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Lecture 23 Basic Blocks

Lecture 23 Basic Blocks. CSCE 531 Compiler Construction. Topics Code Generation Readings: 9. April 17, 2006. Overview. Last Time – Lec22 slides 1-14, 15-16 Finishing touches on Arrays in expressions Project 5 Today’s Lecture Questions on Project 5 – Functions Code Generation

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Lecture 23 Basic Blocks

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  1. Lecture 23 Basic Blocks CSCE 531 Compiler Construction • Topics • Code Generation • Readings: 9 April 17, 2006

  2. Overview • Last Time – Lec22 slides 1-14, 15-16 • Finishing touches on Arrays in expressions • Project 5 • Today’s Lecture • Questions on Project 5 – Functions • Code Generation • References: Chapter 9

  3. Code Generation • Chapter 9 • Issues in Code Generation • Input to code generator • Target programs • Memory management • Instruction selection • Register allocation

  4. Input to code generator • Quadruples • Triples • Indirect triples • Parse trees • Syntax trees • Mixtures – some trees some quads

  5. Target programs • Target Architectures • CISC • RISC • Target Architectures • Object modules • Assembly

  6. Target Machine Architecture • RISC vs CISC • Byte addressable, word addressable? • Byte order? Big Endian vs little • Address Modes supported by architecture • Absolute • Register • Indexed d(R) d + contents(R) • Indirect register *d(R) contents(d + contents(R)) • Cost of address modes in references to memory

  7. Memory management

  8. Instruction selection

  9. Instruction Costs • Floating point costs • FPadd = 2 time units • FPmultiply = 5 time units • FPdivide=9 time units • Page 521 – costs measured in instruction length (somewhat dated, but simple to understand) • mov R0, R1 - cost = 1 • mov R5, mem - cost = 2 • add $1, r3 - cost = 2 • sub 4(R0), *12(R1) - cost =3

  10. Instruction Costs of Blocks: a = b + c • mov b, R0 • add c, R0 • mov R0, a

  11. Other Cost Metrics

  12. Run-Time Storage Managment

  13. Heap • Really Operating System Problem not Compilers job • System Calls • ptr = malloc(size) • free(ptr) • Free space list • What functionality the compiler needs to supply? • Just pointers, function calls

  14. Basic Blocks and Flow Graphs • To generate better code, we will need to analyze the structure of code written as list of quadruples • A Basic Block is a sequence of consecutive statements in which flow of control enters at the beginning and leaves at the end without possibility of branching except at the end • Define/Use: a:= b + c • This statement defines “a” • It uses “b” and “c” • A name (identifier) is said to be live at a given point if its value is after that point in the program

  15. Leaders of the Block • A leader is the first statement of a basic block. • The algorithm for Basic Blocks • Determine the leaders first. • The first statement is a leader. • Any statement that is the target of a branch is a leader. • Any statement immediately following a branch is a leader. • For each leader the block extends from the leader up to the statement just prior to the next leader.

  16. Example 9.3 – Dot Product

  17. Basic Blocks for Dot Product Code

  18. Transformations on Basic Blocks • Common subexpression elimination • Dead-code elimination • Renaming temporary variables • Reordering independent statements

  19. Flow Graphs • Successor block • Predecessor block • Loops • Strongly connected • Inner loop

  20. Example

  21. Computing Next Uses • Scan block backwards from last statement to leader • For statement (i) x := y op z • Attach to statement I, current info from symbol table on the liveness of x y and z • In the symbol table Mark x “not live” = “no next use” • In the symbol table change next uses of y and z to (i) “statement number i”

  22. A Simple Code Generation Algorithm

  23. GetReg

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