Intermediate Language Generation

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Intermediate Language Generation

• COMP431 (Compilers) by Dr Alex Potanin

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JKIL Generation in JKit

• Done inside JavaFileReader
• Note how it resolves scopes using a stack but
  gives each local variable a unique name
• Constructs CFG as it parses the expressions
  inside methods
• Parses a basic block at a time and connects these
  using conditional or exception edges
• Look inside JavaFileReaders comments to see the
  strategies it uses to from from AST to JKIL!
• Some examples are in this lecture

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IL Generation (basic statements)

• Breaking down compound expressions
• Introduce temporary variables!

1 tmp y 2 2 x x tmp
x x (y 2)
1 tmp p.f 2 x tmp 2
x p.f 2
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IL Generation (if statements)

• Breaking down if statements
• Use conditional branch instruction!
• Breaking down if-then-else statements
• Use conditional and unconditional branch
  instructions!

if(x lt 2) x x 1
1 if x gt 2 goto 3 2 x x 1 3
1 if x gt 2 goto 4 2 x x 1 3 goto 5 4
x x 2 5
if(x lt 2) x x 1 else x x 2
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IL Generation (Short Circuiting)

• Breaking compound if-conditions
• Dont forget that and in Java use
  short-circuiting
• When evaluating X Y, evaluate Y iff X
  evaluates to true
• When evaluating X Y, evaluate Y iff X
  evaluates to false
• Useful for statements which have side-effects
  (e.g. methods calls)

class Test int x 0 void f(Test p) 1
if x gt 2 goto 5 2 tmp p.f(p) 3 if tmp
! 0 goto 5 4 x x 1 5
class Test int x 0 int f(Test f) if(x
lt 2 f(p) 0) x x 1 return x
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IL Generation (While Loops)

• Breaking down while statements
• Use conditional and unconditional branch
  instructions!
• Breaking down do-while statements
• Very similar to while statements!

1 if x gt 2 goto 4 2 x x 1 3 goto 1 4

while(x lt 2) x x 1
do x x 1 while(x lt 2)
1 x x 1 2 if x lt 2 goto 1 3
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IL Generation (Field Declarations)

• Parse modifiers
• Parse type
• Parse name
• Parse initialiser
• Create a new Field class
• Special classes in jkit.core take care of storing
  information about classes, methods, and fields as
  a whole

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IL Generation (Variables)

• Important to figure out the right scope given
  just a name
• Checks first if it is a local variable or field
• Checks if it is inherited field
• Checks if it is a filed in outer class
• Check if it is a class
• Otheriwse fail

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IL Generation (Switch Statements)

• Creates a flow graph with a block for each case
• Connects multiple edges to each case, but some
  cases may fall through

switch (ltcondgt) case 1 X case 2 Y break
case 3 Z default W
ltcondgt
1
default
2
3
X
Y
Z
W
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IL Generation (Exceptions)

• To parse try, first translates the entire body
  inside the try statement
• Then creates CFG for each catch clause
• Then adds the exception edges for each possible
  exception and finally block
• See JavaFileReader.parseTry method for more
  details!

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Possible Optimisations
int f(int x) 1 y x 2 z y 3 goto 5
4 goto 6 5 goto 3 6 return z
int f(int x) 1 return x

• Q) Optimisation what is it all about?
• A) Many things! But typical objectives are
• Reducing number of instructions representing
  program
• Reducing number of pipeline stalls by instruction
  reordering
• Improving performance in other ways, e.g.
  constant propagation, strength reduction.

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Possible Optimisations

• Constant Propagation
• Propagate constant values to reduce number of
  local variables
• One of the classic optimisations used by most
  compilers!

int f(int u) 1 w 1 2 x 2 3 y w
x 4 z y u 5 return z
int f(int u) 1 return 3 u
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Possible Optimisations

• Translating IL to Machine Code
• IL statements correspond to 1 instructions on
  most CPUs
• IL has unbounded number of variables
• Most CPUs have a fixed number of registers
• Must allocate each variable to a register - this
  is called register allocation
• Reducing number of registers required can
  preventing spilling

add 1,1,0 // x is 1 mult 2,1,1 // y
is 2 sgt 3,2,1 bne 3,zero,label1 addi
2,2,1 label1
1 x 1 2 y x x 3 if y gt 1 goto 5 4 y
y 1 5
MIPS
IL
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And finally, Java Bytecode

• IL is very similar to Java Bytecode!

• gt javap verbose Test
• public int f(int)
• Code
• Stack2, Locals2, Args_size2
• 0 iload_1
• 1 iconst_2
• 2 iadd
• 3 istore_1
• 4 iload_1
• 5 iconst_2
• 6 if_icmpne 13
• 9 iload_1
• 10 iconst_2
• 11 imul
• 12 istore_1
• 13 iload_1
• 14 ireturn

class Test public int f(int x) x x 2
if(x 2) x x 2 return x
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Java Bytecode

• ClassFileWriter takes care of translation JKil to
  Bytecode
• Well look at bytecode first, before discussion
  the ClassFileWriter