1. Intermediate code generation Jakub Yaghob
Compiler principles
Intermediate code generation
Jakub Yaghob

2. Syntax-directed translation into three-address code
Add several attributes to each grammar symbol
Placement – a name of an object holding a value of the object
Code – a sequence of three-address instructions evaluating the symbol
Label – an absolute or relative address to three-address code

3. Example for our grammar
E → ER + T
E → T
T → TR * F
T → F
F → ( E )
F → id
E.p = newtemp
E.c = ER.c | T.c | gen(E.p=ER.p + T.p)
E.p = T.p
E.c = T.c
T.p = newtemp
T.c = TR.c | F.c | gen(T.p = TR.p * F.p)
T.p = F.p
T.c = F.c
F.p = E.p
F.c = E.c
F.p = id.p
F.c = ‘’

4. Example for while (amateur)
S→ while E do SR
S.L1 = curradr
S.L2 = curradr + E.c.size + SR.c.size + 2
S.c = E.c | gen(JZ E.p, S.L2) | SR.c | gen(JMP S.L1)
S.L1:
E.c
JZ E.p, S.L2
SR.c
JMP S.L1
S.L2:

5. Example for while (professional)
S→ while E do SR
S.L1 = curradr + 1
S.L2 = curradr + SR.c.size + 1
S.c = gen(JMP S.L2) | SR.c | E.c | gen(JNZ E.p, S.L1)
JMP S.L2
S.L1:
SR.c
S.L2:
E.c
JNZ E.p, S.L1

6. Declarations Global objects declarations Local objects declarations
E.g. global variables in C
Local objects declarations
E.g. local variables in a function
At the start of the function × at the start of a block × in the block body
Moving declarations to a “good” place
Global declarations in one well-defined place
Local declarations at the start of a function
Live-variable in the function
Determine the size of the object
Determine the offset of fields in a structure
It does not have to be solved in intermediate code

7. Assignment Type conversion Structure field access Array unfolding
Usually explicit
Intermediate code does not know input language semantic
Structure field access
Use calculated offset from declaration
Direct access using a pointer and added constant
Array unfolding
Multidimensional-array are perceived as one-dimensional (like a memory) – stored in one of the two forms, either row-major (row-by-row) or column-major (column-by-column)
One-dimensional array A[lb..ub] of type with width w
base + (i-lb) * w
i * w + (base – lb * w)
Two-dimensional array A[lb1..ub1,lb2..ub2]
base + ((i1-lb1) * (ub2-lb2+1) + i2-lb2) * w

8. Boolean expression
Sometimes replaced FALSE=0, TRUE=1 (or anything !=0)
Numeric (full) evaluation
All parts of the expression are evaluated
Pascal
Short evaluation
if the result is already known during the evaluation, it won’t be evaluated further
Jumps in the evaluation C

9. Switch What we need Finding the case Evaluate the expression
Find which value in the list of cases is the same as the value of the expression
Execute the statement associated with the value found
Finding the case
Sequence of conditional branches
Small number of cases (<10)
Binary search in table [value,caseptr]
Binary tree of conditions
Table of pointers indexed by value
High density of values in the range of case values

10. Backpatching
Single pass translation: how to set a destination address in a jump to a forward label?
Forward jump
Calling a not yet defined function in a module
Can be resolved by a linker
Generate branches with the targets of the jumps temporarily left unspecified
For each label remember a set of instructions referencing it
Determine the address of the label
Go through the set of instructions and backpatch the determined address

11. Procedure calls Evaluate parameters
Pass parameters by value or by reference
VAR or non-VAR parameters in Pascal
Binding real and formal parameters
Positional
By name
Return value