Issues faced during the design of code generator

1. Presented by,
M.Selva Vinitha
B.E/CSE
M.Kumarasamy college of engineering
Karur.

2. Code generator
 Final phase of compiler design
 Optimized intermediate code is provided as input
 It generates target code
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3. Code generation and Instruction Selection
4
Symbol
table
input output
Front
end
Intermediate
Code
generator
Code
generator
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4. Contd…
 output code must be correct
 output code must be of high quality
 code generator should run efficiently
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5. Pre-requisites
 Instruction set of target machine.
 Instruction addressing modes.
 No. of registers.
 Configuration of ALU
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6. Issues in the design of a code
generator
 Input to the code generator
 Memory management
 Target programs
 Instruction selection
 Register allocation
 Evaluation order
 Approaches to code generation
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7. Input to the code generator
 The intermediate representation of the source program
produced by the front end
 Several choices for the intermediate language
 Linear - postfix nottion
 3 address - quadruples
 Virtual machie - stack machine code
 Graphical - syntax tree &dags
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8. Memory management
 Mapping names in the source program to addresses of data
objects in run-time memory
 Done by the front end and the code generator.
 A name in a three- address statement refers to a symbol-
table entry for the name.
 A relative address can be determined
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9. Target programs
 Absolute machine language
Relocatable machine language
 Assembly language
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10. Contd..
Absolute machine language:
 Produce an absolute machine language program
 can be placed in a fixed location in memory and
immediately executed.
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11. Contd..
Relocatable machine language:
Producing a relocatable machine language program
subprograms to be compiled separately.
relocatable object modules can be linked together and loaded
for execution by a linking loader.
must provide explicit relocation information to the loader, to
link the separately compiled program segments.
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12. Contd…
Assembly language:
Producing an assembly language program
makes the process of code generation easier
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13. Instruction selection
The factors to be considered during instruction selection are:
 The uniformity and completeness of the instruction set.
 Instruction speed and machine idioms.
 Size of the instruction set.
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14. Contd…
Eg., for the following address code is:
a := b + c
d := a + e
inefficient assembly code is:
MOV b, R0 R0 ← b
ADD c, R0 R0 ← c + R0
MOV R0, a a ← R0
MOV a, R0 R0 ← a
ADD e, R0 R0 ← e + R0
MOV R0 , d d ← R0
Here the fourth statement is redundant, and so is the third
statement if ,
'a' is not subsequently used.
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15. Register allocation
• Instructions with register operands are usually shorter and
faster
• Efficient utilization of registers is important in
generating good code.
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16. Contd..
Register allocation phase:
• Select the set of variables that will reside in registers
Register assignment phase:
• Pick the specific register that a variable will reside in.
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17. Evaluation order
 The order in which computations are performed
 Affect the efficiency of the target code.
 Some computation orders require fewer registers to hold
intermediate results
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18. Approaches to code generation
 Most important criteria for code generator is that it
produces correct code
 Correctness takes on special signification
 It contains a straightforward code generation algorithm
 The output of such code generator can be improved by
peephole optimization technique
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19. Conclusion
Output of code generator phase is dependent on:
 Target language
 Operating system
 Memory management system
 Instruction selection
 Register allocation
 Evaluation order
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