Module 15: Reaching Definition
Lecture 30: Analysis
Example
U is the universal set of all expressions appearing in a program
In[B] and out[B] are sets of expressions available at the beginning/end of B
E-gen[B] and e-kill[B] are sets of expressions generated and killed in B
Out[B] = in[B] - e-kill[B]
e-gen[B]
In[B] =
out[P] for B not initial
In[B] =
where B
0
is the initial block
Initialization:
In[B
0
] =
Out[B
0
] = e-gen[B
0
]
Out[B] = U - e-kill[B] if B is not an entry block
Live Variable Analysis
Used for dead code elimination
In[B] and out[B] are sets of live variables at entry and exit
Def[B] set of variables assigned value in B prior to use in B
Use[B] set of variables whose value may be used before definition in B
In[B] = use[B] S (out[B] - def[B])
Out[B] = S in[S] where S is successor of B
A variable is live coming into a block if EITHER is it used in the block before re-definition OR it is live coming out and not re-defined
A variable is live coming out of a block if it is live coming into one of its successors
Initialization:
in[B]=
for all B
e-gen[B]
out[P] for B not initial
where B0 is the initial block
