code generating to next use information

2. Next-use information
Introduction:
we collect next-use information about names in
basic blocks.
if the name in a register is no longer needed,
then the register can be assigned to some other
name.
This idea of keeping a name in storage only if it
will be used subsequently can be applied in a
number of contexts.

3. Next-use information
• Computing Next Uses
• Storage for Temporary Names

4. Computing next uses
• The use of a name in a three-address
statement:
– Three-address statement i assigns a value to x
– Statement j has x as an operand
– Control can flow from statement i to j along a path that has
no intervening assignments to x
– Then statement j uses the value of x computed at i .
– Say that x is live at statement i .
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5. Next-use information
If the algorithms generating intermediate
code or optimizing the code permit certain
temporaries to be used across blocks, these
too must be considered live.
it would be a good idea to mark any such
temporaries, so we do not have to consider
all temporaries live.

6. Next-Use Information
• Algorithm (for x=y+z) : Determining the
liveness and next-use information for each
statement in a basic block.
– INPUT: A basic block B of three-address statements. Assume the symbol table
initially shows all nontemporary variables in B as being live on exit.
– OUTPUT: At each statement i : x = y + z in B, attach to i the liveness and next-
use information of x, y, and z .
– METHOD: Start at the last statement in B and scan backwards to the beginning
of B. At each statement i: x = y + z in B, do the following:
1. Attach to i the information currently found in the symbol table regarding
the next use and
liveness of x , y, and z.
2. In the symbol table, set x to "not live" and "no next use."
3. In the symbol table, set y and z to "live" and the next uses of y and z to i.
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7. Storage for Temporary Names
• Each time separate name has to be created for
using temporary.
• Space and Size increased.
• Pack two temporaries in same location.
• Stored in register than Memory

8. Six temp to two locations t1 & t2
t1:=a*a
t2:=a*b
t2=2*t2
t1=t1+t2
t2=b*b
t1=t1+t2