1. Instruction Scheduling: Beyond Basic Blocks
Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved.
Students enrolled in Comp 412 at Rice University have explicit permission to make copies of these materials for their personal use.

2. Local Scheduling As long as we stay within a single block
List scheduling does well
Problem is hard, so tie-breaking matters
More descendants in dependence graph
Prefer operation with a last use over one with none
Breadth first makes progress on all paths
Tends toward more ILP & fewer interlocks
Depth first tries to complete uses of a value
Tends to use fewer registers
Classic work on this is Gibbons & Muchnick

3. Block from SPEC benchmark “go”
Local Scheduling
Forward and backward can produce different results
Latency to the cbr 8 8 8 8 8
cbr
cmp
store1
store2
store3
store4
store5
add1
add2
add3
add4
addI
loadI1
lshift
loadI2
loadI3
loadI4
Subscript to identify 7 7 7 7 7 2 5 5 5 5 5
Block from SPEC benchmark “go” 1
Operation
load
loadI
add
addI
store
cmp
Latency 1 2 4

4. Using latency to root as the priority
Local Scheduling F o r w a d S c h e u l
Int
Mem 1
loadI1
lshift 2
loadI2
loadI3 3
loadI4
add1 4
add2
add3 5
add4
addI
store1 6
cmp
store2 7
store3 8
store4 9
store5 10 11 12 13
cbr B a c k w r d S h e u l
Int
Mem 1
loadI4 2
addI
lshift 3
add4
loadI3 4
add3
loadI2
store5 5
add2
loadI1
store4 6
add1
store3 7
store2 8
store1 9 10 11
cmp 12
cbr 13
Using latency to root as the priority

5. Local Scheduling Schielke’s RBF algorithm
Run 5 passes of forward list scheduling
and 5 passes of backward list scheduling
Break each tie randomly
Keep the best schedule
Shortest time to completion
Other metrics are possible (shortest time + fewest registers)
In practice, this does very well
Randomized
Backward &
Forward

6. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs a b c d g e f h i l j k B1 B2 B4 B6 B5 B3

7. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs
Only two have nontrivial paths
{B1,B2,B4 } & {B1,B3 }
Having B1 in both causes conflicts
Moving an op out of B1 causes
problems a b c d g e f h i l j k B1 B2 B4 B6 B5 B3

8. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs
Only two have nontrivial paths
{B1,B2,B4 } & {B1,B3 }
Having B1 in both causes conflicts
Moving an op out of B1 causes
problems a b c d g
c,e f h i l j k B1 B2 B4 B6 B5 B3
no c here !

9. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs
Only two have nontrivial paths
{B1,B2,B4 } & {B1,B3 }
Having B1 in both causes conflicts
Moving an op out of B1 causes
problems
Must insert “compensation”
code in B3
Increases code space a b c d g
c,e f h i l j k B1 B2 B4 B6 B5 B3
This one wasn’t done for speed!

10. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs
Only two have nontrivial paths
{B1,B2,B4 } & {B1,B3 }
Having B1 in both causes conflicts
Moving an op into B1 causes
problems a b c d g e f h i l j k B1 B2 B4 B6 B5 B3

11. Scheduling Larger Regions
Superlocal Scheduling
Work EBB at a time
Example has four EBBs
Only two have nontrivial paths
{B1,B2,B4 } & {B1,B3 }
Having B1 in both causes conflicts
Moving an op into B1 causes
problems
Lengthens {B1,B3 }
Adds computation to {B1,B3 }
May need compensation code, too
Renaming may avoid “undo f” a b c
d,f
undo f g e f h i l j k B1 B2 B4 B6 B5 B3
This makes the path even longer!

12. Scheduling Larger Regions
Superlocal Scheduling
How much can we get?
Schielke saw 11 to 12% speed ups
Constrained away compensation code
Why was this harder than DVNT?
DVNT moved information
Scheduling moves ops
DVNT moves forward
Scheduling moves both ways
Value tables partition nicely
Dependence graph does not a b c d g e f h i l j k B1 B2 B4 B6 B5 B3
Value numbering is the best case for superlocal scope

13. Scheduling Larger Regions
More Aggressive Superlocal Scheduling
Clone blocks to create more context a b c d g e f h i l j k B1 B2 B4 B6 B5 B3
Join points create blocks that must work in multiple contexts
2 paths
3 paths

14. Scheduling Larger Regions
More Aggressive Superlocal Scheduling
Clone blocks to create more context
Some blocks can combine
Single successor, single predecessor B1 a b c d B2 B3 e f g B4
B5a
B5b h i j k j k
B6a
B6b
B6c l l l

15. Scheduling Larger Regions
More Aggressive Superlocal Scheduling
Clone blocks to create more context
Some blocks can combine
Single successor, single predecessor B1 a b c d B2 B3 e f g B4
B5a
B5b h i j k j k
B6a
B6b
B6c l l l

16. Scheduling Larger Regions
More Aggressive Superlocal Scheduling
Clone blocks to create more context
Some blocks can combine
Single successor, single predecessor
Now schedule EBBs
{B1,B2,B4 }, {B1,B2,B5q }, {B1,B3,B5b }
Pay heed to compensation code
Works well for forward motion
Backward motion still has off-path problems
Speeding up one path can slow down others (undo) B1 a b c d B2 B3 e f g B4
B5a
B5b h i l j k l j k l

17. Scheduling Larger Regions
Trace Scheduling
Start with execution counts for edges
Obtained by profiling a b c d g e f h i l j k B1 B2 B4 B6 B5 B3

18. Scheduling Larger Regions10
Trace Scheduling
Start with execution counts for edges
Obtained by profiling
Pick the “hot” path a b c d g e f h i l j k B1 B2 B4 B6 B5 B3 7 3 5 2 3 5 5
Block counts could mislead us — see B5

19. Scheduling Larger Regions10
Trace Scheduling
Start with execution counts for edges
Obtained by profiling
Pick the “hot” path
B1,B2,B4,B6
Schedule it
Compensation code in B3,B5
if needed
Get the hot path right!
If we picked the right path, the
other blocks do not matter as much
Places a premium on quality profiles B1 a b c d 7 3 B2 B3 e f g 5 2 3 B4 B5 h i j k 5 5 B6 l

20. Scheduling Larger Regions10
Trace Scheduling Entire CFG
Pick & schedule hot path
Insert compensation code
Remove hot path from CFG
Repeat the process until CFG is empty
Idea
Hot paths matter
Farther off hot path,
less it matters a b c d g e f h i l j k B1 B2 B4 B6 B5 B3 7 3 5 2 3 5 5