Very Busy Expression

• In[B] and Out[B] are sets of VBE at the beginning and end of B
• Use[B] set of expressions b+c computed in B with no prior definition of b or c
• Def[B] set of expression b+c for which either b or c is defined in block B prior to computation of b+c
• In[B] = out[B] - def[B] use[B]
  out[B] = in[S] where S is successor of B
• An expression is VBE coming into a block if either it is used in B or it is live coming out and not defined in B
• An expression is VBE coming out of a block if it is live going into all the successors of B
• Initialization:
  in[B] = U for all B

Common Sub-expression Elimination

For every statement s of the form x=y+z such that y+z is available at the beginning of the block and y and z are not re-defined
prior to s

1. Find all definitions which have y+z that reach s’ block
2. Create a new variable u
3. Replace each w=y+z found in (1) by
   u=y+z; w=u
4. Replace statement s by x=u