Embedded Computer Architecture
VLIW architectures:
Generating VLIW code
TU/e 5kk73
Henk Corporaal
VLIW lectures overview
•
•
•
•
Enhance performance: architecture methods
Instruction Level Parallelism
VLIW
Examples
– C6
– TM
– TTA
• Clustering and Reconfigurable components
• Code generation
–
–
–
–
compiler basics
mapping and scheduling
TTA code generation
Design space exploration
• Hands-on
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2
Compiler basics
• Overview
–
–
–
–
–
–
Compiler trajectory / structure / passes
Control Flow Graph (CFG)
Mapping and Scheduling
Basic block list scheduling
Extended scheduling scope
Loop scheduling
– Loop transformations
• separate lecture
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3
Compiler basics: trajectory
Source program
Preprocessor
Compiler
Error
messages
Assembler
Library
code
Loader/Linker
Object program
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Compiler basics: structure / passes
Source code
Lexical analyzer
token generation
check syntax
check semantic
parse tree generation
Parsing
Intermediate code
Code optimization
Code generation
Register allocation
Sequential code
Scheduling and allocation
data flow analysis
local optimizations
global optimizations
code selection
peephole optimizations
making interference graph
graph coloring
spill code insertion
caller / callee save and restore code
exploiting ILP
Object code
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5
Compiler basics: structure
Simple example: from HLL to (Sequential) Assembly code
position := initial + rate * 60
Lexical analyzer
id := id + id * 60
temp1 := intoreal(60)
temp2 := id3 * temp1
temp3 := id2 + temp2
id1 := temp3
Syntax analyzer
Code optimizer
temp1 := id3 * 60.0
id1 := id2 + temp1
:=
id
+
id
*
id
Code generator
60
Intermediate code generator
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movf
mulf
movf
addf
movf
id3, r2
#60, r2, r2
id2, r1
r2, r1
r1, id1
6
Compiler basics:
Control flow graph (CFG)
CFG: shows the flow
between basic blocks
C input code:
if (a > b)
else
{ r = a % b; }
{ r = b % a; }
1
sub t1, a, b
bgz t1, 2, 3
2
3
rem r, a, b
goto 4
rem r, b, a
goto 4
4
…………..
…………..
Program, is collection of
Functions, each function is collection of
Basic Blocks, each BB contains set of
Instructions, each instruction consists of several
Transports,..
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Compiler basics: Basic optimizations
• Machine independent optimizations
• Machine dependent optimizations
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Compiler basics: Basic optimizations
• Machine independent optimizations
–
–
–
–
–
–
–
Common subexpression elimination
Constant folding
Copy propagation
Dead-code elimination
Induction variable elimination
Strength reduction
Algebraic identities
• Commutative expressions
• Associativity: Tree height reduction
– Note: not always allowed(due to limited precision)
• For details check any good compiler book !
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Compiler basics: Basic optimizations
• Machine dependent optimization example
– What’s the optimal implementation of a*34 ?
– Use multiplier: mul Tb, Ta, 34
• Pro: No thinking required
• Con: May take many cycles
– Alternative:
– SHL Tb, Ta, 1
– SHL Tc, Ta, 5
– ADD Tb, Tb, Tc
•
•
•
•
4/13/2015
Pros: May take fewer cycles
Cons:
Uses more registers
Additional instructions ( I-cache load / code size)
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Compiler basics: Register allocation
• Register Organization
– Conventions needed for parameter passing
– and register usage across function calls
r31
Callee saved registers
r21
r20
r11
r10
Caller saved registers
Other temporaries
Function Argument and Result transfer
r1
r0
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Hard-wired 0
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Register allocation using graph coloring
Given a set of registers, what is the most efficient
mapping of registers to program variables in terms
of execution time of the program?
Some definitions:
• A variable is defined at a point in program when a value is
assigned to it.
• A variable is used at a point in a program when its value is
referenced in an expression.
• The live range of a variable is the execution range between
definitions and uses of a variable.
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Register allocation using graph coloring
Program:
define
Live Ranges
a
b
c
d
a :=
c :=
b :=
:= b
d :=
:= a
:= c
:= d
use
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Register allocation using graph coloring
Inference Graph
a
Coloring:
a = red
b = green
c = blue
d = green
b
c
d
Graph needs 3 colors => program needs 3 registers
Question: map coloring requires (at most) 4 colors; what’s the
maximum number of colors (= registers) needed for register
interference graph coloring?
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Register allocation using graph coloring
Spill/ Reload code
Spill/ Reload code is needed when there are not enough colors
(registers) to color the interference graph
Example:
Only two registers
available !!
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Embedded Computer Architecture
Program:
Live Ranges
a
b
c
d
a :=
c :=
store c
b :=
:= b
d :=
:= a
load c
:= c
:= d
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Register allocation for a monolithic RF
Scheme of the optimistic register allocator
Spill code
Renumber
Build
Spill costs
Simplify
Select
The Select phase selects a color (= machine register) for a variable
that minimizes the heuristic h:
h = fdep(col, var) + caller_callee(col, var)
where:
fdep(col, var)
: a measure for the introduction of false dependencies
caller_callee(col, var) : cost for mapping var on a caller or callee saved register
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Some explanation of reg allocation phases
•
•
[Renumber:] The first phase finds all live ranges in a procedure
and numbers (renames) them uniquely.
•
[Build:] This phase constructs the interference graph.
•
•
•
•
[Spill Costs:] In preparation for coloring, a spill cost estimate
is computed for every live range. The cost is simply the sum of the
execution frequencies of the transports that define or use the variable
of the live range.
•
•
•
•
•
•
[Simplify:] This phase removes nodes with degree < k in an
arbitrary order from the graph and pushes them on a stack. Whenever
it discovers that all remaining nodes have degree >= k, it chooses
a spill candidate. This node is also removed from the graph and
optimistically pushed on the stack, hoping a color will be available in
spite of its high degree.
•
•
•
•
•
[Select:] Colors are selected for nodes. In turn, each node is
popped from the stack, reinserted in the interference graph and given a
color distinct from its neighbors. Whenever it discovers that it has no
color available for some node, it leaves the node uncolored and
continues with the next node.
•
•
[Spill Code:] In the final phase spill code is
inserted for the live ranges of all uncolored nodes.
•
•
•
Some symbolic registers must be mapped on a specific machine register
(like stack pointer). These registers get their color in the simplify
stage instead of being pushed on the stack.
•
•
The other machine registers are divided in caller-saved and callee-saved
registers. The allocator computes the caller-saved and callee-saved cost.
•
•
•
•
•
•
•
The caller-saved cost for the symbolic registers is computed when they have
live-ranges across a procedure call. The cost per symbolic register is twice
the execution frequency of its transport.
The callee-saved cost of a
symbolic register is twice the execution frequency of the procedure to which
the transport of the symbolic register belongs. With these two costs in mind
the allocator chooses a machine register.
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Compiler basics: Code selection
• CISC era (before 1985)
– Code size important
– Determine shortest sequence of code
• Many options may exist
– Pattern matching
Example M68029:
D1 := D1 + M[ M[10+A1] + 16*D2 + 20 ] 
ADD ([10,A1], D2*16, 20) D1
• RISC era
– Performance important
– Only few possible code sequences
– New implementations of old architectures optimize RISC part of
instruction set only; for e.g. i486 / Pentium / M68020
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Overview
•
•
•
•
Enhance performance: architecture methods
Instruction Level Parallelism
VLIW
•What is scheduling
Examples
•Basic Block Scheduling
– C6
– TM
– TTA
• Clustering
• Code generation
•Extended Basic Block
Scheduling
•Loop Scheduling
– Compiler basics
– Mapping and Scheduling of Operations
• Design Space Exploration: TTA framework
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Mapping / Scheduling =
placing operations in space and time
d = a * b;
e = a + d;
f = 2 * b + d;
r = f – e;
x = z + y;
a
b
2
*
*
d
z
+
+
e
f
-
y
+
x
r
Data Dependence Graph (DDG)
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How to map these operations?
b
a
*
Architecture constraints:
• One Function Unit
• All operations single cycle latency
2
*
d
+
e
+
f
r
4/13/2015
z
y
cycle 1
*
2
*
3
+
4
5
+
6
+
+
x
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-
21
How to map these operations?
b
a
*
Architecture constraints:
• One Add-sub and one Mul unit
• All operations single cycle latency
2
*
d
+
e
+
f
r
z
y
cycle 1
2
+
Mul
*
Add-sub
+
*
+
3
+
4
5
-
x
6
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There are many mapping solutions
x
Pareto graph
x
x
(solution space)
x
x
x
x
x
x
x
x
x
x
x
xx
x
x
x
x
x
x
x
x
x
0
x
x
x
x
x
x
x
Cost
Point x is pareto  there is no point y for which i yi<xi
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Scheduling: Overview
Transforming a sequential program into a parallel program:
read sequential program
read machine description file
for each procedure do
perform function inlining
for each procedure do
transform an irreducible CFG into a reducible CFG
perform control flow analysis
perform loop unrolling
perform data flow analysis
perform memory reference disambiguation
perform register allocation
for each scheduling scope do
perform instruction scheduling
write out the parallel program
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Basic Block Scheduling
• Basic Block =
piece of code which can only be entered from the top (first
instruction) and left at the bottom (final instruction)
• Scheduling a basic block =
Assign resources and a cycle to every operation
• List Scheduling =
Heuristic scheduling approach, scheduling the
operation one-by-one
– Time_complexity = O(N), where N is #operations
• Optimal scheduling has Time_complexity = O(exp(N)
• Question: what is a good scheduling heuristic
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Basic Block Scheduling
•
Make a Data Dependence Graph (DDG)
•
Determine minimal length of the DDG (for the given architecture)
– minimal number of cycles to schedule the graph (assuming sufficient resources)
•
Determine:
–
–
–
–
ASAP (As Soon As Possible) cycle = earliest cycle instruction can be scheduled
ALAP (As Late As Possible) cycle = latest cycle instruction can be scheduled
Slack of each operation = ALAP – ASAP
Priority of operations = f (Slack, #decendants, #register impact, …. )
•
Place each operation in first cycle with sufficient resources
•
Notes:
– Basic Block = a (maximal) piece of consecutive instructions which can only be entered at the
first instruction and left at the end
– Scheduling order sequential
– Scheduling Priority determined by used heuristic; e.g. slack + other contributions
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Basic Block Scheduling:
determine ASAP and ALAP cycles
B
ASAP cycle
C
we assume all
operations are
single cycle !
ALAP cycle
A
ADD
<2,2>
SUB
<3,3>
NEG
<1,1> slack
A
LD
<2,3>
ADD
C
ADD
<4,4>
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LD
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A
B
MUL
<2,4>
y
X
4/13/2015
<1,3>
<1,4>
z
27
Cycle based list scheduling
proc Schedule(DDG = (V,E))
beginproc
ready = { v | (u,v)  E }
ready’ = ready
sched = 
current_cycle = 0
while sched  V do
for each v  ready’ (select in priority order) do
if ResourceConfl(v,current_cycle, sched) then
cycle(v) = current_cycle
sched = sched  {v}
endif
endfor
current_cycle = current_cycle + 1
ready = { v | v  sched   (u,v) E, u  sched }
ready’ = { v | v  ready   (u,v) E, cycle(u) + delay(u,v)  current_cycle}
endwhile
endproc
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Extended Scheduling Scope:
look at the CFG
Code:
A;
If cond
Then B
Else C;
D;
If cond
Then E
Else F;
G;
CFG:
Control
Flow
Graph
A
B
C
D
E
F
Q: Why enlarge the scheduling scope?
G
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Extended basic block scheduling:
Code Motion
A
a) add r3, r4, 4
b) beq . . .
Q: Why moving code?
B
C
c) add r1, r1, r2
d) sub r3, r3, r2
D
e) mul r1, r1, r3
• Downward code motions?
— a  B, a  C, a  D, c  D, d  D
• Upward code motions?
— c  A, d  A, e  B, e  C, e  A
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Possible Scheduling Scopes
Trace
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Superblock
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Decision tree
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Hyperblock/region
31
A
B
C
Create and Enlarge Scheduling Scope
D
E
F
G
A
A
C
B
D
F
E
Trace
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C
D
D’
E
tail
duplication
E’
F
G
G
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B
G’
Superblock
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A
B
C
Create and Enlarge Scheduling Scope
D
E
F
G
A
C
B
E
F
G
G’
D
F’
E’
G’’
Decision Tree
Embedded Computer Architecture
C
B
D’
D
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A
tail
duplication
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F
E
G
Hyperblock/ region
33
A
B
C
Comparing scheduling scopes
D
E
F
G
Trace Sup. Hyp.
block block
Multiple exc. paths
No
No
Yes
Side-entries allowed
Yes
No
No
Join points allowed
Yes
No
Yes
Code motion down joins Yes
No
No
Must be if-convertible
No
No
Yes
Tail dup. before sched.
No
Yes
No
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Dec. Region
Tree
Yes
Yes
No
No
No
Yes
No
No
No
No
Yes
No
34
Code movement (upwards) within
regions: what to check?
destination block
Legend:
Copy
needed
I
I
I
I
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Embedded Computer Architecture
Check for
off-liveness
Code
movement
I
add
Intermediate
block
source block
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Extended basic block scheduling:
Code Motion
• A dominates B  A is always executed before B
– Consequently:
• A does not dominate B  code motion from B to A requires
code duplication
• B post-dominates A  B is always executed after A
– Consequently:
• B does not post-dominate A  code motion from B to A is speculative
A
Q1: does C dominate E?
B
Q2: does C dominate D?
C
Q3: does F post-dominate D?
D
E
Q4: does D post-dominate B?
F
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Scheduling: Loops
Loop Optimizations:
A
C
B
A
C’
C’
C’’
C’’
D
D
Loop unrolling
Loop peeling
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C
B
C
B
A
D
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Scheduling: Loops
Problems with unrolling:
• Exploits only parallelism within sets of n iterations
• Iteration start-up latency
• Code expansion
resource utilization
Basic block scheduling
Basic block scheduling
and unrolling
Software pipelining
time
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Software pipelining
• Software pipelining a loop is:
– Scheduling the loop such that iterations start before
preceding iterations have finished
Or:
– Moving operations across the backedge


Example: y = a.x
LD
LD
LD
LD ML
LD ML
ML
LD ML ST
LD ML ST
ST
ML ST
ML ST
ST
ST
3 cycles/iteration
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Embedded Computer Architecture
Unroling (3 times)
Software pipelining
5/3 cycles/iteration
1 cycle/iteration
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Software pipelining (cont’d)
Basic loop scheduling techniques:
• Modulo scheduling (Rau, Lam)
– list scheduling with modulo resource constraints
• Kernel recognition techniques
–
–
–
–
unroll the loop
schedule the iterations
identify a repeating pattern
Examples:
This algorithm most used in
commercial compilers
• Perfect pipelining (Aiken and Nicolau)
• URPR (Su, Ding and Xia)
• Petri net pipelining (Allan)
• Enhanced pipeline scheduling (Ebcioğlu)
– fill first cycle of iteration
– copy this instruction over the backedge
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Software pipelining: Modulo scheduling
Example: Modulo scheduling a loop
ld
mul
sub
st
for (i = 0; i < n; i++)
A[i+6] = 3* A[i] - 1;
(a) Example loop
r1,(r2)
r3,r1,3
r4,r3,1
r4,(r5)
(b) Code (without loop control)
ld
mul
sub
st
r1,(r2)
r3,r1,3
r4,r3,1
r4,(r5)
ld
mul
sub
st
r1,(r2)
r3,r1,3
r4,r3,1
r4,(r5)
Prologue
ld
mul
sub
st
(c) Software pipeline
r1,(r2)
r3,r1,3
r4,r3,1
r4,(r5)
ld
mul
sub
st
r1,(r2)
r3,r1,3
r4,r3,1
r4,(r5)
Kernel
Epilogue
• Prologue fills the SW pipeline with iterations
• Epilogue drains the SW pipeline
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Software pipelining:
determine II, the Initiation Interval
For (i=0;.....)
Cyclic data dependences
A[i+6]= 3*A[i]-1
ld r1, (r2)
(0,1)
(1,0)
(delay, iteration distance)
mul r3, r1, 3
(0,1)
(1,0)
(1,6)
sub r4, r3, 1
(0,1)
(1,0)
st r4, (r5)
Initiation Interval
cycle(v)  cycle(u) + delay(u,v) - II.distance(u,v)
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ld_1
ld_2
ld_3
-5
ld_4
ld_5
ld_6 st_1
ld_7
42
Modulo scheduling constraints
MII, minimum initiation interval, bounded by cyclic dependences
and resources:
MII = max{ ResMinII, RecMinII }
Resources:
 used(r ) 
ResMinII max 

rresources available
(
r
)


Cycles:
cycle(v)  cycle(v)  delay(e)  II.distance(e)
ec
Therefore:


RecMinII minII  N | ccycles,0   delay(e)  II.distance(e)
ec


Or:
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 ec delay(e) 
RecMinII max 

ccycles
 ec distance(e) 
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Let's go back to: The Role of the Compiler
9 steps required to translate an HLL program:
(see online bookchapter)
1.
2.
3.
4.
5.
6.
7.
8.
9.
4/13/2015
Front-end compilation
Determine dependencies
Graph partitioning: make multiple threads (or tasks)
Bind partitions to compute nodes
Bind operands to locations
Bind operations to time slots: Scheduling
Bind operations to functional units
Bind transports to buses
Execute operations and perform transports
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Division of responsibilities between hardware and
compiler
Application
(1)
Frontend
(2)
Determine Dependencies
Superscalar
Determine Dependencies
Dataflow
(3)
Binding of Operands
Binding of Operands
Multi-threaded
(4)
Scheduling
Scheduling
Indep. Arch
(5)
Binding of Operations
Binding of Operations
VLIW
(6)
Binding of Transports
TTA
(7)
Execute
Responsibility of compiler
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Binding of Transports
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Responsibility of Hardware
45
Overview
•
•
•
•
Enhance performance: architecture methods
Instruction Level Parallelism
VLIW
Examples
– C6
– TM
– TTA
• Clustering
• Code generation
• Design Space Exploration: TTA framework
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Mapping applications to processors
MOVE framework
User
intercation
Optimizer
x
x
x
feedback
x
x
Architecture
parameters
Parametric compiler
Pareto curve
(solution space)
x
feedback
x
x
x
x
x
x
x
x x
x
x
x
x x
cost
Hardware generator
Move framework
Parallel
object
code
chip
TTA based system
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TTA (MOVE) organization
Data Memory
load/store load/store
unit
unit
integer
ALU
integer
ALU
boolean
RF
instruct.
unit
float
ALU
Socket
integer
RF
float
RF
immediate
unit
Instruction Memory
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Code generation trajectory for TTAs
• Frontend:
GCC or SUIF
(adapted)
Architecture description
Application (C)
Compiler frontend
Sequential code
Compiler backend
Parallel code
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Sequential simulation
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Input/Output
Profiling data
Parallel simulation
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Input/Output
49
Exploration: TTA resource reduction
•
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Embedded Computer Architecture
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Execution time
Exporation: TTA connectivity reduction
FU stage constrains cycle time
0
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Embedded Computer Architecture
Number of connections removed
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Can we do better?
• Code Transformations
• SFUs: Special Function
Units
• Vector processing
• Multiple Processors
Execution time
How ?
Cost
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Transforming the specification (1)
+
+
+
+
+
+
Based on associativity of + operation
a + (b + c) = (a + b) + c
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Embedded Computer Architecture
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Transforming the specification (2)
d = a * b;
e = a + d;
f = 2 * b + d;
r = f – e;
x = z + y;
a
b
*
d
1
2
+
f
r
b
z
+
x
r
x
Embedded Computer Architecture
z
+
-
y
y
a
<<
*
+
e
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r = 2*b – a;
x = z + y;
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Changing the architecture
adding SFUs: special function units
+
+
+
+
+
+
4-input adder
why is this faster?
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Embedded Computer Architecture
H. Corporaal, and B. Mesman
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Changing the architecture
adding SFUs: special function units
In the extreme case put everything into one unit!
Spatial mapping
- no control flow
However: no flexibility / programmability !!
but could use FPGAs
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Embedded Computer Architecture
H. Corporaal, and B. Mesman
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SFUs: fine grain patterns
• Why using fine grain SFUs:
–
–
–
–
–
–
Code size reduction
Register file #ports reduction
Could be cheaper and/or faster
Transport reduction
Power reduction (avoid charging non-local wires)
Supports whole application domain !
• coarse grain would only help certain specific applications
Which patterns do need support?
• Detection of recurring operation patterns needed
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Embedded Computer Architecture
H. Corporaal, and B. Mesman
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SFUs: covering results
Adding only 20 'patterns' of 2
operations dramatically reduces # of
operations (with about 40%) !!
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Embedded Computer Architecture
H. Corporaal, and B. Mesman
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Exploration: resulting architecture
stream
input
4 Addercmp FUs
2 Multiplier FUs
2 Diffadd FUs
4 RFs
9 buses
stream
output
Architecture for image processing
• Several SFUs
• Note the reduced connectivity
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Embedded Computer Architecture
H. Corporaal, and B. Mesman
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Conclusions
• Billions of embedded processing systems / year
– how to design these systems quickly, cheap, correct,
low power,.... ?
– what will their processing platform look like?
• VLIWs are very powerful and flexible
– can be easily tuned to application domain
• TTAs even more flexible, scalable, and lower
power
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H. Corporaal, and B. Mesman
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Conclusions
• Compilation for ILP architectures is mature
– used in commercial compilers
• However
– Great discrepancy between available and exploitable
parallelism
• Advanced code scheduling techniques needed to
exploit ILP
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Bottom line:
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Handson-1 (2014)
•
•
•
•
•
•
HOW FAR ARE YOU?
VLIW processor of Silicon Hive (Intel)
Map your algorithm
Optimize the mapping
Optimize the architecture
Perform DSE (Design Space Exploration) trading
off (=> Pareto curves)
– Performance,
– Energy and
– Area (= Cost)
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