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sourceCode generation code
end^ front
opt.
endback
target code
IR
IR
Code generation steps
- Source code
(^) intermediate representation
syntax, symbol tables^ generate intermediate representation during parse based on
- Intermediate representation
(^) target code
choose instructions based on target instr set^ instruction selection
decide storage for variables; allocate registers^ memory management
determine prolog/epilog code^ linkage code
choose instruction execution order^ instruction scheduling
CMSC 430
Lecture 9, Page 1
Abstract syntax tree (AST) Abstract syntax trees
(^) stores syntactic structure of program
x
< 2 > < y >
Building the AST
(^) construct node for LHS of production
(^) fill in fields using RHS of production
E^ Example (^0) ::= E
1 (^) ’+’ E
2 { (^) E 0 .val = node(’+’, E
1 .val, E
2 .val);
E
1 (^) ’*’ E
(^2) { (^) E 0 .val = node(’*’, E
1 .val, E
(^2) .val);
id
E
0 .val = node(
id .val);
num
E
0 .val = node(
num
.val);
CMSC 430
Lecture 9, Page 2
Code templates Instruction selection
(^) template for each language construct
(^) ignore surrounding context
(^) simple recursive approach
Language constructs
- complex expressions3. procedure calls2. control structures 1. simple expressions Applying templates
(^) use syntax during parse
(^) apply tree rewriting to AST
CMSC 430
Lecture 9, Page 3
We’ll be targeting RISC-like processors Intermediate representation
(^) load-store architecture
(^) register-transfer language
(^) three-address code
(^) explicit loads and stores
Examples load r1,
addr
$ r
value at
addr
loadi r1,
const
$ r
value of
const
store r1,
addr
addr
r
move r1, r
$ r
r
add r1, r2, r
$ r
r2 + r
sub r1, r2, r
$ r
r2 - r
mult r1, r2, r
$ r
r2 * r
jmp
addr
$ jump to
addr
CMSC 430
Lecture 9, Page 4
Expression trees:Simple expressions
(^) adopt a simple treewalk scheme
(^) assign a virtual register to each operator
(^) emit code in postorder walk
Support routines:
(^) addr( str )
(^) — returns the name of a virtual register that
contains the base address for
(^) str
(^) newtemp()
(^) — returns a new virtual register name
Assume:
(^) assume tree reflects precedence, associativity
(^) assume all operands are integers
CMSC 430
Lecture 9, Page 5
expr( node ) Simple expressions
switch(int result, t1, t2, t3;
type of
node )
case PLUS:
t1 = expr(
left child of
node );
t2 = expr(
right child of
node );
break;emit( add, result, t1, t2 );result = newtemp();
case ID:
break;emit( load, result, t3 );result = newtemp();t3 = addr( node.str );
case NUM:
break;emit( loadi, result, node.val );result = newtemp();
return result
CMSC 430
Lecture 9, Page 6
Simple expressions
PLUS
x ID
PLUS
NUM 4
y ID
load r2, r
$ r
addr(x)
loadi r3, 4
$ constant
load r5, r
$ r
addr(y)
add r6, r3, r
$ r
4 + y
add r7, r2, r
$ r
x + (4 + y)
CMSC 430
Lecture 9, Page 7
Assignment statement Control structures
lhs (^) ←
(^) rhs
Strategy
(^) evaluate
(^) rhs
(^) to a value
(an rvalue
(^) evaluate
(^) lhs
(^) to an address
an lvalue
i)
lvalue
(^) is register
(^) move
(^) it
ii) lvalue
(^) is address
(^) store
(^) it
Registers versus memory
(^) non-aliased scalars
(^) can go in a register
(^) aggregate or potentially aliased
in memory
CMSC 430
Lecture 9, Page 8
Numerical Values Two schools of thought on representation:Boolean expressions
(^) assign numerical values to true and false
(^) evaluate booleans like arithmetic expressions
Control Flow
(^) represent boolean value by location in code
(^) convert to numerical value when stored
CMSC 430^ Neither representation dominates the other.
Lecture 9, Page 13
Numerical Values Boolean expressions
(^) assign a value to true
(say
(^) assign a value to false
(say
(^) use hardware —
(^) and
or , not
, xor
CMSC 430^ Choose values that make the hardware work.
Lecture 9, Page 14
Source Expression^ Numerical Values^ Boolean expressions
Generated Code
b or ( c and not d )
t (^) ←
(^) not d
t (^) ←
(^) c and t
t (^) ←
(^) b or t
a (^) < (^) b
if (a
b) br L
t (^) ←
br L
L1: t
L2: nop
CMSC 430^ A numerical representation handles logic well.
Lecture 9, Page 15
Control Flow Boolean expressions
(^) use conditional branches and comparator
(^) chain of branches to evaluate expression
(^) code looks terrible
Clean up: statements.Control flow representation works well for expressions in conditional
(^) branch to next statement
(^) branch to branch
CMSC 430
Lecture 9, Page 16
Source Expression^ Control FlowBoolean expressions
Generated Code
a < b or ( c
d and e
f )
if a
< b br LT
br L
L1: if c
d br L
br LF
L2: if e
f br LT
br LF
LF:
(^) code under
(^) false
or
t
←
false
br LEXIT
LT:
(^) code under
(^) true
or
t
←
true
br LEXIT
LEXIT:
CMSC 430preserved for later reuse.^ This works well when the expression’s value is tested but not
Lecture 9, Page 17
Do the semantics require evaluating all terms of an expression? What about ”short circuiting” Boolean expressions
(^) once value established, stop evaluating
(^) true or
expr
(^) ) is
(^) true
(^) false and
expr
) is
(^) false
(^) save cycles in evaluation
Order of evaluation
(^) if specified, must be observed
(^) if not, reorder by cost and short-circuit
CMSC 430
Lecture 9, Page 18
Reality Boolean expressions
(^) either approach works fairly well
(^) numerical code reflects logical constructs
(^) control flow code works well for relations
(^) compiler can choose based on context
Control flow
(^) accounting nightmare — tracking labels
(^) backpatching is the right answer
CMSC 430
Lecture 9, Page 19
