This document discusses next-use information, which tracks the subsequent uses of names (variables) in basic blocks to optimize register allocation. It describes computing next-use information by scanning backwards through three-address statements to determine which names are live and have subsequent uses at each statement. It also discusses storing multiple temporary names in the same register or memory location to reduce space and size when generating intermediate code.

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