What simplifications could a compiler, or you, do without sacrifice fast execution

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What simplifications could a compiler, or you, do
without sacrifice fast execution?
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5-7 Code optimization
Two functions f and g
define MAX 10int aMAX, bMAX, cMAX,
xMAX, yMAXint i, j, r, s. . .int f(int
a, int b) int z z 2 a b return
zint g(int a, int b, int c) int z z
a c c b return z
What code optimization can the compiler do? -O,
-O0, -O1, -O2, -O3, -Os ?
With the O or O0 you have to do all
optimi-zations yourself
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Optimization flags
-O, -O0 No optimization-O1 Optimize for
size-O2 Optimize for speed and enable some
optimization-O3 Enable all optimizations as
O2, and intensive loop optimizations-Os Optimi
ze for speed
Default setting!
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Two for loops
. . .for(i 0 i lt MAX -1 i) xi
f(ai, bi) s 2 rfor(j 0 j lt MAX
- 1 j) yj s g(aj, bj, cj)
What can be done?
We want shorter execution time without increasing
the code!
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Loop integration
The two loops have the same range (0, MAX-1), and
no data dependency (x only in loop1, y only in
loop2). Loops can be integrated saves loop
overhead ( only i )!
s 2 rfor(i 0 i lt MAX - 1 i)
xi f(ai, bi) yj s g(aj, bj,
cj)
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Precalculation at compile time
The defined constant MAX is used as MAX - 1 in
the loop. MAX - 1 could be precalculated as 10
1 9 at compile time!
s 2 rfor(i 0 i lt 9 i) xi
f(ai, bi) yj s g(aj, bj,
cj)
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Algebraic simplification
Rewriting function g can save one multiplication
operation
mul sub mul mul sub
int g(int a, int b, int c) int z z c
(a b) return z
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Inlining of functions
Both functions f and g are short and their code
could be inserted directly in the loop.
int a10, b10, c10, x10, y10int i, r,
ss 2 rfor(i 0 i lt 9 i) xi
2 ai bi yj s ((ai bi)
ci)
loop unrolling would give shorter execution time,
but it would also increase the code size, so it
cant be used in this case.
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5-2 Register lifetime
A processor has this instruction type op R1, R2,
R3 all three registers must be different. Code
to run
u c d (1) v a b (2)w a u (3)x
v e (4)
How many registers are needed?
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Register Life Time Graph
u c d (1) v a b (2)w a u (3)x
v e (4)
Four registers are needed!
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Data Flow Graph
A Data Flow Graph can detect data dependencies.
u c d (1) v a b (2)w a u (3)x
v e (4)

• Must be before (3)
• Must be before (4)

(2) and (3) can change execution order!
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New Register Life Time Graph
New instruction order
u c d (1) w a u (2)v a b
(3)x v e (4)
Now only 3 registers needed. Saving 25.
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5-8 CDFG

• Control and Data Flow Graph (CDFG)
• Multiplication takes 3 cycles, all other
  instructions take 1 cycle. Best/Worst execution
  time?

mode 0 TBest 11 2
y 0if(mode 1) for(i 0 i lt 5
i) y ai bi

mode 1 TWorst 11 1(51) 54 5 34
T 31 4
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Multiply Accumulate operation
c) MAC-unit! R1 R1 R2 R3 in one cycle!
y ai bi / one cycle /
TWorst 11 1(51) 51 5 19
19/34 0.56. With MAC 56 of ordinary processor
execution time.
T 1
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Processes on a CPU
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Scheduling states of process
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Priority Driven Scheduling

• Each process has fixed priority
• The ready process with the highest priority
  executes
• Process executes until completion or preemtion
  by higher priority process

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Examples of sampling frequencies and execution
period.
Actuator servo2000 Hz
RTOS
GPS sensor20 Hz
Process periodsGPS1/20 50 ms Speed 1/1000
1 ms Joystick 1/500 2 ms Servo 1/2000
0.5 ms
Speed sensor1 kHz
Joystick500 Hz
Tasks will often run periodicaly with different
process periods.
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Task Triplet
P( max execution time, period, deadline
) deadline lt period RMS deadline period
(simplification)
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6-2 Processor utilization and feasible scheduling
Task TripletP(execution time, period, deadline)
deadline period P1(3, 9, 9) P2(1, 2, 2)
P3(1, 6, 6)
Timeline least-common multiple of process
periods 9, 2, 6 3?3, 2, 2?3 3?3?2 18
CPU utilization
100 ?
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Rate Monotonic Scheduling
RMS shortest period is assigned the highest
priority and so on.
RMS guarantee, feasible schedule exists if
In this case U 1 so there is no guarantee!
n 3 U lt 0.78
( Limit n ? U lt 69 )
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RMS figure
Priorities P2 gt P3 gt P1 (2 lt 6 lt 9)
P1 misses the deadline! No feasible schedule with
RMS!
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Earliest Deadline First Scheduling
EDF guarantee, feasible schedule exists if U
? 1This case U 1, EDF shall produce a feasible
schedule.
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6.3 Scheduling and semaphores
P(execution time, period, deadline) P1(1, 3, 3)
P2(1, 4, 4) P3(2, 6, 6) 3, 2?2, 2?3 3?2?2
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RMS P1 gt P2 gt P3 (3 lt 4 lt 6)
Sem1 is a binary semaphore. accessSem1() and
releaseSem1() takes 0 time.
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RMS with no critical sections
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RMS with critical sections