Compiler Construction Notes have comprehensive overview of Code generation errors and recovery.

1. Submitted by: Momina Idrees
Roll # 1430303
CODE GENERATION, ERRORS &
RECOVERY
Assignment (Finals) of Compiler Construction

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CODE GENERATOR
The final phase of our compiler model is code generator. It takes input from
the intermediate representation with supplementary information in symbol
table of the source program and produces as output an equivalent target
program.
Issues in the design of a code generator
 Input to the code generator:
The intermediate representation of the source program produced by the
front end is input to code generator.
 Memory management:
Mapping names in the source program to addresses of data objects in
run-time memory. A name in a three- address statement refers to a
symbol table entry for the name. A relative address can be determined.
 Target programs:
 Absolute machine language
 Relocatable machine language
 Assembly language
 Selection of instruction:
The code generator takes Intermediate Representation as input and
converts (maps) it into target machine’s instruction set. One
representation can have many ways (instructions) to convert it, so it

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becomes the responsibility of the code generator to choose the appropriate
instructions wisely.
 Register allocation:
A program has a number of values to be maintained during the execution.
The target machine’s architecture may not allow all of the values to be
kept in the CPU memory or registers. Code generator decides what values
to keep in the registers. Also, it decides the registers to be used to keep
these values. The code generator has to track both the registers (for
availability) and addresses (location of values) while generating the code.
For both of them, the following two descriptors are used:
 Register descriptor: Register descriptor is used to inform the
code generator about the availability of registers. Register
descriptor keeps track of values stored in each register.
Whenever a new register is required during code generation, this
descriptor is consulted for register availability.
 Address descriptor: Values of the names (identifiers) used in
the program might be stored at different locations while in
execution. Address descriptors are used to keep track of memory
locations where the values of identifiers are stored. These
locations may include CPU registers, heaps, stacks, memory or a
combination of the mentioned locations.
Code generator keeps both the descriptor updated in real-time. For a load
statement, LD R1, x, the code generator:
 Updates the Register Descriptor R1 that has value of x and
 Updates the Address Descriptor (x) to show that one instance of
x is in R1.
 Ordering of instructions:
At last, the code generator decides the order in which the instruction
will be executed. It creates schedules for instructions to execute them.
Issues a “Code Generator” tackles:
The Code generation module has to tackle a number of issues.

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 Memory management: Mapping names to data objects in the run-time
system.
 Instruction selection: The assembly language instructions to choose to
encode intermediate code statements.
 Instruction scheduling: Instruction chosen must utilize the CPU
resources effectively. Hardware stalls must be avoided.
 Register allocation: Operands are placed in registers before executing
machine operation such as ADD, MULTIPLY etc. Most processors have a
limited set of registers available. The code generator has to make
efficient use of this limited resource.
Common Errors
Program submitted to a compiler often have errors of various kinds. So, good
compiler should be able to detect as many errors as possible in various ways
and also recover from them. Even in the presence of errors, the compiler
should scan the program and try to compile all of it (error recovery). Errors
may be encountered at various stages of the compilation process. A program
may have the following kinds of errors at various stages:
 Lexical: Name of some identifier typed incorrectly.
 Syntactical: Missing semicolon or unbalanced parenthesis.
 Semantical: Incompatible value assignment.
 Logical: Code not reachable, infinite loop.
Error-recovery strategies
 Panic mode
When a parser encounters an error anywhere in the statement, it
ignores the rest of the statement by not processing input from
erroneous input to delimiter, such as semi-colon. This is the easiest way
of error-recovery and also, it prevents the parser from developing
infinite loops.
 Statement mode
When a parser encounters an error, it tries to take corrective measures
so that the rest of inputs of statement allow the parser to parse ahead.

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For example, inserting a missing semicolon, replacing comma with a
semicolon etc. Parser designers have to be careful here because one
wrong correction may lead to an infinite loop.
 Error productions
Some common errors are known to the compiler designers that may
occur in the code. In addition, the designers can create augmented
grammar to be used, as productions that generate erroneous constructs
when these errors are encountered.
 Global correction
The parser considers the program in hand as a whole and tries to figure
out what the program is intended to do and tries to find out a closest match
for it, which is error-free. When an erroneous input (statement) X is fed,
it creates a parse tree for some closest error-free statement Y. This may
allow the parser to make minimal changes in the source code, but due to
the complexity (time and space) of this strategy, it has not been
implemented in practice yet.

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Reference
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https://www.docsity.com/en/simple-code-generation-compiler-construction-
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 Tutorialspointcom. (2018). Wwwtutorialspointcom. Retrieved 15 January,
2018, from
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 Slidesharenet. (2018). Slidesharenet. Retrieved 15 January, 2018, from
https://www.slideshare.net/naparnanayak/code-generation-15188739
 Tutorialspointcom. (2018). Wwwtutorialspointcom. Retrieved 15 January,
2018, from
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