AC in SSA with temporary variables

• Output: Structurally the same SSA with

! registers instead of temporary variables

! additional instructions for storing intermediate results on the stack, if applicable

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Register Allocation Register Allocation by Graph Coloring

Register Allocation by Graph Coloring (2)

Remarks:

• The SSA representation is not necessary, but simplifies the formulation of the algorithm

(e.g.,Wilhelm/Maurer in Sect. 12.5 do not use SSA)

• It is no restriction that only temporary variables are implemented by registers. We assume that program variables are assigned to temporary variables as well, if appropriate.

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Definition (Life Range)

Thelife rangeof a temporary variable is the set of program positions at which it is live.

Definition (Interference)

Two temporary variablesinterfereif their life ranges have a non-empty intersection

Definition (Interference Graph)

Let P be a program part in 3AC/SSA. Theinterference graphof P is an undirected graphG= (N,E), where

• N is the set of temporary variables

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Register Allocation by Graph Coloring

Goal: Reduce number of temporary variables with the available registers.

Idea: Translate the problem to graph coloring (NP-complete). Color the interference graph, such that

• neighboring nodes have differing colors.

• no more colors are used than available registers.

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General Procedure:For coloring the graph, we have two cases:

• If a coloring is found, terminate.

• If nodes could not be colored,

! choose a non-colored node k

! modify the 3AC program such that the value of k is stored temporarily and is first loaded when it is applied

! try to find a new coloring

Termination: The procedure terminates, because by temporarily storing the life ranges and the interferences are reduced. In practice, two or three iterations are sufficient.

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Register Allocation by Graph Coloring (3)

Coloring Procedure: Let rd be the number of available registers, i.e., for coloring, maximally rn colors may be used.

The coloring procedure consists of the steps:

• (a) Simplify by Marking

• (b) Coloring

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Remove iteratively nodes with less than rn neighbors from the graph and put them on the stack.

Case 1: The current simplification steps leads to an empty graph.

Continue with coloring.

Case 2: The graph contains only nodes with rn and more than rn neighbors. Choose asuitablenode as candidate for storing it temporarily, mark it, put it on the stack and continue simplification.

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Coloring

The nodes are pushed from the stack in their order and, if possible, colored and put back into the graph.

Let k be the node taken from the stack.

Case1: k is not marked. Thus, it has less than rn neighbors. Then, k can be colored with a new color.

Case2: k is marked.

• a) the rn or more neighbors have less than rn-1 different colors.

Then, color k appropriately.

• b) there are rn or more colors in the neighborhood. Leave k uncolored.

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In the beginning, t0 and t2 are live.Beispiel: (Graphfärbung)

Einfachheitshalber betrachten wir nur einen Basisblock:

t1 := a + t0 t3 := t2 – 1 t4 := t1 * t3 t5 := b + t0

Am Anfang sind t0, t2 lebendig

0 1 2 3 4 5 6 7 8 9

t5 := b + t0 t6 := c + t0 t7 := d + t4 t8 := t5 + 8 t9 := t8

A E d i d

t2 := t6 + 4 t0 := t7

Am Ende sind t0, t2, t9 leb.

Interferenzgraph: t4

t5 Interferenzgraph:

t0

t1 t2 t3

t7 t6 t8 t1

t9 In the end, t0, t2, t9 are live.

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Example - Graph Coloring (2)

Interference graph:

Assumption: 4 available registers

Simplification: Remove (in order) t1, t3, t2, t9, t0, t5, t4, t7, t8, t6

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t4 t5

t0 t2

t3

t5 t7 t6

t8 t1

t9

Bemerkung:

Es gibt eine Reihe von Erweiterungen des Verfahrens:

• Elimination von Move-Befehlen Elimination von Move Befehlen

• Bestimmte Heuristiken bei der Vereinfachung (Was ist ein geeigneter Knoten?)

• Berücksichtigung vorgefärbter Knoten Berücksichtigung vorgefärbter Knoten

Lesen Sie zu Abschnitt 4.3.2:

A l

Appel:

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Example - Graph Coloring (4)

Remarks:

There are several extensions of the procedure:

• Elimination of move instructions

• Specific heuristics for simplification (What is asuitablenode?)

• Consider pre-colored nodes Recommended Reading:

• Appel, Sec. 11.1 – 11.3

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The introduced algorithms consider subproblems. In practice, there are further aspects, that have to be dealt with for register allocation:

• Interaction with other compiler phases (in particular optimization and code generation)

• Relation between temporaries and registers

• Source/Intermediate/Target Language

• Number of applications (Is a variable inside an inner loop?)

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Register Allocation Further Aspects of Register Allocation

Further Aspects of Register Allocation (2)

Possible global procedure

• Allocate registers for standard tasks (registers for stack and argument pointers, base registers)

• Decide which variables and parameters should be stored in registers

• Evaluate application frequency of temporaries (Occurrences in inner loops, distribution of accesses over life range)

• Use evaluation together with heuristics of register allocation algorithm

• If applicable, optimize again

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Code generation can be split into four independent machine-dependent tasks:

• Memory allocation

• Instruction selection and addressing

• Instruction scheduling

• Code optimization

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Code Generation

Memory Allocation

Modern machines have the following memory hierarchy:

• Registers

• Primary Cache (Instruction Cache, Data Cache)

• Secondary Cache

• Main Memory (Page/Segment Addressing)

Different from registers, the cache is controlled by the hardware.

Efficient usage of the cache means in particular to align data objects and instructions to borders of cache blocks (cf. Appel, Chap. 21). The same holds for main memory.

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Instruction selection aims at the best possible translation of

expressions and basic blocks using the instruction set of the machine, for instance,

• using complex addressing modes

• considering the sizes of constants or the locality of jumps Instruction selection is often formulated as a tree pattern matching problem with costs. (cf. Wilhelm/Maurer, Chap.11)

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Code Generation

Instruction Scheduling

Modern machines allow processor-local parallel processing (pipeline, super-scalar, VLIW).

In order to use this parallel processing, code has to comply toadditional requirements that have to be considered for code generation.

(see Appel, Chap. 20; Wilhelm/Maurer, Sect. 12.6)

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Optimizations of the assembler or machine code may allow an additional increase in program efficiency.

(see Wilhelm/Maurer, Sect. 6.9)