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Intermediate Code Generation
•^
Forms of intermediate code vary from high level^ –
Annotated abstract syntax trees
-^
Directed acyclic graphs (common subexpressions are coalesced)
•^
To the more low level Three Address Code^ –
Each instruction has, at most, one binary operation
-^
More abstract than machine instructions
- No explicit memory allocation• No specific hardware architecture assumptions -^
Lower level than syntax trees
- Control structures are spelled out in terms of instruction jumps -^
Suitable for many types of code optimization
Three Address Code •^
Consists of a sequence of instructions, each instruction mayhave up to three addresses, prototypically
t1 = t2 op t
•^
Addresses may be one of:^ –
A name. Each name is a symbol table index. For convenience, wewrite the names as the identifier.
-^
A constant.
-^
A compiler-generated temporary. Each time a temporary address isneeded, the compiler generates another name from the stream t1, t2,t3, etc.
- Temporary names allow for code optimization to easily move
instructions
- At target-code generation time, these names will be allocated to
registers or to memory.
Three Address Code Instructions •^
Unconditional jump: goto L^ –
L is a symbolic label of an instruction
•^
Conditional jumps:
if x goto L
and
ifFalse x goto L
–^
Left: If x is true, execute instruction L next
-^
Right: If x is false, execute instruction L next
•^
Conditional jumps:
if x relop y goto L
•^
Procedure calls. For a procedure call p(x1, …, xn)
param x1…param xncall p, n
Three Address Code Instructions •^
Indexed copy instructions:
x = y[i]
and x[i] = y
–^
Left: sets x to the value in the location i memory units beyond y
-^
Right: sets the contents of the location i memory units beyond y to x
•^
Address and pointer instructions:^ –
x = &y sets the r value of x to be the location (l value) of y.
-^
x = *y, presumably y is a pointer or temporary whose r value is alocation. The r value of x is set to the contents of that location.
-^
*x = y sets the r value of the object pointed to by x to the r value of y.
SDD of Intermediate Code •^
Incremental Translation^ –
Instead of using an attribute to keep the generated code, we assumethat we can generate instructions into a stream of instructions
- gen(<three address instruction) generates an instruction• new Temp() generates a new temporary• lookup(top, id) returns the symbol table entry for id at the
topmost (innermost) lexical level
- newlabel() generates a new abstract label name
Translation of Expressions •^
Uses the attribute addr to keep the addr of the instruction for thatnonterminal symbol.
E.addr = lookup(top, id.text)
| id
E.addr = E1.addr
| ( E1 )
E.addr = new Temp()Gen(E.addr = minus E1.addr)
| - E
E.addr = new Temp()Gen(E.addr = E1.addr plus E2.addr)
E
E1 + E
Gen(lookup(top, id.text) = E.addr)
S^
id = E ;
Short-circuit Boolean Expressions •^
Some language semantics decree that boolean expressionshave so-called short-circuit semantics.^ –
In this case, computing boolean operations may also have flow-of-controlExample:
if ( x < 100 || x > 200 && x != y ) x = 0; Translation:
if x < 100 goto L2ifFalse x >200 goto L1ifFalse x != y goto L L2: x = 0L1: …
Flow-of-Control Statements •^
Suppose that we have a grammar:
S
if ( B ) S1 | if ( B ) S1 else S
| while ( B ) S
and that we have defined an attribute for code
B.CodeS1.Code
B.trueB.false
to B.trueto B.false
B.CodeS1.Code Goto S.next
S2.code
B.true B.FalseS.Next
to B.trueto B.false
Control-Flow Boolean Expressions
B.Code = gen(goto B.false)
B
false
B.Code = gen(goto B.true)
B
true
B.Code = E1.code || E2.code
|| gen( if E1.addr relop E2.addr goto B.true)|| gen( goto B.false)
B
E1 rel E
B1.True = B.false; B1.false = B.true;B.Code = B1.code
B
! B
B1.true = newlabel(); B1.false = B.falseB2.true = B.true; B2.false = B.falseB.Code = B1.code || label(B1.true) || B2.code
B
B1 && B
B1.true = B.true; B1.false = newlabel();B2.true = B.true; B2.false = B.false;B.Code = B1.code || label(B1.false) || B2.code
B
B1 || B
Avoiding Redundant Gotos, Backpatching •^
Use ifFalse instructions where necessary
•^
Also use attribute value “fall” to mean to fall through wherepossible, instead of generating goto to the next expression
•^
The abstract labels require a two-pass scheme to later fill inthe addresses
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This can be avoided by instead passing a list of addressesthat need to be filled in, and filling them as it becomespossible. This is called backpatching.
