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CPSC 325 - Compiler

CPSC 325 - Compiler. Tutorial 8 Code Generator (unoptimized). What is Code Generator?. Intermediate Representation. What Code Generator should do?. Translate all the instructions in the intermediate representation to assembly language Allocate space for the variables, arrays etc.

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CPSC 325 - Compiler

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  1. CPSC 325 - Compiler Tutorial 8 Code Generator (unoptimized)

  2. What is Code Generator?

  3. Intermediate Representation

  4. What Code Generator should do? • Translate all the instructions in the intermediate representation to assembly language • Allocate space for the variables, arrays etc. • Create the necessary symbolic information • Gather all of the necessary information

  5. What does computer understand?

  6. Assembly language • Advantages • Simplifies code generation due to use of symbolic instructions and symbolic names • Logical abstraction layer • Multiple Architectures can describe by a single assembly language • Can modify the implementation • Macro assembly instructions • Disadvantages • Additional process of assembling and linking • Assembler adds overhead

  7. Assembly langauge • Relocatable machine language (object modules) • All locations (addresses) represented by symbols • Mapped to memory addresses at link and load time • Flexibility of separate compilation • Absolute machine language • Addresses are hard-coded • Simple and straightforward implementation • Inflexible – hard to reload generated code • Used in interrupt handlers and device drivers

  8. Assembly example

  9. Memory Registers ALU Control Modern CPU • ALU • Control • Memory • Registers

  10. Arithmetic and Logic Unit (ALU) • Performs most of the data operations • Has the form: • OP Rdest, Rsrc1, Rsrc2 • Operations are: • Arithmetic operations (add, sub, mulo) • Logical operations (and, sll) • Comparison operations (seq, sge, slt)

  11. Arithmetic and Logic Unit (ALU) • Many arithmetic operations can cause an exception • Overflow and underflow • Can operate on different data types • 8, 16, 32 bits • Signed and unsigned arithmetic • Floating-point operations (separate ALU)

  12. Control • Handles the instruction sequencing • Executing instructions • All instructions are in memory • Fetch the instruction pointed by the PC and execute it • For general instructions, increment the PC to point to the next location in memory

  13. Control • Unconditional Branches • Fetch the next instruction from a different location • Unconditional jump to an address • j label • Unconditional jump to an address in a register • jr rsrc • To handle procedure calls, do an unconditional jump, but save the next address in the current stream in a register • jal label jalr rsrc

  14. Control • Conditional Branches • Perform a test,if successful fetch instructions from a new address, otherwise fetch the next instruction • Instructions are of the form: • brelop Rsrc1, Rsrc2, label • relop is of the form: • eq, ne, gt, ge, lt, le

  15. Control • Control transfer in special (rare) cases • traps and exceptions • Mechanism • save the next (or current) instruction location • find the address to jump to (from an exception vector) • jump to that location

  16. Others, additional information… • Please refer to your CPSC231 text book… In the book; there is all of the details.

  17. Memory layout Stack Heap Data segment Text Segment Reserved

  18. 0 zero hard-wired to zero 1 at Reserved for asm 2 – 3 v0 – v1 expr. eval and return of result 4 – 7 a0 – a3 arguments 1 to 4 8 – 15 t0 – t7 caller saved temporary 16 – 23 s0 – s7 calliee saved temporary 24 – 25 t8, t9 caller saved temporary 28 gp pointer to global area 29 sp stack pointer 30 fp frame pointer 31 ra return address Register

  19. Stack (cont.) • Please refer to the hand out for more details..

  20. Guidelines for the code generator • Lower the abstraction level slowly • Do many passes, that do few things (or one things) • Easier to break the project down, generate and debug • Keep the abstraction level consistent • IR should have ‘correct’ semantics at all time • at least you should know the semantics • Use assertions liberally • Use an assertion to check your assumption

  21. Guidelines for the code generator • Do the simplest but dump thing • it is ok to generate 0 + 1*x + 0*y • Make sure you know what can be done at… • Compile time in the compiler • Runtime in a runtime library • Runtime using generated code • Runtime library is your friend! • Don’t generate complex code sequences when it can be done in a runtime library assembly hack

  22. Guidelines for the code generator • Remember that optimizations will come later • Let the optimizer do the optimizations • Think about what optimizer will need and structure you code accordingly • Example: Register allocation, algebraic simplification, constant propagation • Setup a good testing program

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