Live Variables Analysis

107 Pages·2017·0.39 MB·English

by  Uday Khedker

Live Variables Analysis
Uday Khedker
(www.cse.iitb.ac.in/˜uday)
Department of Computer Science and Engineering,
Indian Institute of Technology, Bombay
Dec 2017
WSSE Pune Liveness Analysis: Outline 1/25
Outline
• Live Variables Analysis
• Strongly Live Variables Analysis
• Some Observations
Dec 2017 IIT Bombay
Part 1
Live Variables Analysis
WSSE Pune Liveness Analysis: Live Variables Analysis 2/25
Defining Live Variables Analysis
A variable v is live at a program point p, if some
path from p to program exit contains an r-value oc-
currence of v which is not preceded by an l-value
occurrence of v.
Start Start Start
p p p
v =a∗b v =a∗b
a=v+2 v =a+2 vv==v +2
End End End
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 2/25
Defining Live Variables Analysis
A variable v is live at a program point p, if some
path from p to program exit contains an r-value oc-
currence of v which is not preceded by an l-value
occurrence of v.
v is live at p
Start Start Start
p p p
v =a∗b v =a∗b
a=v+2 v =a+2 vv==v +2
End End End
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 2/25
Defining Live Variables Analysis
A variable v is live at a program point p, if some
path from p to program exit contains an r-value oc-
currence of v which is not preceded by an l-value
occurrence of v.
v is live at p v is not live at p
Start Start Start
p p p
v =a∗b v =a∗b
a=v+2 v =a+2 vv==v +2
End End End
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 2/25
Defining Live Variables Analysis
A variable v is live at a program point p, if some
path from p to program exit contains an r-value oc-
currence of v which is not preceded by an l-value
occurrence of v.
v is live at p v is not live at p v is live at p
Start Start Start
p p p
v =a∗b v =a∗b
a=v+2 v =a+2 vv==v +2
End End End
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 2/25
Defining Live Variables Analysis
A variable v is live at a program point p, if some
path from p to program exit contains an r-value oc- Path based
currence of v which is not preceded by an l-value specification
occurrence of v.
v is live at p v is not live at p v is live at p
Start Start Start
p p p
v =a∗b v =a∗b
a=v+2 v =a+2 vv==v +2
End End End
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 3/25
Defining Data Flow Analysis for Live Variables Analysis
INk = Genk ∪ (OUTk − Kill k)
Genk , Kill k
OUTk = INi ∪ INj
INi INj
Geni , Kill i Genj , Kill j
OUTi OUTj
Dec 2017 IIT Bombay
WSSE Pune Liveness Analysis: Live Variables Analysis 3/25
Defining Data Flow Analysis for Live Variables Analysis
Single statements or Maximal groups
Basic Blocks ≡
of sequentially executed statements
INk = Genk ∪ (OUTk − Kill k)
Genk , Kill k
OUTk = INi ∪ INj
INi INj
Geni , Kill i Genj , Kill j
OUTi OUTj
Dec 2017 IIT Bombay